Introduction to Voltammetric Analysis: Theory and Practice

Introduction to

Voltammetric Analysis

Theory and Practice

F. G. Thomas and G. Henze

CS I RO

PUBLISHING

©CSIR02001 All rights reserved. Except under the conditions described in the Australian Cop y ri ght

Act 1968 and publication may be reproduced, stored in a retrieval system or transmitted in any form or by any means, electronic, mechanical, photocopying, recordi n g , duplicating subsequent amendments, no part of this

or otherwise, without the prior permission of the copyright owner. Contact

CSIRO PUBLISHING for

all permission requests.

National Libra ry of Australia Cataloguing-in-Publication entry Thomas, F. G. (Francis George),

1933-. Introduction to voltammetric analysis. B i b liography.

Includes index. ISBN 0 643

06593

1. Voltammetry. 2.

tl.

Electrochemical analysis. I. Henze, Gunter.

543.087I Available from:

CSIRO

PUBLISHING

150 Oxford Street

(PO Box 1139)

Collingwood VIC 3066

Australia

Tel:

+61 3 9662 7500

Email:

[email protected]

Website:

http://www.publish.csiro.au

Fax:

+61 3

9662 7555

Set in Adobe Minion and Frutiger Cover design by Melissa Gibson Edited by Brenda Hamilton

Typeset by Desktop Concepts

P/L,

Melbourne

Printed in Australia by Brown Prior Anderson

(1932- ) II. .

Title.

Foreword

The subject of electrochemical analysis is essentially as old as the invention of the battery by Volta more than 200 years ago. In this 200 year history, the subject has been fashionable at times and under threat of extinction in other periods. For example, it has often been said that voltammetry, which is a very important sub-discipline of the electrochemical analysis, almost disappeared in the middle of the last century and then underwent a renaissance in the 1 960s when high quality com­ mercially available instrumentation became widely available at a low cost. This book reflects applications and concepts that have predominantly emerged since the 1960s renaissance period. The authors have a very acute sense of applications that are important and have produced a remarkabl e collection at the end of the book which covers the determination of organic, inorganic and biologically important materials such as pharmaceuticals. Applications using the classical polarographic method and the ultra-sensitive forms of stripping voltammetry are all included. The usc of the dropping m ercury electrode in polarography is presented to enforce the important historical aspect of this subject and also as an excellent tool for teaching the subject of voltammetry. Extensive data col lections, not available elsewhere, are also contained in a compact form in this book. The availability of this book should permit a teacher of the discipline to present a course on the topics described in the seven chapters and then immediately use the applications in a laboratory exercise, in order to enhance the teaching component of the subject. Voltammetric analysis is not widely taught in present-day university courses. In addition to assisting in rectifying this deficiency, it can be noted that a technician having to undertake practi­ cal voltammetric analysis could use this book as a self-help teaching guide. That is, undertake the experiments described in the book, whilst at the same time examine the theoretical principles on which they are based. A book of this kin d can only be produced by authors such as Henze and Thomas who have had wide experience in both the fundamental and applied aspects of the sub­ ject. Personally, I wi ll find this book on the introductory aspects of voltammetric analysis invalu­ able in both the areas of teaching and research. I am therefore delighted to be able to recommend this book as being a valuable addition to the literature on analytical aspects of voltammetry. Professor Alan M. Bond Head, School of Chemistry, PO Box 23, Monash University, Victoria 3800, Australia 27 February 200 1

v

Contents

v

F o reword

Preface 1

xi

Introduction 1.1 1.2 1.3 1.4

Defin ition The cell Electrode processes Volta mmetric cu rrents

References

4 7 7 10 12 14 17

Techniques

18

1.4.1 1.4.2 1.4.3

Diffusion currents

Kinetic an d catalytic currents Capacitive currents

1.4.4 Adsorption currents

2

1

2.1

Direct current polarography

2.l.l

The diffusion current

2.1.2 Reversible processes: the half-wave potential

2 . 1.3

2.1.4

2.2 2.3 2.4

2.1.5

Irreversible processes

Polarographic maxima

Selectivity and sensitivity limitations Amperometry Linear sweep and cyclic voltammetry Pulse techniques 2.4.1 Square-wave polarography

2.4.2 Sq uare - wave vo l tam m et ry 2.4.3

2.5 2 .6

Normal pulse polarography and voltammetry

2.4.4 Differential pulse polarography and voltammetry

Alternating current polarography and voltammetry

2.5.1 Tensammetry

Chronopotentiometry References

3 Stripping analysis 3.1 Electrochemical stripping techniques 3.2 Voltammetric stri ppi ng 3.2.1

Electrolytic accumulation 3.2.1.1

3.2.1.2

Anodic voltammetric stripping

Cathodic voltammetric stripping

3 . 2 .2 Adsorptive accumulation

18 19 22 27 27 28 31 33 38 39 39 43 43 46 51 52 56

58 58 60 60 60 70 72 vii

viii

Contents

3.3 3.4

Chronopotentiometric stri pping analysis 3.3.1 Data presentation Potentiometric stri pping analysis References

4 Practical aspects 4.1 4.2

4.3

4.4 4.5

5

Reference electrodes

4.5.2

Working electrodes

4.5.4

Modified electrodes

4.5.3

4.6 4.7

90 90 92 97 98 99 99

Sources of error Sample preparation Supporting electrolytes Voltammetric cel ls Electrodes 4.5.1

101 107 11 3 1 17 1 19 1 24

Micro-electrodes

Instrumentation and automation Evaluation and calculation References

Flow-through techniques 5.1

5.2

Am perometric and volta mmetric flow t hrou g h detection Flow-through stripping voltam metry -

References

6 Applications: inorganic species 6.1 6.2

6.3

7

Polarographic m et hod s Stripping voltammetry Amperometric methods Refe rences

Applications: organic species 7. 1 72 7.3 7.4 .

Pola rog raphic and voltam metric methods S tri ppi ng m eth od s Amperometric methods Flow-through detectors References

8

80 82 85 88

126

128

139

142

144 1 44 153

170

174

176

176 206

212 213 215

Appendices Appendix 1 Appendix 2 Appendix 3 Appendix 4 Appendix 5 Appendix 6 Appendix 7 References

Index

Inorg a nic species-water sam ples Inorganic species-biological and medical samples Inorganic species-agricultural, industrial and other samples Organic species-general Organic species-biological and medical samples Abbreviations Internet addresses of instrument supp l iers

220 225 227

230

233 237 241 24 1

247

List of applications

1

2

3

4

5

6

7 8

9

10

11

12

13

Determination of chromium in the presence of ni trate

Determination of s ulfide and sulfite P olaro graphic determination of cyanid e

Determination of cadmium, copper, lead and zinc after U.V. digesti o n using an HMDE

ASV determination of cadmium and lead u s in g an U l t r a tr a ce graphite electrode

Determination of antimony and bismuth by anodic s t ri p ping v oltammet r y ASV d e termina t ion of mercury on a gold electrode Determination of arsenic( III) and arscnic(V) by

cathodic s t rip p i ng vo l t am m etry

Determination of sel enium by cathodic st ri pp ing vo ltam me t ry Determination of iron

by adso rp t ive str ippin g v olta mmetry

Determination o f p l a t i num by adso rp tiv e stripping voltammetry of c h ro mium as its DTPA c omp l ex by a ds orp ti ve

Determination

st ripp ing volt a nune t r y

Determination of u r a niu m by ads orp t iv e stripping voltammetry

IS

Determination of aluminium as its alizarin-S comp l ex by AdSV Determination of cadmium and lead in high orga n ic content fluids by

16

Polarog raph ic determination of for mald ehyde

14

17

differential PSA

Polarographic

determin ation of as c orbic acid (vitamin C)

18

Pol a rogr ap h i c determination of nitrilotriacetic acid (NTA) and ethylenediamin-

20

Polarographic deter min at i on

19 21

22 23

24

25

26

27

etetraacetic acid (EDTA) [Din38 413, part 5]

Polarogr ap hi c determination of d i a zepam in body fluids and pha rmaceut i ca l p r od u c t s

Polarographic

of nicotine

dete rminat ion of cin ch o cain e

prep ar at ion s

(dibucaine)

Polaro grap hic determination of folic acid

Polarographic determination of riboflavin ( vi t a m in B2) of free styrene in p o l ys tyrene

1 53 1 57 159 1 61 162 1 62 164 164 1 66 166 168 169 1 80 184 1 86 194 1 97

in pharmaceutical

Polarographic determination of thiamine (vi ta m in B 1)

Determination

1 50 150 151

and mixed polymers

Determination of fenchlorazol-ethyl by adsorptive stripping vol tam me try

Determination of thiourea by cathodic stripping v ol t ammetry

198 200 201 201 204 207 209

ix

Preface

This text is a t ra nsl ation of the German text Polarographie und Voltammetrie: Grundlagen und Analytische Praxis ( Sp ringer 2001) by Gunter Henze. This E nglish lan g uage version, wr itte n con­ curren t ly with the German version, is a c o operat ive effort by the tw o authors i nc orp or atin g essen­ tially the same m aterial as in th e German text but p res e nt ing it in an alternative o r d er. The group o f a n alyti cal tech niques that are i n cl ud e d within the term vo l t a m me t ry are argu­ ably the most versatile of all analytical t echn i qu e s . Th ey may be used to determine sim pl e metal ions, complex m etal species, inorganic anions, and a wide ran ge of organ ic compounds at concen­ trations ran gin g over nine o rd e rs of magn it ude from 10-2 mol L _, (g L-1) down to t h e 1 o-11 molL-I (ng L-') r egi o n with recent deve l o pment s emphasisi n g analyses in the l ower concent r atio n ranges . S am p les from a vari e ty of sources-marine, p otab le, surface and waste waters, plant and animal tissue, b ody fluids, pharmaceuticals, foodstuffs, so i l s, agric ult u ral prod ucts and a range of indus­ trial products-m ay be an al ysed using vo l t a m metri c methods. V oltamm e t ri c techniques are of partic ular value to those involved in en viro nmen t a l studies-particularly i n studies of me t al spe­ ciation and of p o ll ut an t s and their metabolites, clin i cal analyses, and d r ug evaluation and metab­ olism st udie s. Furthermore, the eq u ip m en t r e qui re d for voltam metry is si mple a nd relatively inexpensive-in many cases b ein g an order of magnitude lower in cost than spectrophotometric equipment. Despite the many advantages of voltammetric tech niq u es, the authors believe that th ey are under-represented in many a n alyt i c a l laboratories. This is probably due to two factors-the lack, until re c e nt ly , of fully automated eq u ipmen t and the limited coverage of electrochemistry, p a r tic ­ ularly polarograp hy and voltammetry, in many un dergradu a te courses and g e n era l an alytical chem ist ry texts. This text was written in order to try and overcome this latter d efi c i e ncy . It is aimed at students in final year undergra duat e courses and those b egin n in g research p r og ram s which req ui re the student to have a kno wledge of voltammetric t e chn i qu e s . The text has also been written for grad ua tes w o rk i n g in industrial and commercial analytical laboratories who have little or no experience of voltammetric methods and who wish to extend their p r ofes si o n al kn owledge and exper tise . The authors consider that good pra c t i c e in analytical c hemis t ry is based on a s o und under­ standing of the physical and chemical principles underlying the techniques used and so present the theoretical basis of polar ogr aphy and voltammetry before c o ve ri n g the practical aspects and applicati o ns . In ch apter 1, th e basic electrochemical c o n c e p ts relevant to an understan d in g of polarograp hy and vo l tamm etry arc introduced. I n chapter 2, the u n d erlyi n g th eo ry of the various voltammetri c t e ch nique s is presen ted . A detailed a cco unt o f the original technique, direct c urre n t polarography disc o ve red in Prague by H e yr ovsky in 1923 a n d put on a sound theoretical basis by llkovic ten years later, is prese n ted first. The various modifications made to this t ec hn iq ue in order to imp rove its sen s i tivity and s el e c tivity and dec r eas e the time r equ ired for a measurement are then discussed. Th ro ugho u t, a sem i-quantitative approach is adopted in presenting the theory in which the mathematical rel ati o n ship s between the relevan t physical q u an t ities and the analyte xi

xii

Preface

concentration are introduced for understanding the technique but with the derivation of the equations being omitted. In chapter 3, the sensitive stripping voltammetric techniques are discussed. Then follows a chapter dealing with the practice of voltammetric analyses. Sample pre-treatment and equipment including recent developments in electrode materials and construction are presented. In chapter 5, voltammetric and amperometric flow-through detectors are described. These detectors, which commonly have volumes in the f...LL range, are the key component for the development of minia­ turised and automated microprocessor controlled voltammetric equipment and procedures. Furthermore, the combination of chromatographic separation techniques with flow-through vol­ tammetric or amperometric detectors, results in powerful analytical procedures which are very sensitive and selective and which can be performed rapidly. In the remainder of the book applications of the various voltammetric methods are presented. An outline of the voltammetric behavio ur of the elements, interspersed with details of 15 applica­ tions to the determination of inorganic species in a variety of samples is the subject of chapter 6. In chapter 7, the voltammetry of organic functional groups is summarised and includes a selec­ tion of 12 applications of voltammetry to the determination of organic compounds. A series of appendices which list the important features of a selection of some 300 applications of voltamme­ try and polarography to the determination of i norganic and organic species in a wide range of samples concludes the book. The authors wish to thank Metrohm Ltd and their staff in Switzerland and Australia, espe­ cially Dipl. Ing. Uwe Loyall, Product Manager for voltammetric equipment, Herisau, Switzerl and. Francis G. Thomas MSc, PhD formerly Associate Professor of Chemistry, James Cook University, Townsville, Australia

Gunter Henze Dr.rcr.nat.habil. Professor of Analytical Chemistry, Technical University of Clausthal, Germany

Introduction

1.1 DEFINITION

to that group of electroanalytical t echniques in whic h the current (ampere) that fl ows t h ro u gh an electrochemical cell is meas ured as the poten tial ( volt ) applied to

The term voltammetry is appl ie d

the electrodes in the cell is varied. The term, first used by Kolthoff in 1940 [ 1), is derived from the units of the electrical parameters measured-volt-am (pere)-metry. It is im p o rt an t a t th i s p oi nt to distin guish between the term s voltammetry spelt with 'mm' and voltametry sp dt with on ly one 'm'. The term v olt amet ry has been used to describe the te c h n i qu e ge n e r al ly known as 'controlled current p o ten tiom e tric ti trat io n ' . The essential d iffere nce between voltammetric and other p oten tiodyn a mic tech n i ques, such as constant cu rren t co ulo metry, is that in vo lta m metry an elec tro de with a small surface area ( < 10-5 m2) is u sed to monitor the cu rrent p ro d u c ed by the spec ies in so lu tion reacting at this electrode in resp onse to the potential appli ed to i t . Because the electrode used in vol t am m etry is so small, the amoun t of material reacting at the electro de can be igno red . This is in c ont rast to t h e case in co ulo me try where large area el ect r o des are used so that all of a spec ie s in the cell may be oxidised or red uced . When mercury, flowing through a fi n e capillary so t h a t it emerges as droplets, is used as the small electrode in a vol tamm et ri c cell the technique has the sp ecia l name polarography. This nam e is derived from the fact that the electrode can be polarised . An electrode is said to be polar i sed when no direct current flows across its interface wi th th e solution even th ough there is a p otential difference across this interface. In his definitive w ork p u blished in 1922 [ 2) on 'Elect rolysi s with a dropping mercury electr ode ', J aro slav Heyrovsky referred to this p he n o men o n and called the recorded current-po te nti al polarisation curve s polarograms. The recommen dation of IUPAC [ 3] is that voltammetry is the gen e ral term to be used when current potent ial rel ati on shi p s are being in vest i g ated and th at o n ly when a flowing conducting liquid electrode ( such as a dro pping mercury e l ec t ro d e) is used as the wo rk ing electrode should the term polarography be u se d. P o larography, the o r igina l t ech n ique, is thus a s p eci al case of voltammetry.

1.2 THE CELL

cell u se d in vo lt a m m et ry is a multi-phase system in which electrical energy is used to bri ng about a che mical ch an ge ( el e ctr o lys is ) in species in the cell. In its si m p l est fo r m , su ch a cell consists of two electronic conductors called electrodes i m me rsed in an el ectrolytic conductor (ionic solution) co n tai ning the substance of analytic a l interest called the analyte. This ionic solution c o m pletes the el ec tr ical circuit. It is the processes which occur at the solution-ele ctro de interface involving the analyte and the factors that influence these interfacial processes that arc expl o i ted in electro-analytical ch e mis t ry. The application of a vo ltage or a current fro m an exte rna l source to the electrodes p r o d uce s an elect ri cal r esponse fro m the analyt e The electrochemical

1

2

Introduction to Voltammetric Analysis

in the cell solution. The nature and magnitude of this response may be used to both identify and quantify the analyte. The sm all electrode used to monitor the response of the analyte is known as the working elec­ trode (WE). Even though only a negligible amount of material is involved in the processes occur­ ring at the working electrode, its small size ensures that a high current density develops at its surface. Consequently, it is the processes which occur at this small electrode that control the cur­ rent flow through the cell. The small current-controlling WE may be constructed from a wide variety of conducting materials. The more common materials used include various forms of graphite and metals such as mercury, gold or platinum. These electrodes may be stationary, rotating or, in the case of mer­ cury, flowing with respect to the cell solution. They are usually named after the material from which they are made and in some cases after other physical characteristics. For example, the drop­ ping mercury electrode or DME; the mercury thin film electrode or MTFE; the ha ng ing mercury drop electrode or HMDE; the glassy carbon electrode or GCE; the carbon paste electrode or CPE; the rotat­ ing platinum electrode or RPE; a chemically modified electrode or CME; and so on. The second electrode in the simple cell, called a counter electrode (CE),-serves two purposes. It is used to control the potential applied to the working electrode and to complete the circuit for carrying the current generated by the processes occurring at the WE. In the former role it must act as a reference electrode (RE). The ideal reference electrode must be a completely depolarisable electrode, i.e. it must be able to maintain a constant potential at its interface with the cell solution irrespective of any cu rrent that may flow across this interface. This can only be achieved if the reaction that controls the potential of this electrode is very fast and if there are no significant changes in the ion concentration profile in the vicinity of the interface-conditions that cannot be realised in practice unless the current that flows th rough the cell i s extremely small and the composition of the cell solution does not change significantly from sample to sample. In the simple two-electrode cell the counter electrode must be much larger in area th an the working electrode so that ( i ) the current density at its surface is so small that the electrode processes occurring at the CE do not affect the current signal generated at the WE in any significant way, and (ii) the flow of current through the cell as a result of the processes at the small WE has a minimal effect on the concentration profile at the CE surface so that its function as a reference electrode is not seriously impaired. A simple arrangement for measuring the current that flows through a two- electrode cell as the voltage applied to the electrodes is varied is shown in Figure 1.1. Accurate measurements are only possible with this basic set-up when no voltage drop occurs in the cell and, as indicated above, the counter electrode is maintained in a constant-composition solution . The solution in the cell will have a resistance to the flow of current and as a consequence there will be a potential drop across the cell due to the current flow generated by the electrode process. Although this may be mini­ mised by adding a high concentration of a supporting electrolyte (SE) to the cell solution it can never be eliminated. The voltage applied to the cell, V•PP' then is given by: (l.la) where

£..11E and E,er arc the potential drops across the working electrode-solution interface and reference electrode-solution interface, respectively, when the reactions at both electrodes are wri tte tl as reductions. iceii is the current flowing through the cell. When the analyte is reduced electrons flow from the WE to the analyte and the current is called a cathodic current. When the an alyte is oxidised the electron flow is reversed and the current is called an anodic current. R is the cell resistance in ohms.

Introduction

3

Working electrode D. C. Potential source

(Potentiostat)

Cell

Ammeter Figure 1.1

Counter electrode

Two-electrode arrangement for measuring current-potential curves.

Since the absolute values of s ingle electrode potentials cannot be measured, it is normal to all working electrode pot ent ials to that of the reference electrode so that equation l.la

refer

becomes:

vapp where

=

E' WE

+ icell R

(l.lb)

E' wr is the potential of the working electrode relative to the reference electrode being used.

is only under zero current conditions when no reaction is occurring at the working elec­ that the voltage applied to the two-electrode cell is identical with the sum of the potential drops at the electrode interfaces. Since in a real measurement situation a current flows through the cell and Vapp differs from the sum of the two electrode potentials by iceuR-the so-called 'iR drop'-it is preferable to refer to the plot of icen aga ins t V,,PP ob tain e d from a two-electrode cell as

It

trode

Historical note. The pol arographic cell comprising of a UME and a mercury pool electrode was introduced by f. Hcyrovsky. In his book Polarographisches Praktikum [ 4] he describes the following si m p le arrangement tor mea­ suring current-voltage curves: 'Connect th!! ends A and B (Fig.l.2a) of a 10 to 20 ohm slide-wire e.g. a mea su r i n g bridge or Kohlrausch drum used for conductometry or a potentiometer wire used for compensation measurements, to a 2 or 3-volt lead batter y, C. The potential necessary for the electrolysis in the cell is obtained by connecting the end of the wire, A, and the slide contact, S, (fig.l.2b) to the (respective) mercury electrodes in the cell. The cell is made from a 5 to 20 em·' glass beaker in which the solution to be studied is placed. M ercur y to a hei gh t of 5 mm is poured into the beaker and the dropping (mercury) electrode p laced in the solution.' a

8

A z Figure 1.2

(a) Schematic arrangement for elect rol ys is with the dropping mercury electrode. (b) A simple practical apparatus [4].

4

Introduction to Voltammetric Ana lysis

a current-voltage curve. A very simple experimental a rr a nge men t for the two-electrode system is to use the m ercury c o l l e c t in g in t h e bottom of the cell as the counter electrode (i.e. a m e rcury pool electrode or MPE). However, as the c ell s olu tion comp osi tion varies so will the p ot e n t i al of th e MPE. For more pr ec i se control of E,'v"E it is necessary to use as a reference e l e ct ro de an el ectrode that can maintain a more constant poten t ial than the mercury pool electrode. In modern me a su ri ng systems the current carrying r ol e of the counter electrode is sep ar at e d from its potential control role by intr od u cin g a third electrode, th e auxiliary electrode, AE, to the cell to com p l et e the current carrying circuit. The a r e a of t his auxiliary electrode must be l a rge com p a red to t h at of th e working electrode for the reason (i) given above. The addition of the aux­ iliary elect ro de means that the counter electrode now is u sed only to control the potential of the worki ng electrode and so becomes a t ru e reference electrode. Since no current flows thro ugh t h e potential co n t rolli n g circuit, t he size of the reference electrode can be reduced and a co nducti n g salt br idge can be placed between the vari a ble compo s ition cell solution and the reference elec­ t ro de so th a t th e co m posit ion of its solution will remain constant. The in crease in resistance resulting from these ch anges does not affect the p otential control of the work ing electrode. Two electrodes wh ich ar e c o m monly used as reference electrodes for the p recis e control of the working electrode pot e nti al in aqueous media are the silver-silver chloride electrode in a solution of fixed chloride concentration and the saturated calomel electrode or SCE ( m ercury-mercurous chl o ri de electrode in a s atur ated KCl solution ) . Th e se ele ctro des are robust eas i l y constructed and main­ tain a constant po t enti a l over lo n g perio d s of time. The three-electrode system will be discus se d in more detail in chapter 4. ,

1.3 ELECTRODE PROCESSES

The t er m elect rode process includes a l l p he no m ena which, as a result of applying a voltage to an el e ctro de lead to a cu r re nt flow through that electrode. The course of an electrode process is g r e a t ly influenced by the nature of the phase b ound ary of t h e working electrode and its associated electrical double layer(§ 1.4.3). Th e importance of this re gio n cannot be over-emphasised s inc e the poten t ial appli ed to the c ell exists o nl y across the double layers of th e working and reference el ectrodes when there is no current flow throug h the cell; there bei n g no p otential drop across th e cell soluti on under zero current conditions. In cells wit h s u p port i n g electrolytes the double la ye r s are on ly a few nanometers thick y e t all the electrochemical reactions occur there. Most voltammetric and polarographic an a l yses utilise electrode processes in which electron s or ions are exchange d between the two phases. As a result of this the analyte will be either oxidi s ed or reduced d e p end i ng on the potential of th e WE. Such an electrode process is re ferred to as a charge transfer reaction a n d p rod u ce s a flow of charge that is a current, thro ugh the electr ode This is su m m a ri s ed in equation 1.2: ,

,

.

(1.2)

Ox +ne- �Red where

Ox and Red represent the analyte in its oxidised and reduced forms respectively, and n is t h e n umber of electrons, e-, that react with each molecular uni t of the analyt e .

and illustrated in Figur e 1.3. A c urrent flow that results from t he oxidation or reduction of a sub­ stance is known as a faradaic current, i F, a nd its mag n i tude depend s on the conc entratio n profile of t h at substance in t h e re gio n of th e electrode s ur face T his pr o fil e depe nds on the concentration of the substance in th e cell solu t io n and on the kinetics of all steps in the associated electrode pro­ cess. If the profile is controlled by the diffusi on of th e an al yte to the electrode, the faradaic current is known as a diffusion current, if controlled by the kinetics of a step in th e electrode process, as a kin e tic current, a nd if controlled by a cat alytic process, as a catalytic current. .

Introduction

5

Solution

Ox

Figure 1.3

Schematic representation of the electrode reaction (for a reduction) as an electron exchange at the electrode-solution boundary.

As a result of the elec t r od e process, the concentration of an a n alyte at the electrode surface drops until it r eac h e s an equilibrium value determined by the potential of the working electrode and the standard electrode potential, J!>oxireu' of the charge transfer reaction ( equatio n 1 .2). Once

equilibrium is reached it might be expected that the current would fall to zero. But this is not

observed. It is fo u n d

th at the charge transfer reac ti o n can be maintained even in a stationary solu­

tion. Hence the decrease in concentration of the an alyte at the electrode surface due to the elec­ trode reaction must be countered by supplementation of the analyte from the bulk solution in the

cell1• In addition, any soluble electrode reaction products must be moving out into the bulk solu­ tion. This movement of mat e ri al to and from the electrode surface m a y be achieved by a number

of mechanisms.

The analyte in the test (cell) solution may be t r a nsp o rt e d to the electrode surface by convec­ This is fa c ili t ated by movement of t h e solution relative to the electrode, for example by stir­ ring or by the action of the mercury dr oppin g from the capill a ry of the DME. The most important process for transporting mo le c ule s through the thin laye r of solut i o n (10-100 f.lm) immediately in c onta c t with the electrode surface is diffusion. Thi s is the case for both the transport of analyte t o the electrode surface and the transport of the electrode reaction p ro du c ts away from the electrode surface into the bulk of the solution. Nernst called this impor­ tant region adjacent to the electrode surface the diffusion layer (figure 1.3 and§ 1.4). Another mechanism by which ionic analytes may be transported i n a cell arises when t h ere is a current flow th ro ugh the cell. The positive i o ns m ove towards the worki ng electrode w he n it i s negatively charged and the negative ions move in the opposite direction. This process is kn o w n a s migration and leads to the m igratio n current, im. All ions present in the cel l solution contribute t o the migration cu rren t in proportion to their ch arge , concentration and mobility. Because the conce ntrati o n of the supporting electrolyte is usually more than one thousand times that of the tion.

The electrodes use d for voltammetric and polarographic analyses have small surti1ce areas ( 1-10 x I o-� m2), and the faradaic current flow is normally in t he nAto f!A range. This is so small that even after repeated measurements there is no perceptible change in the analyte concentration in the bulk of the test solution in the cell. A small non-faradaic current can also be measured, even in an analyte-frec solution of the supporting electrolyte alone. Th i s s mal l cur­ rent is required to p ola rise the electrode, i.e. to e s t a bl ish the electrochemical double laye r and potential difference across the electrode-solution p hase boundary and is referred to as the charging or capacitive current (see § 1.4.3 ) .

6

Introduction to Voltammetric Analysis

analyte, the contribution of the analyte itself to the migration current can be neglected. In effect, in addition to reducing the cell resistance, the supporting electrolyte carries the current through the cell solution although it is not involved in the current-determining electrode process at the working electrode. Other processes which may affect the m agnitude of the current at the working electrode include: adsorption onto the working electrode of the analyte, its reaction products or other sur­ face active materials in solution; the de- solvation of solvated species; the formation of an insolu­ ble electrode reaction product; reactions with the electrode material (insol uble salt formation); and chemical reactions involving the analyte prior to the charge transfer reaction or its immediate reduction-oxidation products. In order to fully understand the electrode process, both mechanis­ tic and kinetic information about all the individual steps are required. An i m portant factor which influences the efficiency of a particular voltammetric analysis (limit of detection, sensitivity and/or resolution of adjacent current signals) is the degree of revers­ ibility of the electrode process. The reversibility of an electrode process will depen d on the kinetics and mechanism of the various steps in that process. An electrode process may be irreversible because of a slow charge transfer step, a slow chemical step, a slow adsorption step or a thermody­ nam i cally non-equivalent pathway for the reverse electrode process. If the charge transfer step is slow and rate determining the process is electrochemically irreversible, if a chemical step is rate determining then the electrode p rocess is chemically irreversible. However the experimentally observed degree of reversibility of an electrode p rocess not only depends on the kinetics of the electrode process i tself, but also on the particular voltammetric technique being used. The various voltammetric techniques differ in the dynamics of the poten­ tial applied to the cell and i n their measurement rates (time between individual current measure­ ments). Each technique has a characteristic time as has each electrode process. If we identify the half-life of an electrode process with its characteristic time, then if this is short (a fast reaction) compared to the experimental measurement interval equilibrium will be attained wel l within this time interval and the process will be seen to be reversi ble by this technique. On the other hand, if the half-life of the process is long on the experimental time scale, it may have little or no apparent effect on the measured response. For example, the characteristic time of a direct current polaro­ graphic experiment ( § 2.1 ) is related to the drop time of the DME, say about 1 s, whereas that of an alternating current polarographic experiment ( § 2.4) is related to the frequency of the super­ imposed a.c. voltage, say about 1 0 ms, so that an electrode reaction which appears reversible in d.c. polarography, may appear irreversible in a.c. polarograpy. The rate at which the charge transfer step ( oxidation or reduction of the analyte) occurs at a particular potential will determine whether or not an electrode process is electrochemically reversible. The time dependence of the electrode reaction may be estimated from the heteroge­ neous rate constants k,.ed or kox for the charge transfer step. For a reduction: k

red

ko

_

-

exp

and for an oxidation: k

ox

where

=

o

k exp

{-a.nF(ERT

l:,. o ) 1

f

{(1-a.)nF(E-�)} RT

( 1.3)

(1.4)

k" is the standard heterogeneous rate constant (em s-1 ) for the charge transfer reaction, i.e. the value of the rate constant at the standard electrode potential, E', for the charge

Introduction

7

transfer reaction at the electrode in usc. Often the polarographic half-wave potential , £112 , ( § 1.4) is used as t he reference potential instead of 1::.-o. a is the ch ar ge transfer coefficient and is that fraction of the total free energy change of the reduction step which is due to the p o tentia l difference at the W E-s o lut i on interface. 1 n is the number of electrons involved in the rate determining charge transfer reaction, E is the potential of the working electrode ( = E..v h ), F i s the F a raday constant ( F 96485 C mor- 1 ) , R is the molar gas constant ( R 8.314 J K- 1 mor- 1 ) , and T is the absolute temperature ( K ) . =

=

Using e quati o n 1 .3 or 1.4, t h e rate of a particular c h arge transfer reaction c a n be calculated from a k nowledge of its standard rate constant. In pola rography, c h a rge transfer reactions are 1 co nsidere d to be reversib l e when k0 > 0. 1 - 1 em s- \ as irreversible when k0 < 1 0-4- 1 0-5 em s- and as quasi-reversible for interm ediate values of k0• H owever as indicated above, the deg ree of reversibility also depends on the particular voltammetric techn i q ue being used . The in fl uence of the reversibility of the charge transfer reaction on the voltam metric response is dealt with in chapter 2.

1 .4 VOLTAM M ETRIC CURRENTS 1 .4 . 1 Diffusion currents

that flows through the working electrode depends upon the kinetics of the o vera ll and its magnitude will depend on the rate of the slowest individual step in that process. The r ate of the charge transfer step is potential dependent ( equations 1 .3 and 1 .4) so that if the w o rking electrode potential is such that this step is fast then diffusion of the analyte from the bulk solution to the electrode surface across the diffusion layer is the slow step which controls the current flow. This is the situation most often encountered in analytical voltammetry, and the cur­ rents that flow un d er these co nditi o n s are c a lled diffusion curren ts. The diffusion current depends on the limiting concentratiotl gradient of the analyte at the electrode surfa ce ( dc/dx)x=o• which is related to the d ifferen ce in analyte concentration at the surface of the electrode, c,, a n d in th e bulk solution, ca, and the thickness of the diffusion layer,
The

cu rrent

electrode process

The charge transfer co e ffi cien t ,

u, is a param eter

electrode p ro ce s s . Wh i l e it a p pears in volta mmetric relationships for irreversible el ec trode processes and is i mp o rta nt in any discussion of el t'c tro de kinetics, further consideration of this parameter is not n eces s a r y for understanding ana lytical voltam m etry. Readers seeking a more deta i l ed discussion of

helps in the m athematical description o f the

this para meter are referred to reference [ 5 ] .

which together with the standard heterogeneous rate constant

8

I ntroduction to Voltammetric Analysis

.,...---- ca a

ell u

� :I f/1 ell

-e

X

t) ell G) ell .c -

iii

c 0

Cs

� -

c ell u c 0 u

I I t1 I I I

<

t2

<

I I I I t3 I I I I

b

Distance from the electrode surface Figure 1 . 4

Concentration profi les i n the diffusion layer. (a) Constant layer thickness at different potentials: £1 < £2 < £3 for a n oxidation and £1 > £2 > £3 for a reduction. (b) Layer thickness increasing with time, t, at a constant potential ( £3) . c. - b u l k solution concentration of analyte; Cs" - electrode surface concentrations at different potentials En ; On - diffusion layer thicknesses at different ti mes tn .

An expression for the diffusion controlled charge transfer current can be obtained from the general faradaic relationship for cu rrent as a function of the number of moles reacted, namely: i ( t)

where

=

( d N/ d t) nF

( 1 .5)

i( t) is the current at any time, t, is the number of moles of analyte reacting, n is the number of electrons which react with each molecular unit of the analyte, F is the Faraday constant. N

and Fick's first law of diffusion: ( d N/ d t) A

where

=

- D( d c/ d x)

A is the area of the electrode, D is the diffusion coefficient of the diffusing species, x is the distance ordinate ( i .e. the distance from the WE surface).

( 1 .6)

I ntroduction

9

Combining equations 1 . 5 and 1 .6 and substituting {-( c. - c, ) / 0 } for dc/dx in accordance with Nernst's assu mption, the following relationship is obtained for the current at any time: . z ( t)

=

n FAD( c, - c, )

0

( 1. 7 )

·

The concentration gradient increases with voltage a s indicated above and shown i n Figure 1 .4a. When the electrode potential reaches a value where the charge transfer reaction is so fast that the analyte molecules are reduced ( oxidised) as quickly as they arrive at the working electrode surface, the value of c, is effectively zero and equation 1 . 7 becomes for c, = 0:

.

z( t)

=

.

'"

=

n FADc.

( 1 .8)

0

In this case the current is known as the limiting diffusio n current, i0, the value of which remains consta nt as the voltage is further increased. This is the maximum value of the diffusion current and since it is proportional to the concentration of the analyte it is of paramount importance i n analytical voltammetry. In such cases where the analyte concentration i s held constant at the edge of the fixed diffusion layer by convection and where the motion of the solution relative to the electrode does not disturb the diffusion layer, then the current reaches a constant value at the increased potentials. The plot of current versus voltage gives the characteri stically shaped wave shown in Figure l . Sa. The potential at which the current is one-half of the value of the limiting current, ( i id/2), is called the half-wave potentia� E112• For the simple reversible charge transfer reaction (equation 1 . 2 ) , the concentrations of the analyte and its electrode reaction product on the electrode surface are equal at this potential. EJ /2 is related to the standard electrode potential of the analyte and so can be used to confirm the identity of the analyte. In the second case, the solution is stationary with respect to the working electrode so that the only way the charge transfer reaction can be maintained in the absence o f convection i s by diffu­ sion alone. This results in the continual expansion of the diffusion layer into the cell solution in accordance with Fick's second law of diffusion: =

( 1 .9) Consider the situation where the potential of the working electrode WE is suddenly stepped a value in region 1 to one in region 3 (Figure l .Sa), that is from where c, c. (no charge transfer} to where c, 0 ( constant current region when 0 is constant) . When the potential is first stepped, dc/dx will be a maximum but with time the diffusion layer thickness increases and con­ sequently the concentration gradient at the surface of the working electrode (dcldJx�o decreases. In the simplest case of linear diffusion of the analyte up to th e surface of a planar worki ng elec­ trode, the diffusion layer i ncreases with time as (7t D t) 1 12 so that the relationship for the current now becomes:

from

=

=

l

"( t )

=

n

FAD

I/2

Ca

( 1t t).- J /2 ·

( 1 . 1 0)

This fundamental relationship in electrochemistry is known as the Cottrell equation [ 6 ] . The decrease of current with the square root of time, is characteristic of diffusion controlled currents when using stationary electrodes with surface areas > I o-7 m 2 in stil l solutions.

10

Introd uction t o Volta m m etric Ana lysis

a

- E

b

Fig ure 1 . 5

- E (a) Current-potential curve as the p otent ia l is sca nned and the d iffusion laye r thickness is held id is the limiting diffusion current and fw the half-wave potential. 1 Potential re g i o n of double layer charging only, 2 potential dependent charge transfer region and 3 potentia l region of diffusion limited current. (b) Current-potential curve obtained when the potential is s ca n n ed for the case of a growi ng d i ff u s i on laye r; iP is the peak cu rre nt and fP is the peak pote nt i a l All curves are for a constant area electrode.

co n s tan t

.

.

When the pot en t i al of the working electrode i s gr a d u al ly increased in a still solution, the result is a combination of th e two cases discussed above. On reaching the potential range in which the electrode reaction occurs ( ( 2 ) in Figure l .Sa) t h e c urr en t fi rs t r i ses ( i ncrea sing ( dc/dx)x--0) du e to the decrease in the surface concentration, c, . A p o t e nti al (time) is then reached where the increase in (dc/dx) """ 0 d u e to the decrease in c, is j u st balanced by the decrease in (dc/dx)"""0 due to the i n c reas i n g thickness of t he diffusion layer, 8, w i t h time. At this p oin t both (d c/dx)x=o and the current reach a maximum va l u e As the po ten t ial is further inc r eas e d c, � 0 and the current decreases in accordance w i th e q u ati o n 1 . 1 0 due to the continuing increase in 8. The resulting current-potential curve, illustrated in Fi gu r e l .Sb, has the sh ape typi c al of a voltammogram. Like the l im it i n g diffusion current, id, th e maximum or peak current, ip, is proportional to the analyte concentration and is used t o quantify t h e amount of a n alyte in the cell solution. The peak poten­ tial, EP, like £1 1 2 , gives qu a l it a t i ve information about the analyte. .

,

1 . 4 . 2 Kineti c and cata lytic c u r rents

In a d d it ion to t h e charge transfer reaction, many electrode processes include associated chemical reactions of the analyte and/or its ch a r ge transfer rea c ti on p rod u c t s These reactions may o ccu r .

only within the double l a yer re gio n at the electrode interface or they may involve reactions of the analyte in the bulk o f the solution. If these chemical reactions are fast and diffusion of the analyte

Introduction

11

to the electrode surface i s the slowest step in t h e overall electrode process and s o deter m i n e s the concentration of the electroactive s p e c i es at t he electrode su rface, th e n a d i ffu sio n controlled current is pro d u c e d as described above. However when the a cc om p an yi n g chemical reactions have a m a rked i n fl uen c e on th e m ag ni t u d e of the current, the current is called e ith er a kinetic current, �in' or a catalytic current, i.at· C h em ic al reactions prior to the ch arg e transfer step may convert an otherwise electrochemi­ cally inert (electroinactive) co m p o un d i n t o a fo r m which can be either oxidised or reduced at t h e WE so th at it can t h en be determined by voltammetric methods. Such electrode processes in which the c h arge tran sfer rea c ti on i s p r ec e d e d by a chemical step are said to p r oc ee d via a chemi­ cal-electrochemical mechanism o r CE mechan ism. Typi ca l re act i ons i n c l ude t h e p roto n a t i o n of bases, the de - pr o t o na t ion of acid s p ec i es and the formation or d i ssoc i at i o n of i n o rga n i c com­ plexes. A well known example is the reduction of fo r m al de h yd e to m ethan ol [ 7 ] . Formaldehyde is p resent in solution ma i nly as the electroinactive h yd ra t e d form wh ich is in s l u ggi sh e qu i l ib r i u m with a sm a ll amount of the reducible u nhydrated form. Reduction of the unhyd r ate d form di s ­ turbs the e qu il ibr i u m at the el ec tr o d e in te rfac e . Co n sequ en tly, some of the electroinactive hydrated form i s converted to the electroactive form at a d e fi n ite r at e a c c o rd i ng t o : ( 1. 1 1 ) of formation of the ele ct ro ac tive H2C=O i s slower than the rate of diffusion of to the wo r k i ng elec t r ode , t h e ma gnit u d e of the cu rrent is determined b y the rate of the reaction which p rece des t h e ch arge tran s fer step ( e qu a tion 1 . 1 1 ). That is, the c u r r en t dep en ds on kdehvd and s o is a ki ne t i c current. (The c o n c e n tra t i o n of free H2C=O in solution is so . low that th e con tri b ut i on to the current due to its diffusion from the s ol ut io n and reduction at the electrode is n eg l i g i bl e . ) Electrode processes in which the ch arge transfer reaction is followed by a chem i ca l reaction are said to p roc eed by a n electrochemical-chemical or EC mechanism. Such fo l l o wi n g chemical reactions c an give rise to two different e ffec t s . First, the p ro d u c t of the charge t r an sfer reaction is con ve rt ed into an electro inactive form by a chemical react i o n . An example of this o cc ur s in t h e electro-oxidation of ascorbic ac i d [ 8 ] . In the charge transfer step the c o mp ou n d is r eve rs i b l y oxidised to an electroactive i n term e di a t e pro d u c t which undergo es a rap id irreversible chemical reaction to form the electro inactive product, dehy­ droascorb ic acid. While the l i miting current is not i nfl uenc ed b y th is re actio n , t h e oxidati on potential is, E1 12 b ein g shifted to more n egat ive po te n t i als. Second, a co mp o n en t of t he cell solution ( o th e r than the analyte) reacts with the pr o du ct of the charge transfer reaction of the analyte at or very close to the el e ct ro de surface and c o nve rt s this p ro duc t back to the o r igi n a l electroactive form o f the analyte. In t h i s situation the analyte is act i ng as a catalyst for t h e r edu ct i on o r oxi dat i o n of the other component and the current is refe rred to as a c at a lytic current, i,31• When this c h e m i c al step is very fast , th e a n a lyt e concentra­ tion at the elec tr ode surface is b eing r eple n i sh ed by the follo wi ng chemical reaction at a m u ch faster ra te than by diffusion from the b u lk solution. This leads to a la rg e increase in the c u rr en t. The oxidants used for these reactions are co m po un d s tha t a r e n o t e l e c t r oa ct ive un d e r t h e p r eva il ­ ing experimental conditions or else are only re d uce d at much more ne g a t ive p o te n t ials than that at which the analyte i s reduced. The reduction of Mo(VI) in the presence of nitrate s up p ort i ng electrolyte a cc o r d in g to the fo l lo win g scheme [9] is a typical example (no rmal ly , nitrate is n o t reduced at a mercury electrode ) . Since the rate

hydrated H2C ( O H ) 2

Mo (VI ) + 3e-

Polarographic

___:_:_ •ed.:.::u.:.:: cti= o ":...� ._ M o ( l ll) ;

_

Mo(III) - 3e-

No3

) Mo(VI)

( 1 . 1 2)

12

I ntroduction to Voltammetric Analysis

Another mechanism is that which prevails when catalytic hydrogen waves are produced at a mercury working electrode. The reduction of the hydrated proton to hydrogen gas occurs at quite negative potentials because of the high overvoltage of the charge transfer step on mercury. In the presence of certain compounds, for example plati n um salts, a range of organic bases and many organic thio compounds, the hydrogen evolution reaction is shifted to more positive potentials and a catalytic hydrogen wave is observed in the polarogram or voltam mogram. A typical mecha­ nism involving an organic nitrogen base, B, is as follows: ( l . l 3a) BH+ + eBH"

+ BH"

� BH" �

H2 + 2B

( 1 . 1 3b) ( 1 . 1 3c)

in which the base B (which may be electro inactive) is acting as a catalyst for the reduction of the proton. Equation 1 . 1 3 is an example of a chemical-electrochemical-chemical or CEC mechanism. Catalytic hydrogen waves have been used for the estimation of a n umber of organic bases [ 1 0 ] . A ch aracteristic o f both catalytic and kinetic currents is that generally they increase at a much greater rate with increasing temperature than do diffusion currents. Because other components of the cell solution are involved, their magnitude is often more sensitive to changes in solution com­ position than is the case with diffusion currents. In summary, kinetic currents arc of little use in analytical voltammetry but catalytic currents have been exploited (a) for the determination of the catalyst at very low concentrations and (b) for the indirect voltammetric analysis of electroinac­ tive compounds that react with the catalyst. 1 .4 . 3 Capacitive c u r rents The very simple representation of the electrode-electrolyte solution interface presented in Figure 1 . 3 omitted any detail of the electrode double- layer as this was not essential for an understanding of the diffusion process. In order to understand the origin of the charging or capacitive current, i,, and the effects of adsorption it is necessary to consider this important region at the phase interface in more detail. When a potential is applied to an electrode from an external source, the charge that flows to the electrode resides on its surface. Because of electrostatic attraction, ions of opposite charge in the electrolyte solution will be attracted to th e immediate vicinity of the electrode to form the electrochemical d ouble layer. The generally accepted model of the resulting charge distribution in the double layer is based on that proposed by Stern [ 1 1 J and shown in Figure 1 .6 for the case where the electrode is negatively cha rged and there is no adsorption of ions or molecules (other than solvent) onto the electrode surface. According to this m odel, there is a layer of solvent molecules in contact with the electrode sur­ face and these molecules have their dipoles orientated by the electric field. The plane in which the centres of these adsorbed solvent molecules lie is referred to as the In ner Helmholtz Plane, T H P . There is then a layer of solvated ions of opposite charge to that on the electrode ( cations in this case). These ions lie in a plane referred to as the Outer Helmholtz Plane, OHP, and the region between this plane and the electrode surface is referred to as the compact layer. Most of the charge on the electrode is co untered by the charge on the ions in the OHP, the remainder being coun­ tered by the ions dispersed in the region immediately adj acent to the OHP by the thermal motion of the solution. This region where the remainder of the charge on the electrode is balanced and the potential asymptotically approaches that of the homogeneous bulk solution phase is called the diffuse layer. The thickness of this layer decreases with increasing electrolyte concentration and is

Introduction

8

8

8

8

8

E lectrode

8

8

Com pact laye r

�!

�� �

Solution

8

8

I

13

8

8 I

:

laye r

�

Outer H e l m h oltz lay r

L_------l• I n n e r H e l m h oltz layer

0

Adso rbed so lve nt molec u l e

Figure 1 .6

8 Anion

8

Hyd rated cati o n

Schematic representation of the charge d istribution at the electrode-electrolyte solution interface when the electrode is negatively charged.

just a few nanometers thick for 0. 1 to 1 .0 m o l L- 1 soluti o n s . Outside this layer is the homogeneous bulk solution phase where electroneutrality prevails. Thus the po t e n t i a l difference between th e electrode and so l uti o n occurs across this very n m:row r e gi on ne a r th e i nte rfa c e . Th i s ch a r ge d interfacial reg i on is e qu iva le n t to a p a r a l le l p l a te c a pa cito r , one plate being the electrode surface an d the other the ch a rge in the compact and d iffus e regi o ns of the double l ayer . Like a capacitor, the doubl e l ayer at the interface has the ability to store c h arge and is characteri sed by a capaci­ tance-the dou b le layer cap a citance, edt · The capacitance of a parallel plate c a p a c i tor is p ro p o r t i on al to the area of the plate s and the dielectric constant of the m a te rial between them and inversely p r o p orti o n a l to the distance between the p la te s but i s i n dep e n d e n t of the vo lt age . The structure of th e do uble l ayer is not constant but varies with the nature and c o n c entr at io n of the el e ct ro lyte solution and with the p oten t i al of the el e ctro d e . Thus for a given solution, even for one c o n t a in ing o n ly a single electro ­ lyte, the cap aci tan c e of the electrode d o ub le l a ye r, Cd(, not only varies with the area of the ele c ­ trode and t hickn ess of the double layer itself, but is also a fun cti o n of the electro de p ot ent i al . The

value of th e double layer c ap ac i t an c e of an electrode is n o r m a lly expressed as t h e double l aye r

14

Introduction t o Voltammetric Analysis

capacitance per unit area, Cdl. It is related to the electrode potential and the charge on the elec­ trode by the general formula for a capacitor, namely:

which, b ec au se

Cd1

Q ( � EA )

( l . l 4a)

va ri es with voltage, is usually expressed in the differential form:

( U 4b)

where

C,11 is the double layer capacitance per u ni t area in F m-- z ,

Q is th e charge on the electrode in coulombs, �E is the potential applied to the cell in volts, r e l a t iv e to Em, the potential of the electro­ capi llary maximum at which the charg e on the electrode is zero, and 2 A is the area of the electrode in m •

The rate at which the charge flows to the working electrode to establish EwE is ca l l ed the charging current, ic ( d Q/dt) . There will always be a charging current associated with a potentiodynamic tech nique (i.e. one in which the potential is changing during the course of the measurement) or a technique in which the area of the electrode is changing ( DME) . In the case of the DME the charg­ ing current is given approximately by the equation: =

( 1 . 15)

The first term allows for the effect of the rate of cha n g e of the electrode potential o n t h e charging cu rrent while the second term gives the increase in i, due to the growi ng mercury drop. The charging current observed with a DME in oxygen-free l mol L- 1 KCl and th e charge on t h e drop­ ping m e rc ury electrode are shown in Figure 1 .7. The po ten t ia l of zero c ha rge , which is determined by the el ectrode material and the composition of the cell solution, occurs at -0.56 V (versus normal calomel electrode) for t h e DME in 1 .0 mol L-1 KCl. Since i, is not associated with a cha rg e transfer process, it is a non -faradaic current and is not used analytically. It does, however, define the analytical limit of detection and limit of determination of a voltammetric method. Thus i, is the technique noise, above which the analytical sign al ir must be detected and measured (§ 2. 1 . 5 ) . I n order t o decrease the magnitude o f i, and improve the sensitivity o f t h e measurement, a number of variations on the original polarographic technique have been developed so that the current is measured under conditions where i, is minimised and the signal-to-noise rat i o , iF/ (, is maximised ( cha pte r 2 ) . 1 .4.4 Adsorption cu rrents The adsorption of surface active substances or surfactants at the electrode-electrolyte interface significantly changes the structure of the double layer as shown schematically in Figure 1.8 [ 1 2] . Adsorption is potential dependent and the adsorbed surfactant may be either a charged ion o r an uncharged, but usually polar, molecule. The adsorbed surfact an t molecules displace solvent mol­ ecules from the electrode surface a nd may themselves be partly de-solvated in the process. Since in aqueous systems the a ds o rb e d molecules are normally larger tha n the disp laced solvent mole­ cules, th e average thickness of the double layer increases. In addition, the adsorbed molecules usually have a lower dielectric constant than water. The increased sepa ration of the charge laye rs

Introduction

} .1. 1 I � I_ I l l } � 1. I _�-1

15

��- � � � w. -

-

Em I

+

+

I

0

Figure 1 .7

- 0 .2

- 0 .4

:- 0. 6 I I I I I I I I I I

- 0.8

- 1 .0

- 1 .2

[V]

The charging cu rre n t and charge on the d ro pp i n g mercury electrode as a fu n ct i o n of the electrode potential (versus normal calomel electrode) for oxygen-free 1 mol L� 1 KCI solution.

when th e larger molecul es are adsorbed together with the decrease in dielectric constant of the

adsorbed l ayer results in a decrease in the dou ble l ayer cap ac itance and a consequent decrease in the ch arging cu rrent . Ad s orp tion , by affecting the base current in vo l t a m met ric studies, is essen ­ tially an i n terfering matrix effect and must be allowed for i n analytical app l i c a ti o ns [ 1 3 ] . Surfactants can ch an ge their adsorption con diti on (adsorb, desorb o r c h ange their orientation with respect to the el ectro de sur face ) su ddenly over a few millivolts or gradua lly over several cen­ tivolts. The po ten tia l at which these changes occur u s ua ll y varies with the concentration of the surfactant [ 14] . The ch a n ge s in do u b l e layer cap ac itan ce b ou ght about by adsorption can be observed as a function of poten tia l using a.c. polarography ( § 2 . 5 ) . I n ad d it i o n t o c h angi ng t h e c ha r g i ng c urr en t, ad sorp t io n phenomena can h ave a m a r k ed influence on far a da ic c urrents. The ad so r p t ion of either the re d u c e d or oxidised forms of t h e analyte pro d u c es q u ite distinctive effects in the volt a mmo gram s and p o l arogr am s . When th e analyte co n c entrati o n is in excess of that needed t0 form a mo n o l ayer of the a d so rbe d form on the electrode surface, two waves are observed in the d . c . p olaro grams as shown in Figure 1 .9. One wav e is associated with t h e electrode reaction i n whi ch the analyte i n so l ut i on gives the products in solution while the other is a s s o c i a t ed with t h e electrode reaction which includes the adsorption of the an alyte or its electrode reaction p roduc t [ 1 5 ] . When the r e d uce d form of the analyte is adsorbed an a d s or p tion pre-wave is observed at m o r e positive pot e n t i a l s than the 'normal' fa r ad a i c reduction wave ( F ig ure 1 .9- 1 ) . On t he other hand, when the oxidised form is adsorbed the adsorption wa ve appears a s a p o st - wav e at more negative potentials ( Figure 1.9-2}. It is th e sum of the he i gh ts of the two waves that is p roportio n a l to the concentration o f the analyte an d so can b e use d for a n a l ysis . Another complication resulting from an a dsorp tion process occurs when an clectroinactive surfactant, present in t h e cell solution with the analyte, is adsorbed onto the electrode surface. The adsorbed m o l ecu le s hinder and in some c as e s to t ally block t he charge transfer reaction of the

16

Introduction to Voltammetric Analysis

)

r::\ \.:J su rfactant molecule Solvated

Figu r e 1 . 8

� \.:._)

+ nO

Adsorbed

Solvent

s u rfactant molecule

O mol ecule

Schematic representation of the ads orption of surface active molecules onto the surface of the electrode showing the displacement of solvent molecules from the surface. The ion distribution has been omitted for clarity.

1

2

t

-E

Figure

1 .9

Polarograms obtained for the reduction of an analyte when (1) t h e electrode reaction product is adsorbed on the electrode and the analyte is not adsorbed and (2) when the analyte is adsorbed on the electrode and the red uction produ ct is not. iad - adsorption current; Ox, - analyte in solution; Oxact - analyte adsorbed on the electrode; R, - red uced analyte in sol ution, Rad - reduced ana lyte adsorbed on the electrode.

analyte over the potential range in which the surfactant is adsorbed. This causes a reduction in the current and/or a shift in the potential of the voltammetric wave [ 1 3 ] . Occasionally when two elec­ troactive species which are reduced or oxidised at similar potentials are together in a solution with the surfactant, the adsorbed surfactant may inhibit the electrode reaction of one of the electroac­ tive species but not the electrode reaction of the second species. This enables the second species to be analysed in the p resence of the first. Such behaviour is referred to as electrochemical masking. Unfortunately surfactants more commonly interfere with voltammetric analyses and it is usually necessary to remove such substances from the sample prior to the electrochemical measurement.

Introduction

References 1 2 3

4 5

6 7 8 9 10 11 12 13

14 15

l.M. Kolthoff and H .A. Laitinen, Scier1ce, 1 940, 92, 1 52. }. He yrovs k y, Chemicke Listy, 1 922, 1 6, 2 5 6 ; Philosophical Magazi n e, 1 92 3 , 45, 303. N.M.N.H. Irving, H. Freiser and T.E. West, editors, Co mpendium ofAnalytical No m e n cla t u re, I . U . P.A.C. Analytical Chemistry D ivisio n , Pergamon, Oxford, 1 978. ) . Heyrovsky, Polarographisches Pra k t ik u m , Springer Verlag , 1 960. See for example the discussion in P. Delahay, Double Layer and Electrode Kinetics, Interscience, New York,

1 965, Section 7 . 2 . F.G. Cottrell, Z Physika lis ch e Chemie, 1 903, 42, 385. N. Landqvist, Acta Che mica Scandanavica, 1 95 5 , 9, 867. S.P. Perone and W.j . Kretlow, Analytical Ch e m istry, 1 966, 38, 1 760. M.G. John so n and R.J. Robinson, A n a lytical Chemistry, 1 952, 24, 366. S.G. Mairanovskii, f. Electroanalytical Chem istry, 1 963, 6 , 77. 0. Stern, Z. Elektrochemie, 1 924, 30, 508. F.C. Anson, Acco u n ts of Chemical Research, 1 9 7 5, 8, 400. F.G. Th om a s and L. G ie r st , /. Electroanalyt ical Ch emistry, 1 98 3 , 1 84, 239. F.G. Thomas, C. Buess- Herman and L. Gierst, /. Electroanalytical Ch e m is t ry, 1 986, 2 1 4, 597. R. Brdicka, Z. Elektrochemie, 1 942, 48, 278.

17

2

Techniques

T h e physico-chemical studies of phenomena occurring at the electrode-solution interface during the 1 9 50s and 1 960s led to considerable advances in voltammetric theory and the development of a range of new voltammetric t ech n i q ues. In the la t e sixties, relatively low cost instruments which used solid state operational amplifier circuitry became available and led to an upsurge in the application of the newer po l a ro gr aph i c and voltammetric methods of a n alysis which is still con­ tinuing after 30 years. The introduction of mi c rop ro c esso r controlled digital eq u ipm e n t in the mid-eighties has fu rth er extended the versatility of voltammetric techniques and fa ci lita te d the processing of the electrical response from the analyte. The newer techniques which provide a marked improvement in sen sitivi t y , versatility and speed of analysis over the original direct cur­ re nt polarography ( DCP) find ready application in the growing demand for the reliable analysis of trace and ultra-trace levels of pharmaceuticals and their metabolites, and of a wide variety of e n v i ronmental pollutants. Although the newer vo lt a m me t ric techniques h ave replaced d.c. polarography in the analyti­ cal laboratory, the relative simplicity of the t e ch n iqu e and of its theoretical basis [ 1, 2) make DCP a convenient starting point for the discussion of analytical voltammetry, particularly since many of these newer techniques were developed in order to ove rc o me the limitations of DCP. In addi­ tion, the classical DCP experiment may give inform ation about the electrode behaviour of a new analyte ( reversibility, adsorption, etc.) that is useful in the subseq u en t development of the opti­ mum methodology for its analysis. In the following discussions it is assumed that a three-electrode cell ( c hap te r 4 ) is being used, in which the iR drop is compensated for so that the recorded polarogram or voltamm ogram is a plot of c u rren t i, against the po t en t i al of the working electrode ( Ewr:) relative to the reference electrode being used. While most of the discussion refers to reduction or cathodic processes, t he arguments apply equally to oxidation or anodic reactions with allowance being made for the reverse direction of the current flow. ­

,

2 . 1 D IRECT CURRENT POLAROGRAPHY As indicated in c h a pte r 1, direct current polarography involves recording the current that flows through a cell contain ing the an a lyt e and supporting electrolyte when the potential of a DME (as working electrode) is varied slowly in a linear manner with time. Under these conditions the sig­ moid s haped current-potential curve or polarogram, with its marked oscillations due to the growth and fa ll of the mercury drops, is obtained ( Figure 2. l a ) . Because the ra t e of change of potenti al is small (typically 2 mV s- 1 ) , the actual potential change experienced by the electrode during the life-time of an individual drop (ca. 5 s) is sufficiently small so that when developi ng the theory, the p ote nti al may be considered constant du ri ng the life of a drop.

19

Tech niques

b

- E [V ]

- E [V] Figure 2 . 1

Classical d.c. polarogram (a) a n d smoothed sampled or tast polarogram (b) using the dropping mercury electrode.

2. 1 . 1 The d iffusion current At potentials where the charge transfer step is fast and the current is determined by the rate of dif­ fusion of the analyte to the electrode surface, the current at a particular potential is given by equa­ tion 1 .7, namely: . 1( t)

=

nFAD( c. - c. ) (5

For the simplest case of linear (one-dimensional) diffusion of the analyte to a planar electrode equation 1 . 7 becomes

(2. 1 ) which gives the Cottrell equation ( 1 . 1 0 ) for potentials where c. = 0. When the working electrode is a D ME, this equation must be modified to allow for: (i) the electrode area, A, increasing continuously during the life of a drop; (ii) the electrode expanding into the growing diffusion layer which results in the diffusion layer thickness being less and the concentration gradient being greater than is the case for a stationary electrode under the same experimental conditions; (iii) the electrode surface being spherical so that the diffusion is no longer li near but spheri­ cal ( three-dimensional ) . The surface area o f the DME i s a function o f the 'mass flow rate o f mercury through the capil­ m (g s-1 ) , and at any time, t( s), in the life of a mercury drop is given by

lary,

( 2.2 ) where

k

=

2 -2 0.85 1 5 cm g 13 at 25°C and A is in

cm2•

The growth of the drop into the diffusion layer has been calculated to reduce the thickness of the diffusion layer by a factor of J377 [ 3 , 4, 5, 6] thus increasing the concentration gradient and hence the current by a factor of J773 . Incorporating this factor and substituting for A (equation 2.2) and F (96485 C) into equation 2. 1 leads to: i( t)

=

70 . 8 tl D 1

1 1 m 213 t 1 16

( Ca - c,)

(2.3 )

20

I ntroduction to Voltam metric Ana lysis

which in the limiting current region ( c, becomes: 1 c1 =

70

=

.

8

0) and at the end of the drop life ( t n

D1 /2

m

2/3

t l /6

D

c.

=

tD; i( t)

=

id )

( 2 .4)

which is known as the Ilkovic equation. Thi s relationship gives the current in amperes, A, when D is expressed in cm2 s- 1 , m is in g s-1 and c is in mol L- 1 • Modification of equation 2. 1 to allow for the spherical diffusion of the analyte to the DME is not so straightforward. The fundamental equation for three-dimensional diffusion to a stationary spherical electrode of radius, r, is [ 4] :

.

l( t ) = n FA ( c. - c. )

{(D) 1 12 D } 1t t

· + -;:-

(2.5)

Using equation 2.5 instead of the linear diffusion equation 1 . 7 , gives:

ld .

_ -

/ ( 70 . oo n 0 1 t 2 m 2 J t1!6 0 c. I

+K

D112 t 1 !6 0

m

-113 )

( 2 .6)

with K = 4.45 g1 1 3cm- 1 • The second term in the brackets, often referred to as the sp heric i ty cor­ rection, is of the order of 0 . 1 for the DME under normal operating conditions. The above approach however is an over- simplification of the situation. The glass of the capillary inhibits diffusion to the DME from above, the mercury drop is not tru ly spherical and a small section of its surface is attached to the mercury column in the capillary. These factors tend to counter the increase in id predicted by equation 2.6. A detailed discussion of the various attempts to allow for all these and other factors, such as the movem ent of the centre of the drop, i s p resented by Meites [ 1 ] . From the viewpoint of applying DCP to chemical analysis, the importan t feature that emerges from the theoretical studies is t h at they all predict the limiting diffusion current id to be proportional to the concentration of the analyte ( id k1 L c. ; where k1 L represents all the terms in equation 2.6 other than c.) . This is observed experimentally over a wide range of concentra­ tion ( four orders of magn itude) for many a nalytes. Since it is found that the simple Ilkovic equation ( 2 . 4 ) describes the observed behaviour very well in most cases, the various factors dis­ cussed above m ust tend to cancel each other out. It i s noted from equation 2.3 that the current increases with time since the e ffect of the increasing surface area o f the DME more than com­ pensates for th e decrease in diffusion current with the sq uare root o f time observed with sta­ tionary electrodes ( equation 2 . 1 ) . Another feature is that at any given potential, except for the first drop [ 7 ] , the current-time profile observed on one drop is reproduced on the next drop. This, together with the fact that there is no long-term accumulation o f reaction products on the surface of a D M E , are reasons why this physically cumbersome el ectrode is still used for some studies. One o f the con sequences o f the dependence of id on the drop time is that its limiting value varies slightly with potential, particularly at voltages more negative than - 1 . 0 V relative to the saturated calomel reference electrode ( SCE) . This arises because the surface tension of the mercury-electrolyte solution interface changes with potential, decreas i n g as the potential changes in either directio n from its maximum val ue which occurs at Em , the potential of zero charge. As the surface tension decreases, the drops get smaller and so the drop time, tn, decreases. In practice, this effect is overcome by using a mechanical drop h ammer to dislodge the drops a t a constant pre-determ i ned time. Much of the data p ublished in various compilations [ 1, 6] was obtained when recording equipment was heavily damped both electrically and mechanically. Accurate values of the diffu­ sion current at drop fall as given by equation 2.4 were not obtainable with such equipment and so =

Tech niques

the average of the rec o rded c u rre nt o s c i l l ation s wa s re p o rte d This was identified with .

21

the mathe­

matical average of the diffusion current over the life of a d ro p which is given by:

t�1 I i( t) at = 6/7 id

(2.7)

0

60 7 n Dl/2 rn 2/3 t vJ /6 '• 0

.

where

id is t h e value of the diffusion current at the end of the dro p l i fe ( t = tD) and has a maxi ­

mum val u e at th is t i m e .

Since modern equipment can accurately record the value o f id at tv, all further discussion wi ll reminded t o ch e c k when using mathematical relationships or referring to literature data, whether or not the relationships and d er ived parameters quoted are for average or m axi m u m currents. For exa mple, th e Diffusion 1 12 Current Constant, lv ( 60. 7 nD ) , q u ot e d i n m a n y t a b u l a tion s i s no r m a lly c o m p u ted from the

use this val ue o f c u rre n t a t th e e n d of t h e drop life. Readers a r e

=

average current, ( i,1)avc· The Il kovi c equation 2.4 a pp l ie s wherever diffusion controls the current such as in the limit­ ing current region w h e r e c5 0 . According to eq uat i on 2.4, the diffusion current is affected by the =

diffusion coefficient of the analyte, the flow rate of mercury and the dr o p time in addition to the

concentration of the analyte. c o e ffi c i e n t s increase with temperature by 1-2% per degree K so that the c e l l temper­ ature must be c ont roll e d for accurate work. Diffusion coefficients are a l s o affected by the vis c o s i ty of the solution, so th a t it is i m p orta nt to ke e p the cell so l u t i on comp o s i t i o n co n s tant wi t h res p ec t to t h e major com po n e n t s when working with m ixed solvent systems and those minor compo­ nents , such as gelatine, which affect the v is c os ity 213 1 16 The p ro du c t rn t , referre d to as the capillary characteristic, is de t e r m i ne d by the physical parameters wh ich influence the flow of mercury and the drop size: the c a p ill a ry diameter, the length o f the capillary, the press u re fo r ci n g th e m e rcu ry t h r o u gh th e c a p i l l a ry the surface te n sion of the mercury-cel l s o l ut i o n interface and the electrode p o te n t al ( wh i c h a ffe cts the s u r fa ce ten­ sion) . From th e P oi s e u ille e q u at i on or l iq ui d flow through a capillary, th e flow rate of m erc u r y 1 m (g s- ) is give n by:

Diffusion

.

i

f

rn

where

=

4.64

x

1 0 6 r4 { h Hg - 7.75

,

,

x

-4

1 0 y ( rn t

)-1/3 } 1 - 1 ·

(2.8)

r is the radius of the capillary in em, l is the l e ngth of the c a p illary in e m , hHg is the height of the mercury c ol u m n ab o ve th e cap i l la ry tip in e m , a nd y the interfacial tension between mercu ry and the cell s o l ut i on in m N m- 1 •

is

A s the second term i n the brackets am o un ts to o nly 2-51Vo of the first te r m rn i s approximately t o th e h e igh t of the mercury column for a g ive n capillary. Si n c e the drop weight ( rn tu) i s constant at a gi ven p oten t i a l in a p a rt i c ul a r solution, then rn = Kh1 1 g and . ' -1 h -I t0 K rn = K (K 11 ) an d from equation 2.4, g ,

pr o p ort iona l =

'

,

( 2.9)

Thus a pl ot of the diffusion current against the s q u a r e root of the heig h t of the mercury column should be very nearly linear for a d i ffu si o n controlled electrode process. This is a useful diagnostic t e s t for a diffusion controlled current.

22

Introduction to Voltammetric Analysis

2 . 1 . 2 Reversible processes: the half-wave potential The characteristic shape of a DCP wave ( Figure 1 . 5 ) fo r a d iffus i o n co ntro l l ed rev ers ib le ch a rge transfer process (equation 2. 1 0) ( 2 . 1 0a) (2. 1 0b ) ( a red ) , -..=::=;:;__4 ( ared ) cell diffusion

where

( a0,)cell and ( ared )ce11 r e fe r to the a na lyte in it� o xidi sed and reduced form s , respectively, in the cel l solution, a nd ( a0x) s a n d ( ared ) s a re the corresponding s pe c i es at the electrode s urface ,

is de sc r ibed by the

N e rn st

equation:

Eorv!E where

=

,...o t:'.

+ { RT n

F 1n

ox/red

{ ( c,)0, + ( c, ) rcd }

=

( c. )ox ; and ( c. ),.d

}

( 2. 1 1 )

=

0 fo r the case of a reduction,

(cJox and ( c. ) ,.d a re the concentrations of the oxidised and reduced forms of the a n a lyte , respectively, in the cell s ol ut i o n , then from e qu a t i o n 2 . 3 and 2.4

so that

( c, ) ox

and

( c, ) rcd

where

{ c..)ox fox ( c, ) red lrred

En ME is the p o te nt i a l of the DME relative to the reference electrode, E0ox/rcd is the s ta n da rd p ote nt i al for reaction 2. 1 0b relative to the reference electrode, R is t h e gas constant, 1 ' is the absolute temperature, F is the Faraday c o n sta nt , n is the number of e le c tro n s involved in eq u a t i o n 2 . 1 0b, ( c5 ) 0, and ( c. ),.d a re the electrode surface concentrations of the oxidised and r e d u c ed forms of the a na lyt e, r e spe c ti vely, and J;,. and fre d are the c orr es p on d in g a ctivi ty c oe ffic ie n ts .

Since for equati on 2 . 1 0,

where

( 2 . 1 0c )

=

ox

( id - i) (k )

( 2 . 1 2 a)

i (k )

(2. 1 2b )

=

IL

I L red

( k11)ox and ( k1 L )red rep rese n t all the terms i n the Ilkovic eq u at i o n ( 2 .4) (other than con­ centration) for t h e oxidised and r e duc e d forms of the a n alyte, respectively.

Substituting into e qu a t i on 2 . 1 1 gives:

EDM J:. =

E

0

RT

ox/red + n F i n

{( ') } ld - t

-t. ·

+

RT ln nF

{

D

1 12

r J ox

(voJ (j;.J red

}

( 2. 1 3 )

Techniques

Figure

2.2

when

i

where

=

23

2 Variation in the half-wave potential for the reduction of Zn + as a function of the concentration of different supporting electrolytes .

id/2, th en by defini tion,

fnME = E ' 1 1 2

E' 112 is known as the half-wave potential for a reve rsi b l e electrode reaction ( relative to the reference electrode) and is g ive n by: E l /2 = E' r

ox/red + {. ( )112 ( )} R ],

nF

r

D red

Jox

Dax

In

fred

( 2 . 1 4)

Because it is related to the standard electrode potential for the electrode process, the reversible a characteri stic of a gi ve n an a l yt e in a p a rt i cu l a r s upp o rt i n g electrolyte solu­ tion and so can be used to id e nt ify t h at a nal yte From equ a t i o n 2 . 1 4 it can be seen that H 112 i s independent of the concentration of the analyte for a reversible process. In many cases the oxi­ 1 12 dised and reduced forms of the an a lyte have similar diffusion coefficients so t h at ( D,ed/ Dux) is close to un ity and has little effect on I! 1 1 2 . On the other hand, the c h a r ge s on Ox an d Red differ by n el ec tron s so th a t thei r respective acti v i ty coefficient-s will be d i ffe re n t ly affected by changes in the concentration and n a ture of the supporting el e c tro l yt e s This may become s i gnifi can t at high con­ centrations and will be reflected in changes in 1! 1 1 2 such as those illustrated in Figure 2.2 for t h e reduction of zinc( I I ) i n the presence of non-complexing p e r c h lor at e ions. At lower s u p p ort i ng electrolyte concentrations ( ca 0. 1 m o l L- 1 ) th e s e effects are negligible and the second term on t h e right-hand side of e quat i o n 2. 1 4 is small e n o u gh to be n egl ec te d so that E 'l/2 may be i de nt i fi ed with the st an dard po te nt ial for the electr o d e reaction, J::"'ox/ red· Equ a t i o n 2. 1 3 no w b ecom es :

.

half-wave po te n t i al is

.

.

( 2 . 1 5a)

,

which is often called the Heyrovsky-Ilkovic equation [ 8 ] . Changing to b as e 1 0 l o g a rit h m s and substituting for the ph ysi c a l co n sta nt s gi ve s at 25°C:

24

Introduction to Voltam metric Analysis

Em:v!t

=

E llz r

0 . 0592

+ -n- l o g

{

( id - i )

--i-

}

( 2. 1 5b)

When the cell solution contain s b oth the oxidised and reduced forms of the analyte, that is both ( calox > 0 and ( c. l red > 0, th en equation 2. 1 5b becomes:

Eo::v!E where

=

{

' 0.0592 r ( id )cath - i ] l og [ . ( ) ] E112 + -n I l d an '

_

}

(2 . 1 5c )

( id lcath a n d ( id ) a n a re the limiting cu rre n ts for the forward cathodic ( reduction) and of equation 2 . 1 0, r es p e ctivel y, and !! 1 12 is the p o ten tial at w h ic h i ! ( id l cath + ( idlan l / 2

reverse anodic ( oxidation) r e a ct ions =

For a r eve rsible electrode process, I! 1 12 is the same for both the reduced and oxidised forms of the analyte, and when both the oxidised a n d reduced forms are present t oge th e r in so l u t io n, only a single wave is observed with thi s same !!112• The effect of the num ber of electrons involved in the electrode react ion , n, ( e q u ati o n 2. 1 Ob) on the slope of the polarographic wave is shown in Figure 2.3a. For a reve r s i b le electrode process the value of n m a y be d et e r m i n e d from a plot of EnME ve r s u s log { ( id - i ) / i ) which will be a straight line of slope 59.2/ n mV. Alternatively, n may be e s tim ated directly from the p ol a ro graphi c wave u s ing the Tomes [ 9 ] rel at i o n s hi p which for a reduction at 2 5°C is E3t4 - E 1 ;4

=

Ej 1 4

-56.4 -- mV n

(2. 1 6)

and £114 are t h e po t e n t ials where i 3 V4 an d i id / 4 re s pe c t iv e l y. other hand, i f t h e value o f n i s known from other experiments (e.g. co u l om et ry) , then either of the above two methods can be used to check wh e t h e r or not the electrode process i s reve rs ib l e . I t sho u l d be noted that for reversible electrode processes with more c om pl ex stoichiometries t h an that of e q u a t i o n 2 . 1 0 , the plot o f equations 2 . 1 5 m a y give slopes other th a n 59.2/ n mV ( see discussi on in r e fe re nce 5 ) . When the reduced form of the analyte i s a metal soluble i n m e r c u ry the wave i s also described by e q ua t i o n 2 . 1 5a . However e q u a ti o n 2 . 1 4 for E ;12 mus t be modified to allow for the diffusion of t h e reduced form i n t o the m e r c u ry instead of th e solution phase and eox/red m u s t be r epl a c ed with the equivalent e xp r e s s i o n fo r the m e t a l a m a l ga m [ 1 ] . I f t he reduction product forms an insoluble p h a s e o n the electrode su rface, the half-wave potential is n o longer independent of the analyte concentration but varies with log c. a c co r din g to t h e re l a t i o n s h ip [ 1 ] :

where

=

=

,

On the

E

o

oxtred

+

or Eo

ox/red

-

R T1

F n

R T1 F 11

n

{fux(c.Jox}

(kn) + -f

n T ) ox

( 2 . 1 7a )

2

R T1 ( . n ld n

.

- 1)

( 2 . 1 7b)

Tech n iques

25

a

- E

b

- 0.5 Figure 2.3

- 1 .0

E

- 1 .5 [V] (vs. SCE)

(a) The effect of the n umber of electrons involved i n the electrode process, n (=1 , 2 and 3, respectively), on the slope of the d.c. polarogram. (b) The effect of the degree of reversibility on the shape of the d.c. polarograms for two-electron reactions. The electrode reactions are, 1 , Cd1+ + 2e- � Cd(Hg)

and 2,

H,O,

+

2e - ---> 201 1

and the p o ten t ia l of the

( reversible )

( irreversible)

DME is give n by Emvt F

=

r

E l /2

+

RT nF ln

{ id --: i) } 2(

jd

(2.18)

The majority o f metal ion reductions which give metals insoluble i n mercury are not reversible. The above equations have been shown to describe the behaviour of the anodi c waves observed in the oxidation of the mercury electrode in the presence of low concentrations ( up to 1 mmol L-1 ) of halide ions. In these cases the electrode reaction (2. 1 9)

of the mercury ( I) halide on the electrode surface and the value of in equation 2. 1 7 is t ha t of t h e respective mercury-mercury(I) h alide electrode. �ox/red

produces an insoluble layer

26

Introduction to Voltam metric Ana lysis

In a d d i t ion to the small changes in the h alf- wa ve p o t enti a l due to the a ct i vi ty coefficient effect at high el ectrolyte concentrations, £" 1 12 may c hang e s i gnifi cantl y when the co mpo s i ti o n of the sup p o r t i n g electrolyte solution is varied. If a species, L, wh i c h can react with a m etal ion to form a complex is added to the solution, t h e value o f f!1 1 2 for the re d u c ti o n of t h a t metal ion may be s hifted markedly. Since the co mpl ex e d metal ion is a more stable species than the free metal ion it will be more difficult to reduce so that £" 1 2 is s h i fte d to m ore n e ga tive p ot en ti als 1 the str o ng e r the c o m p l e x , the greater the shift. If the c omp l exing agent, L, reacts with th e metal ion according to:

­

(2.20)

where

Kr is

the s tab i li t y constant for the for m at i on of the complex metal i o n MLp"+, and p is th e number of l i ga nd s bound to the m e ta l ion in the complex,

then the s h i ft in t: uz is g iv en by [ 1 ] : .:\ E �/2

whe re

=

( E �/2 h.1L - ( E�/2 )M

=

-

RT RT ln K- - p ln cL nf 1 nF

(2 . 2 1 )

( I:."' u2 ) M and ( Eruz ) ML is the reversible half-wave potential of the metal ion in the absence and presence of the li ga n d , respectively; C1 . is the bulk c o nce n t r at i o n of the l iga n d in the cell solution, and th e other symbols have their usual m eaning .

Using e q u a t i o n 2 . 2 1 , the stability constant of the metal ion c o mp lex formed can be calcu­ lated from t h e shift in E r 1 12 on adding t h e ligand to the metal ion solution. The effect of com­ p lex i ng agents on e l /2 can be used to improve th e selectivity of a voltammetric or polarographic analysis. Two m etal ions w h os e polarographic wave s overlap beca use th ey have similar half-wave p o ten ti al s may be sep a rate d if a c o m p le xi n g agent which forms a strong com­ plex with one of the m etal ions but not the o th e r is added to the solution. The reduction wave of the for mer is shifted to a m u c h more negative p o t en tia l while that of t h e latter is only slightly affe c te d so that two distinct waves a re now observed. Note that equation 2 . 2 1 ass u m e s that the effects of the activity c o e ffi c i e n t s and diffusion c o e ffic i e nts of the various species do n o t con­ tribute sign i ficantly to E r 1 1 2 • Another situation where changes in the solution composition c an cause significant c h a nge s in the half-wave p o t en tial arises wh en the a n alyte is an o rganic compound, Org, wh ich reacts at the electrode acc o r di n g to the equation:

( 2.22)

In such cases the half-wave p o tential is pH depe n de n t and for a reversible process at 2 5°C is g i v e n by [ 6 ] E'1 /2

=

Eo oxired - --n

(0. 0 59m)

PH

Again , it i s a s su m ed that the diffusion coefficients of Org and OrgH111{ 111 that their activity coefficients do n o t significantly affect E r 1 12.

(2.23)

n)+ are similar and

Techn iques

2 . 1 . 3 I r revers i b l e processes

27

­

When the cha r ge

transfer reaction i s slow th e s h ap e of the wave is de t e rm i ned by the rate of the charge transfer reaction and not by diffusion o f the ana lyte to the electrode surface. As eq u ili b rium cannot be attained at the electrode s u rfa c e d u ri n g the t i m e of the measurement, the Nernst equation does not a p p ly and i nst e ad the appropriate rate expressions must be used to obtain the relationship between th e current and surface c on c ent r a t i o n of the a n a lyt e . Since the rate constants are p ote nt i al d e pe nd ent ( e qu at i ons 1 .3 an d 1 .4) an i n c r ea s e in the po tential ( more ne ga ti ve for a reduction and more p os it ive for an oxidation) causes the re a c ti o n rate to i ncrease, so that eventu­ ally a diffu sion controlled limiting current re gio n is reached where id k1 L c• . For the case of an i rreve rs ib le reduction, the potential-current relationship is given by [ 4, 5 ] : =

E

•

=

E

o

oxir< d

+

(0. 9 16R!1) o. n . F

-

In

{ } ( RT ) {

and E

_

E J /2

+

( ir i ) i

+

0.9 1 6 R o. n . F

�

---

(

( t0)112}

I n 1 . 3 5 9 kred D

o. n . F

{(id - i)}

ln --.1

( 2 .24)

(2.25)

w it h

( 2 . 26)

where

n. is t h e n umber of electrons in vol ve d i n th e ra t e d e t e r m in i n g step ( n. :S: n ) . (These equa­ tio n s apply fo r the c u rrent measured at t0, t h e end of the drop life . )

Th e sha p e o f t he wave is affected b y both the t ra n s fe r co e fficient , o. , an d the r a t e constant, k,.d· The effect is to r edu c e the slope of the wave so th a t it is s p r e a d over a wider p ote n t i al range than in the case of a reve rsib l e wave ( Figu re 2.3h ) . The half-wave potential now contains a kinetic term in addition to the thermodynamic t e r m E0 ox/rod and also depen ds upon the d rop time. Because o f these fac to rs it is i m po rt an t when stu dyin g irreversible systems t o re p r o d u ce all exp e ri m e nt a l con­ ditions carefully if £112 i s to be u se d t o id e n ti fY the analyte.

2. 1 .4 Po larographic maxima When r e co r d ing d.c. p ol a rogra ms , the current is oft en observed to rise st e e p ly ini ti a lly to a maxi­ mum value a t the head of th e wave before dropping back to t h e limit in g c urrent p l a te au ( F i gu r e 2.4). This p h e n o m e n o n is the result of a d d i t ional analyte b e ing brou g h t to t h e electrode surface by the movement of the s ol u t i o n past the drop surface. The movement is attrib uted to the m ot i o n of the double l ayer as a re su lt o f t h e a sym m et ric electrical field around the drop. Such m axi m a are referred to as maxima of the first kind and in t e r fer e with the determination of the analyte. Substances w h ich arc adsorbed a t the electrode interface h i n der this m o t i o n and can be used to suppress these u nwa nte d maxima. Good maximum suppressors in co mm o n use include Triton X- 100 and ge l at ine at concentrations of 1 0-2 to 1 0-.lo/o. The second type of maxi m a appear on the limiting current p l a t e au and also interfere wi t h the measuremen t of id . They are referred t o a s maxima of the second kind and arise from too strong a flow of mercury fro m the c a p i ll a ry durin g dr o p growth. Such maxima are fo u nd to be m o r e prevalent at high supporting e le c tr ol yt e concentrations. They c a n be red u c ed or eliminated by

28

Introduction to Voltammetric Analysis

Figure 2 . 4

- E

[V]

Polarographic maxima. A d . c . polarogram ( 1 ) with a maxi mum o f t h e fi rst k i n d ( 2 ) a n d with a maximum of the second kind (3) .

lowering the mass t1ow rate of mercury from the capillary and by working with supporting elec­ trolyte con centrations below 1 mol l .- 1 where possible. 2 . 1 . 5 Selectivity and sens itivity l i m itations

With DCP, inorganic and organic analytes may be determ i ned at concentrations as low as 1 0- 5 to 1 0-6 mol L- 1 • When more than one analyte is present in solution the currents are additive and pro­ vided that their half-wave potentials are separated by at least 1 20 mY they can be resolved and the concentrations of the various species determined. Figure 2.5 shows the polarogram obtained for Pb 2+ , Cd2+ and Zn 2+ ions in admixture in oxygen-free 0 . 1 mol L -I potassium chloride solution. The concentrations of the three m etal ions can readily be determined from the heights of each individual wave in the polarogram since the waves with E'u2 values of -0.40 V, -0.60 V and - 1 .00 V ( relative to the SCE ) respectively are well separated. The d . c. polarogram of an oxygen-free solution of the supporting electrolyte alone consists of three parts. When the working electrode is anodically polarised, a large anodic current is observed. As the potential is scanned towards more negative values, the anodic current rapidly rises to a small value. Then follows a potential region where the small current increases gradually passing through i 0 to become a small cathodic current, until at a sufficiently negative potential a steeply rising cathodic current develops. The large anodic current is due to the anodic dissolu­ tion of the mercury of the electrode and the potential at which it occurs is determined by the anion of the supporting electrolyte, whi l e the cathodic current arises from the reduction of the cation of the supporting electrolyte. Between these two limits is the polarograph ically usejiJI pote n ­ tial range of that parti cular supporting electrolyte-solvent system and the small current that t1ows in this potential range is called the residual current, i,« ( Figures 1 .7 and 2 . 5 ) . The useful polaro­ graphic potential range of some common supporting electrolytes for the DME in various media are listed in Table 2. 1 . The residual current is important because it is the 'noise' level and as such determines the sensitivi ty of the polarographic measurement. The residual current is the sum of the capacitance or charging current, i,, ( § 1 .4.3 ) and the faradaic currents produced by the reduction or oxidation of electroactive impurities. The latter contribution is made insignificant by using high purity solvents, high purity salts as supporting electrolytes, and removing oxygen from the system by passing high purity nitrogen or argon =

Techniques

29

i cath

-----

-0

.

2

- 0.6

Figure 2.5

_ _ ....

/

/

/

I

I

Residual c u rrent

- 1 .0

E [V] (vs. Ag/AgCI,

M e rc u ry dissolution

----

/

- 1 .4

3 M

KCI)

i an

D.c. polarogram for the reduction of a mixture of Pb2 + ( £1 1 -0.40 V), C d 2 + ( £1 12 -0.60 V) and 2 - 1 . 00 V) in 0.1 mol L-1 KCI. Potentials are relative to the saturated calomel electrode. zn 2 + ( £1 1 2 The polarogram h a s been displaced vertical ly for clarity. The potentia l at which t h e residual current is zero, Em• occurs at ca. -0.3 to -0.4 V, i .e . at the foot of the Pb2 + wave. =

=

=

through the cell an d solution pri o r to, and over the solution wh il e , re co r di n g t h e p o l a ro gra m . Oxygen is a nuisance in polarographic s tu d ies and m ust be removed from the cell so lu t io n because it is redu c e d in two steps, first to H202 and then to hydroxide ions. In air-saturated 0. 1 M KCl solution, th e two waves each have a h e igh t of ca. l .9 1-1a and have half-wave potentials of -0.05 V a nd -0.85 V (versus SCE ) , respectively. On t h e other h a n d , exce p t at the potential of zero cha rge, Em, t h a t is the p o t e nt i a l rel ative to the reference electrode at which the c ha r g e on the electrode is zero ( F i g u r e I . 7) s o that i, 0, th e c a p ac i t ive current will always have a finite v a l u e . When the contri b u t i o n from c l e c ­ troactive impurities is insignificant, iros can be iden tified with ic ( Note: I t i s only when the contribution from impurities is z er o at E111 that the p o t en t i a l at w h i c h i,•• 0 will be Em. ) The capacitive c u rr e nt depends on the electrode p o t e n t i a l and the rate o f change of the area of the dro pping mercury electrode. It i s given approximately by equation 1 . 1 5 which at constant po t e nt i al b e c o m e s : =

=

(2.27) O n combining e q ua t ion s 2.2 a n d 2 . 2 7 one ob t a i ns : .

lc

( t)

=

0 . 5 6·s cdl m 213 t-11 3 ( E' m - EDME )

( 2.28)

In a quantitative polarographic measurement, the a n aly t ic all y relevant p a r t of the c u rr en t is the diffusion controlled fa r a d a i c current, ir ( id ) , arising from the ch arge transfer reaction o f t h e analyte. The c ap ac itive current, i,, is the in terference or noise signal (§ 1 .4 . 3 ) . A c om p a r is o n of equatio ns 2.4 and 2.28 sh ows that d u rin g drop g rowth , iF an d ( cha n ge i n o p posi te directions, iF =

30

Introduction to Voltammetric Analysis

Table 2 . 1

Polarographically useful potential ranges o f some supporting electrolyte - solvent systems.

Solvent

Anodic limit

Cathodic limit

l .O M KCl

+ 0. 1

-1.9

l . O M HCl

+ 0.1

-1 .3

-0.4

- 1 .0

Electrolyte

( Vvs SCE)

( Vvs SCE)

Water

1 2 M HCI * I . O M NaOH

0.9 M NaF

1 . 0 M N H 1 + 1 .0 M NH4Cl l . O M LiOH

1 .0 M HCI04

0 . 5 M Na 2Tartrate ( pH 9) l .O M KCN

*

-0. 1 5

+ 0. 2

-1.85 -

1 .85

-0. 1

- 1 .9

-0. 1 5

-2. 1

+ 0.4

0 -0 .6

-1.3 - 1 .85

- 1 .9

0 . 5 M Na2S04

+ 0 .3

0. 1 M Et4NOH

-0. 1

-2.4

+ 0.8

-1 .9

- 1 .85

Dichloromethane n-Bu4NCI04 n - B u4NI

-o.s

Acetonitrile NaCI04

+ 0.6

Et4Ncto.

+ 0.6

NaCI04

+ 0.5

Et4NCI04

+ 0. 5

n - Bu.1CI04

+ 0.5

- 1 .7

- 1 .7 -

2.8

D i methylformam ide

•

From

[ reference 1 0 ) .

-2 .0

-3 .0

-3 .0

increasing with time while ic decreases with time as shown in Figu re 2.6. With decreasing analyte concentration, iF becomes smaller until ( for the case of a 4 s drop time) when if < 2 ic it becomes no longer visually detectable. Note that the small amounts of oxygen and any trace impurities that may be present in the cell solution will not be noticed because their combined faradaic currents are smaller than i,. The iF : i, signal- to- noise r a tio is the determining factor for the sensitivity ofDCP. If more sensitive polarographic determinations are to be performed, this ratio must be increased by using electrical circuitry or techniques in which either the faradaic signal is enhanced or the charging current is minimised or where both can be achieved. Inspection of Figure 2 . 6 shows that if the current is not measured over the whole drop life as in normal DCP, but over a short period of time towards the end of the life of a drop when the rate of increase of surface area is least, then the current averaged over the sampling interval, tm, should have an improved iF : ic ratio. This is the basis of one of the earlier attempts to improve the signal­ to- noise ratio, namely, tast or sampled polarography using a linear potential scan [ l l ] . In this method, the current is read over the sampling interval (typically 5-20 ms ) , recorded, and 'held' by the circuitry until the next current sample is taken towards the end of the subsequent drop. The result, shown in Figure 2 . l b , is an 'envelope' of a normal d.c. polarogram without the usual oscil­ lations and is described by the same equations, 2.4 and 2. 1 5, and so on. There are two developments which have resulted in a major reduction in the charging current contribution to the sampled current. The first is the replacement of the DME with the static

Techn iques

31

D rop fall

I -c ! ..

u :I

measu rement t i m e

T i me

Figure 2.6

�-- D rop l ife -----.!

The va riation of iF and i, during the l ife of the drop. In tast polarography the current is sampled over the me a s u rem e n t interva l, tm, nea r the end of the drop life where the ratio iF : ( is a maximum .

mercury drop electrode, SMDE.

In this electrode the mercury drop is extruded rapidly over 40 to

200 ms and its growth then stopped, after which the area of the mercury electrode rem ains con­

stant. The area-time plot for the SMDE and a typical s am p ling pe riod is sh own in Figure 2 .7a. This el ec t rod e h as the advantage of the constant area of a solid electrode with the lower charging current baseline while still retaining all the advantages of the DME such as a renewable clean sur­ face b ein g ge n e ra te d with each d ro p . An other advantage of thi s electrode is that the voltage range can be covered mo re rapidly since the several second wait for each new drop of the DME to grow to a suitable size has been replaced by the b rief extrusion period. The second has come with the introduction of modern digitally controlled instruments. The linear d.c. potential ramp applied to the working electrode has been replaced with a stepped or so-called staircase vo ltag e waveform in which the applied vo ltage is ch an ged in s m a ll discrete steps at re gular intervals ( Figure 2 . 7 b ) . Th e voltage steps are syn c hro n i sed with the dro p formation and the current is usually sampled over the last 5 to 20 ms of the drop life. Since the voltage is constant (a E/a t 0) when the current is being sampled, there is no contribution to the ch a rging current from this source (first term of equation 1 . 1 5 ) . Because the current i s now measured at constant potential and con stan t electrode surface area, a major reduction in charging current is achieved resulting i n an i m p rovem e n t in sensitivity over DCP to ca. 5 x 1 0- 7 mol L- 1 • However there is no im p ro vement over DCP in the resolution of adj a ce nt w ave s . =

2.2 AMPEROMETRY

The d i rect proportionality between th e an alyte concentration and the current that flows when the potential of the working electrode is h eld constant at a value in the limiting current regi on of the analyte ( Figure 1 . 5a, region 3) is the basis of the technique kn o wn as amperometry. Amperometry, which might be c on s i de red a s vo l ta m me t ry at constant potential, has been utilised in a number of ways. First, in amperometric titrations as an i ndi c ato r to m o nitor the course of the titration. In this app l i cat i on e i t he r the a n a lyte, A, or the titrant, T, or both must be electroactive. The working

32

Introduction to Voltammetric Analysis

a

I -I I

I

:-

D rop life

E

Initial voltage

' ' ' ' -+-:

: ,

tstep

:'

- - - - - - - - - - - - - - - 1' ­

' ' ' :+--

b

c

Figure 2 . 7

-E

The variation of surface area with time for the static mercury drop electrode, SMDE, (a) and the potential-time profile for the stepped or stai rcase voltage applied to the worki n g electrode (b) and the resu ltant polarographic wave (c). tm, measuring interval; �rep• time interval of the voltage step (= to): !1 f,1•P' voltage step of the applied stai rcase waveform (adapted from [ 1 2]).

electrode is normally a rotating platin um or carbon electrode. The current signal is used to follow the disappearance of the analyte during, or the appearance of excess titrant o n completion of, the chemical reaction used for the titration . A

+

T � Products

(This may be either a precipitation, complexation or redox reaction . ) A plot of current versus titrant volume will give two linear sections and the end point of the titration is the volume corre­

sponding to the intersection of the extrapol ation of these two linear sections.

Techniques

33

Second, two current-carrying working electrodes, usually platinum wire or sheet, may be used in a technique referred to as biamperometry or dead stop titrations. The current flowing through the two electrodes (one an anode and the other a cathode) is the same. A plot of current versus titrant volume is of the same form as that for an amperometric titration and the current at the end point is a minimum. An important biamperometric method is the Karl Fischer titration for the determination of low levels of water in organic solvents. The titration is carried out in anhydrous methanol. The titrant is a methanolic solution of pyridine, iodine and sulphur dioxide. The reac­ tion involved is

(2.29) so that each mole of water reacts with one mole of l2• Since l2 is the only species that can be reduced at the cathode a sudden flow of current occurs when, on reaching the end point, the first excess of l2 is added to the solution. Third, the proportionality between the current and concentration is used to determine the analyte concentration directly after calibration using standard solutions. The determination of oxygen in a variety of waters and biological fluids using the Clarke cell ( Figure 6. 1 7) is an example of direct amperometry. The probe has a gas-permeable membrane separating the test solution from an inner electrolyte solution. A fixed potential is applied to the two electrodes, usually a plat­ inurn wire cathode and a silver chloride coated silver wire anode, in the inner electrolyte solution. Oxygen diffuses through the membrane and is reduced at the platinum cathode. The fourth and increasingly important application of amperometry discussed in chapter 5 is in the detection of analytes in flowing liqu ids after they have been chromatographically separated from the matrix and other analytes in the original sample. Detectors based on hydrodynamic amperometry using flow-through cells are important in high performance liquid chromatogra­ phy. Applications of amperometry are given in chapters 6 and 7.

2.3 LINEAR SWEEP AND CYCLIC VOLTAMMETRY

Another approach to measuring the faradaic current under conditions where the ip : i, ratio is increased is to apply a rapid potential scan to the DME over the last seconds of the drop life when the rate of increase of area and hence i, is lowest. The potential profile is shown in Figure 2.8a. In this technique, referred to as single sweep polarography, SSP, a current peak is observed in the i-E plot rather than a wave for the reasons outlined in section 1 .4. In developing the theoretical description of the current peaks it was assumed that because of the rapid potential scan rate (v = SO to 500 mV s- 1 ) , the DME behaved as a stationary electrode over the duration of the scan (0.5 to I second) . However, this was not a good assumption. For example, if the fast potential scan is applied at 3 seconds after drop birth, the electrode area increases by approximately 20% during the next second. This results in a corresponding increase in the charging current above that required to change the potential during the period of the scan (equation 1 . 1 5 ) . The result is that SSP has a base line with a significantly large curving slope (Figure 2 . 8a) which limits its analytical applicability. With the introduction of the SMDE, the HMDE and the use of solid electrodes all of which have a constant area during the voltage sweep, SSP has been superseded by linear sweep voltam me­ try, LSV. For the case of a reversible process, the peak current in a linear sweep voltammogram is given by the Randles-Sevcik equation [ 4, 1 3- 1 5 ] ( 2.30)

Introduction to Voltammetric Analysis

34

a

t

b

t

-E

-E

icath

-

Figure 2 . 8

where

--

�

- - -

-

---

-E ian -E

, .- s7 mV I

I

Ep[ox]

Linear sweep (a) and cyclic voltammetry (b). Top: variation of the potential applied to the working electrode with time. Bottom: resulting current-potential curves .

k contains all the physical and mathematical constants and is constant at a given temper­ ature, Dux is the diffusion coefficient of the analyte in its oxidised form, 1 v is the potential scan rate in V s- , and the other symbols have their usual meaning.

This equation also applies for an oxidation except that ( ip )an is negative. The peak potential, EP, is independent of scan rate and is related to the DCP half-wave poten­ tial, 1!112 for a cathodic process, by [ 5 ] (2.3 I a)

Tech niques

35

and for an anodic process, by . mv at 2soc E r112 + -28 5 n

while th e hal f- peak potential process by

( Epdcath

EP12, i.e. th e potential at which i

=

E�1 2 +

1 .09 R T

nF

=

=

(2.3 1b)

ip/2' is rela ted to l! 112 for a ca th o d ic

E�12 + 2 8 · 0 mV at 25°C n

(2. 3 1 c)

and for an a no d i c pro c ess, by 0 Er1 12 - -2 8 . m v at 2soc

n

( 2 . 3 1 d)

The differe nce between EP and EP 12 is th us 56.5/ n mV for a reversible process at 25°C. Th e p eak current is considerably larger than the diffusion current for the s a m e system because the rapid ra te of ch a n ge i n vol ta g e means that th e diffusion layer is much th i nne r and the concen­ tration grad ien t correspondingly greater. The an alyt i c a l detection limit is m uch im p roved over DCP being about 5 X 1 0-7 m o l L- 1 • In addition, since LSV p r o d u ces peaks in the i-E p l o ts , better resolution is obtained a n d p e a ks separated by as l itt le as 50 mY can be r ea d i ly resolved. The peak current increases with the square root o f th e s c an rate , v , and so greater p eak cu rr e n t s a n d h ence greater sensitivity might be e xp e ct e d a t h i g he r scan r a t es . However the charging current increases linearly with v ( equat i o n 1 . 1 5 ) , that is at a greater rate tha n does th e p e ak current. In o rder 112 to optimise the ir: ic r ati o in LSV, sc an rates are l im i t e d to the range wh ere v > v, that is to v < 1 .0 V s-1 • For an irreversible electrode reaction, both the ch arg e transfer coefficient and the rate con­ stant of the c h arge transfer reaction influence the peak c ur re n t which is given by: ( 2.32) where

n.

is the number of electrons involved in the rate determining ch arge transfer step ( n. � n) , k ' c on t a i n s all t h e mathematical and physi c al constants and is a fu nc tion of t h e rate con­ stant of the e l ec t r ode reaction, and the other sym b o l s h ave their usual meaning.

The reader is refe r re d to referen ces 4, 5 or 15 for a detai led discussion. In su m m a ry, irreversibility decreases iP. For exa m pl e , for an irr evers ib le one-electron process wi th a. 0.5, (�);,..)Cip),• • 0 . 78 [ 5 ] . This con trasts with DCP where the l imiting diffusion current, iu, is not affected by the reversibili ty of the electrode process. Because of the dependence of current h eigh t on the reversibility of the e lectrode process, care m ust b e t ake n wh en us i n g LSV for analytica l purposes. Solution conditions and electrode s urfa ces must be reproduced as closely as possible for the sample and standard. Also, for an irreversible process t he ma gnit u d e of the difference between El/2 a nd EP is a function of scan rate and the rate constant of th e electrode reaction. The difference i ncr ea s e s with increasing scan rate and d ec reas i n g values of k,ed (or k0x) . T hi s results in a s p read i n g of the current peak shape so that as the el ect ro de process becomes more irreversible, t h e resolution between peaks decreases. Experimental c o ndi tio n s should be adjus ted so t hat the electrode process is as close to =

=

36

I ntroduction to Voltammetric Analysis

c

b

A

-E

a

ii

8

b

Figure

2.9

-E

[V]

-E

[V]

(A) Cycl ic voltammog rams for a reversible (a), a quasi -reversible (b) and a n i rreve rs ibl e (c), electrode p rocess . (B) T he effect of (i) the number of ele ct ro ns involved, and (ii) t he reversibility, on the sh a pe of a l i near sweep voltammogra m . I n cre a si n g i rreversibil ity (a) � (c) .

reversible as can be achieved when using fast scan methods for voltammetric a nalysi s The effects of reversibility and n o n the shap e of a linear swe ep volt amm o gram are shown in Figure 2.9B. If the p otent i al is first scanned in one direction ( negative) as in LSV and then returned to the initial p otenti al by a scan at the same rate in the o pp o s i te direction (positive ) , the technique is known as cyclic voltammetry, CV. F i gure 2 .8 compares the variation of potential wi th time and the responses fo r LSV a n d CV. Cyclic votammetry is used mainly for studying the mechanisms and reversibility of elec t r o de p rocesses and only sometimes used for a n alytica l purposes. The current-voltage cycle illustrated i n Figure 2 . 8b is obtained when the reaction products, formed during th e forward ( cathodic) s ca n as a result of the electrode reaction that gives rise to the peak at ( Ep ) calh' are oxidised on the revers e scan as shown by the peak at ( Ep )an · From equ ations 2 . 3 l a an d b, for a reversible redo x process at 25°C: .

(2.33)

Tech n iques

37

1

E p [red]

i cath

- E

2

Ep[ ox]

ian Figure

2. 1 0

1

E p [ox]

Cyclic voltam mogram for a n electrode reaction involvi ng a n E C mechanism i n which a reversible charge transfer step is followed by a chemical reaction to give a second electroactive species.

a n d LSV, the p o sit i o n of th e current peak is i nd ep e nd en t of the vo ltag e s ca n r a t e , v . In a d di t io n the ca t h od i c and anodic peak c u rre n t s are e qu a l . For quasi-reversible s ys t e m s , that is t h o s e which appear reversible in DCP but give an irrevers ­ ible response when faster m easuring t ec hn i qu e s are u s e d , the difference in the p e ak potentials is about (60/ n) mV at low s can rates of 10 to 20 mV s-1, b ut becomes greater as the scan r a te is increased. Also, tlt.� i n c re a s e s as th e irreversibilty of th e electrode p rocess i n cre a s e s , that is as kred and as for SSP

and k0, ( equations 1 .3 , 1 .4) decrease.

For a totally i rr eve r s i b l e process, that is o n e in which the electrode r e act i o n pro d u ct s cannot be oxidised b ac k to the initial a n alyt e because kox is ext r e m el y small or because th ey have u n d e r ­ gone a subsequent i r r eve rs ible chemical c han ge to compounds that are no t electroactive, no anodic c u r re nt peak is observed ( F i g u re 2.9A ) . F o r th e case where a reversible c ha rge transfer i s fo ll o we d b y a c he m i c a l reaction in which electroactive p rod uc ts are formed, a CV of the type shown in Fi gure 2 . 1 0 may b e obtained . In the first cat h o di c sweep the peak at ( E/red is observed. On the reve r s e (oxidation) s c a n a p e a k is observed at (t.� ) 10,. However, a l t h o u g h th e peak sep a r at i o n is the p re d i c t e d 57/n mV for a reve rs ­ ible e lec t r o d e process, the ratio of the peak heights is n ot as e x p ec t ed . Rather ( iP) 1 ox i s smaller than ( iP) red b e c au s e part of the electrolysis pro d u c t has been c h em i c al ly converted to co m p o u n d 2 and is therefore no l ong e r available for oxidation a t ( E/ ox· The ratio of t h e s e peak h e i gh t s de pe n ds on the rates of the s ub s eq u e n t chemical reactions and is a fun c t io n of these ra t e constants and the p ote n t i a l scan ra t e . If th e anodic scan is c o n t i n u e d a second oxid at i o n peak a ppe a r s at (E/ox if the pro duct of th e following c h em ical reacti o n is electroactive. If a fte r the second anodic peak the po t e n t i al scanning c o n t in ue s into a s ec o n d cathodic sweep, then a new c a thod ic peak appears at ( E/ red due to the reduction of the oxidised product of th e chemical step that fol l ow e d the initial reduction { at ( EP ) 1 red } . If the s ec o n d redox system is r ev e r sib l e then these two p e aks will also be separated by 57/ n mV. Cycl ic voltammetry gives inform ation on the re do x behaviour of electrochemically active spe­ cies, on the kinetics of el ectr o de reactions, an d in many cases enables the identity of re ac t i ve inter­ mediates an d subseq u e n t r e ac t i o n p r o d uc ts to be ascertained. CV also g ives q u al i ta t ive

1

1: l

38

I ntroduction to Voltammetric Analysis

t

...

t

Principle of pulse methods showing the decay of the faradaic, iF, and capacitive, ic, currents fol lowing the application of a potential pulse, A fpl• to the working electrode.

Figure 2 . 1 1

indications of more complex electrode processes incorporating adsorption and as such i s a valu­ able diagnostic tool in the development of analytical procedures.

2.4 PULSE TECHNIQUES Following on from his earlier work with square-wave polarography, SWP, in which a small a mpli­ tude alternating sq uare-wave voltage was superimposed on the slow d.c. potential ramp, Barker [ 1 6 ] found that applying a short duration rectangular or square potential pulse to the WE also proved to be a very effective means of reducing the unwanted capacitive (noise) current and achieving a marked improvement in the sensitivity of polarographic and voltammetric determi­ nations. A number of polarographic and voltammetric methods have been developed from the original SWP [ 1 7 ] , however only the more widely used techniques will be presented here. In these techniques a square-wave potential pulse (or alternating square-wave voltage in SWP) with either a constant or an increasing amplitude, is periodically applied to the working elec­ trode. Following the initial potential step at the beginning of the pulse, the faradaic and capacitive currents that flow in response to this potential change decay in different ways. The faradaic current, 12 ip decreases with t- 1 in accordance with the Cottrell equation (equation 1 . 1 0 ), whi le the capacitive current, (, decreases according to the relationship for charging a capacitor:

AEP1,

.

where

R

l

c

_

(AEP1) [ J

- R

exp

-t

R Cd (

is the ohmic resistance of the polarographic circuit, and 1 Cd 1 is the double layer capacitance of the working electrode, WE. *

(2.34 )

Techniques

39

The variations of iF and i, over the duration, tpt• of the square pulse, A Ept• are illustrated in Figure 2 . 1 1 If the current is measured towards the end of the pulse, essentially only ir is recorded since ic has decayed effectively to zero. Methods based on the application of square voltage pulses to the working electrode use pulses of differing frequen cies and magnitudes and utilise a range of measurement techniques. Pulse methods may be used with the whole range of working electrode types, the DME, the HMDE and the SMDE, as well as with solid electrodes and the different types of modified electrodes. Except for the case of SWP, normally only one pulse is applied to each drop in analytical studies with the DME. This pulse is synchronised with the drop period and is applied in each case towards the end of the drop life. 2.4. 1 Square-wave polarogra p hy In the original square-wave polarographic method, the rectangular alternating voltage superim­ posed on the linear d. c. ramp potential usually has a frequency of 225 Hz and an amplitude in the range AEsq 5-30 mV. The current is sampled over a measurement interval, tm, near the end of each square wave half cycle. This current is then rectified, amplified and recorded as a function of the applied d.c. voltage. Sampling the current near the end of each half cycle ensures that the charging current arising from th e sudden step in potential at the beginning of the half cycle has decayed away and does not contribute to the measured current. However when using the DM E , there is still a charging current flowing during tm as a result of the slight in crease in area of the mercury electrode during tm ( equation 1 . 1 5) . This is minimised by ( i ) measuring the current towards the end of the drop life when the rate of increase in electrode area is least, and ( ii) by using a tilted square wave as shown in Figure 2 . 1 2 . The p ulse tilts are omitted if a static mercury drop electrode with its constant surface area is used instead of the DME. A further reduction in the contribution of charging current can be achieved in modern instru­ ments with digital control when the linear voltage ramp is replaced by a stepped voltage ramp (staircase waveform ) . The square-wave pulses can then be applied to the plateau (constant volt­ age) region of the staircase waveform. The current-potential plots obtained in SWP are peak-shaped ( Figure 2 . 1 3 ) with the peak potential corresponding to the half-wave potential of DC P . For reversible electrode processes, the peak current is dependent on the amplitude and frequency of the superimposed rectangular volt­ age and is given by [ 5 ] : =

(2.35)

where k i s constant a t a given frequency. The square-wave polarograms shown in Figure 2 . 1 3 highlight the influence o f n and A�,q o n the height and width o f the peaks. For reversible pro­ cesses, sensitivities of the order of 1 0-8 mol L- I can b e achieved with SWP and peak resolution is 40-50 mV. The peak height and hence the sensitivity decreases rapidly with increasing irrevers­ ibility of the electrode process. 2.4.2 Square-wave voltammetry The characteristic feature of the technique described by Osteryoung [ 1 8) and known as square­ wave voltammetry, SWV, is that the whole measurem ent process can be carried out using a very rapid voltage scan rate. The method thus enables very short analysis times to be achieved and is

The product RC of a circuit comprising a resistance, R, and a capacitance, C, in series is known as t h e t i me constant of that circuit, t,. S ince after the potential is stepped in such a circ u i t ( will decay i n an exponential manner ( equa­ tion 2.34), and after 12 t, seconds it will be only 6 x 1 0-4% of ils initial value. The time constant of a typical cell with an electrolyte concentration of 1 mol L-1 and a mercury drop work ing electrode is - 40 IJS so that 12 t, is reached in - 0.5 ms, which i s much less than the duration of the p ulses used in practice.

40

- �- -

Introduction to Voltam metric Analysis

-

i

�E

�J p i

1

t

i

I

� � tm

Figure 2 . 1 2

a

b

c

iF

I

I

I

I

I

I

I

c

I

�! !+---

�! !+---

�! �

tm

tm

tm

Alternating square-wave voltage and current transients in square-wave polarography: (a) waveform with pulse tilt, (b) capacitive current, (, (c) faradaic current iF . � measuring interval. =

X A6

- E

Fig ure 2 . 1 3

Square-wave polarography. The dependence of the peak height on I'>.E,q, the amplitude of the square-wave voltage, and on n, the number of electrons involved in the electrode reaction. (1 )-(3) n = 1 (thallium); (4)-(6) n 3 (bismuth); (1 ) and (4) M,q 40 m V; (2) and (5) M,q 20 mV; (3) and (6) l'>. f,q 1 0 mV. =

=

=

=

Techniques

41

Meas u ring point 1

_j

_ _

� Esq

-1--T �I 1

I

1+--

I

tstep

Figure 2 . 1 4

Meas u ri n g po i nt 2

Potential-time profile and current sampling program i n square-wave voltammetry a s proposed by Osteryoung [1 8] . The duration of each square-wave cycle, �. is the same as that of each voltage step of the stai rcase waveform ( -) . �Este = voltage step of the staircase waveform, p �Esq amp l i t u de of the supe r imposed square-wave voltage ( -). The current is measured at ti me s 1 and 2 . =

particularly important as a detection method i n flow systems. In this technique, the potential applied to the working electrode consists of a relatively large rectangular wave of amplitude .1.E,q = -50 mV being superimposed onto a stepped voltage ramp (staircase) with voltage steps of -10 mV ( for the case of a cathodic scan ) . The duration, t,q, of the rectangular wave cycle is equal to that of the staircase voltage steps and is usually within the range of 5-1 0 ms. figure 2. 1 4 illus­ trates the voltage-time profile applied to the electrode and the current sampling times within this profile. Two values of current are measured in each rectangular pulse cycle; one at point 1 at the positive end of the pulse and one at point 2 at the negative end of the pulse. The difference between the two current values, Lli ( i2 - i1 ) , is plotted against the potential of the working elec­ trode and gives a peak shaped current as in SWP. The pulse times being in the millisecond range enable the potential to be scanned very rapidly at rates up to 1 200 m V s- 1 • Only one mercury drop is required for each measurement sequence. Because of the rapidity of the measurement, irrevers­ ible processes do not give a significant current signal. This enables measurements to be made in the presence of oxygen. This is of particular advantage when making rapid measurements in flow­ ing systems as it is often difficult to remove oxygen from such systems. However the rapid analy­ ses are achieved at the expense of sensitivity, since the use of short pulse times results in a lower iF to i, ratio. An alternative version of square-wave polarography developed for digitally controlled instru­ ments is based on Barker's SWP. The linear d.c. potential ramp is replaced by a d.c. staircase potential and a SMDE replaces the DME. (If the DME is replaced by an HMDE or a solid elec­ trode the technique is square wave voltammetry. ) The drop life is synchronised with each step in the staircase potential. The constant potential of each voltage step is modulated with a small amplitude, L1E,q, alternating square-wave voltage of frequency f.q, toward the end of the step as shown in Figure 2. 1 5 [ 1 2 ] . The current, in I ' inz• is measured over a number of cycles, n, twice in each cycle for a short period, tm P t012, just prior to each polarity change of the square-wave voltage cycle. The average of the differences between in1 and i112 for each measurement cycle: =

42

Introduction to Voltammetric Analysis

I

A o Ill G) li G) u

1

I

I

_. I tm 1 +-

a.. 0 ... "D � ::s

I

I

a

�::1 !::!G)

0 E

I

1

_. I

Meas u ring point 1

Drop life

I 1 +I

t

E

b

Sqm

c

Figure 2 . 1 5

Digitally control led square-wave polarography using the static mercury drop electrode. A number of square-wave cycles is superimposed on each step of the staircase waveform towards the end of the voltage step. (a) The variation of the mercury drop surface area with time. (b) The voltage applied to the electrode as a function of time. (c) The resulting cu rrent-potential curve. � Estep = voltage step of the staircase waveform; � fsq amplitude of the alternating square-wave voltage modulation, sqm, n cycles of frequency fsq; tstep duration of each step (same as the drop life, to); �,. tm 2 , etc, = current measuri ng intervals (only two labeled); (adapted from [1 2]). =

=

(2.36)

is plotted against t he potential of the working electrode to give a peak as in SWP. W ith digital con­ trol, the parameters can be v a r ie d over a wide range of values. Typical settings are: t,tep t0 0.3 s; �Estep = 6 mV; �Esq = 20 mV; .fsq = 50 Hz; n = 4; tm 1 tm2 2 ms. These settings give a voltage scan =

=

=

=

Techn iques

43

20 mV s- 1 , but m u ch higher scan rates up to 1 000 mV s- 1 can read ily be programmed m a k in g the method ideal for fast voltamm etric detection in fl ow - thro u gh cells. Because of the high fr equen cy of t h e square wave mo d u l a t i on , the technique is only suitable for use with reversible systems. As w it h a.c. polaro gr ap hy ( § 2.5) i rreversi b l e electrode processes are unable to k e e p up with the voltage changes of a 50 Hz cycle (a voltage s tep every 10 ms) . The square wave modulation of the pote n t i al has virtually no effect on the concentration pro fil es established by the d.c. potential at the el ect ro d e interface for an irreversibly r e d uc e d sp e c i es a nd so tli - 0. Ad v an tage can be taken of this fe at u r e when a sample contains two electroactive s p ec i es in its ma t r ix which are reduced at similar p o te n t ia ls . If the e l e c t ro de reaction of one of the species is reve rsi b l e and that of the other irreversible, then the former can be rea d il y detected in the pres­ ence of the latter which gives no signal in SWV. The state of development and ca p a bilitie s of SWP and SWV have been well reviewed [ 1 9, 20 ) . rate of

2.4.3 Normal pu lse polarography and vo ltammet ry In normal pulse polarography, NPP, the vo l tage of the workin g electrode is c h a n ge d, not by me a n s of a d.c. ram p or staircase vo ltage as in SWP, b ut by a seri es of rectangular pulses of continuously increas ing magnitude superimposed on an i nit i a l c on sta n t vo ltage as sh own in F igur e 2. 1 6b. The application of each p u l se is synchronised with the drop-life of the SMDE and o cc u r s just before the drop is dislo dg e d . Only one voltag e pulse with a duration o f 3 0-60 ms i s a pplied to e ach drop. The am plitude, �EP'' increases from one p u l s e to t h e next up to a maximum of about 1 000 mV. To minimise i,, th e current measurement is made in the i n t e r v al tm at about 10 ms b efo re the completion of the p uls e ( F i gure 2. 1 6a). N ote : �EP1 i s negative for a cathodic and positive for an anodic process. The measure d current is plotted again s t the p u lse am p l i t u d e and the r e s u ltin g i-E plo t ( Figure 2. 16c) i s a wave similar to that obtained in sampled po l aro gra p hy ( F i gure 2. 1 b ) . The height of t he wave is proportional to the analyte concentration, a n d th e h a lf- wave p o t e nti a l c o rre spon d s to that for DCP. Peak - sha p e d curves are o b tain ed when the difference in current between s ucc e ssiv e pulses is p lotte d ag a in st potential. A favourable feature of the normal p ul se te c h n ique is that it can be u sed with solid electrodes, that is it b e c om e s normal pulse voltammetry, without th e p ro b l ems that arise from the ac cu m u latio n of e lec tro d e reac tion p ro d u cts on such electrodes. This is because the electrode pot ent i a l is returned to an initial valu e which is in th e range where t he reverse el e c tro de reaction occurs, so that any electrode re a cti on p roduc ts acc u m u l a ted d u rin g the short pulse will be cleaned from the electrode when it is at the initial pot e nt i a l for the tim e between pulses. S en sitivi t i es are reported down t o 1 o-7 mol L- J and wave resolution is of the or der of 1 00 mV. The s ens i t ivi ty is i m pro ved and the time of ana l ys i s decreased by u si n g a SMDE in place of th e DME. 2.4.4 Differential pu lse polarography a n d voltammetry The most im po rta n t and sensitive of the puls e p ola r ogr a p h i c methods is differential pulse polarog­ raphy, DPP. In this t e chn ique , a linear ramp ( older i n st r u m ents ) or staircase ( m o de rn instru ­ ments) voltage rise is used ( F igure 2 . 1 7b) an d a rec tan g ul ar vol t a ge p u l s e with a co n s tant amplitude, tlEpl> of 1 0- 1 00 mV is ap p l i ed to each mercury drop (DME or SMDE) j us t prior to the end of its life, tD. T h e pulse d u ra t ion , tpi• is in the range 40-60 ms and drop times of 2-6 s are used. Synchronisation between the d ro p life and time of application of the pulse is a ch i eve d by means of the electronic c ircui try. If an HMDE or a solid el e c t ro d e is used instead of the DME then the te ch ­ nique is differential pulse voltarnmetry, DPV. In DPP, two current measurements are m ade : the first, i" in the measu rement interval, t m P immediately before the a p p l i ca t io n of the vol t age p ul s e and the second, iz in th e interval tm2 n e ar the end of the pulse ( F ig u r e 2. 1 7 ) . Both m eas u reme n t s are made on the same m erc ury drop.

44

I ntroduction to Voltammetric Analysis

tm

I �I 1 I

A

I �I I

Drop life

I 1 +I I

a

I I 1 +I

E

t

f

L

Starting potential

b

6 EP1

t

c

-E

F i g ure 2 . 1 6

Cu rrent sampling progra m (a) and potential-time profile (b) for normal pu lse polarography using a S M DE. Resulting normal pu lse polarogram (c) . 6fP1 = pulse amplitude; tP 1 = pulse duration; tm current sampling interval .

=

( Because of the small pulse duration these are a t almost the same electrode area when using a DME, thus minimising charging current contributions from this source . ) In DPP with the SMDE and DPV with the HMDE the surface area is the same for both measurements. ln both DPP and DPV, the difference, 11i i2 - il' is plotted against the applied d.c. voltage ( ramp or staircase) and gives a peak-shaped polarogram/voltammogram ( Figure 2. 1 7c) as !1i reaches a maximum in the region of R 12• 1 For a reversible process, the peak current is proportional to the an alyte concentration and is given by =

( 2 . 37 )

and the potential at which the peak occurs is (2. 3 8 )

Techn iques tm1

tm2

-+l :.--- : : : : -+j �

A o a.. ca

45

o

I!! ca "C Q) � u :II

a

...

� e m E :II

Q)

'

-+ ;

D rop

l ife

:+' '

f---� - - - - - -

t

b

di

c

Figure 2. 1 7

Drop surface area-time plot and cu rrent meas u ring program (a) potential-time profile for differential pulse polarography (b) using a SMDE and the resulting differential pulse polarogram (b) using a SMDE and the resulting differential pu lse polarogram (c) tm1 and tm2 cu rrent sampling i ntervals; tP1 = pulse d urati on ; tstep staircase voltage step duration ( drop life time); dfstep voltage step of the staircase waveform and the d£P1 = pulse amplitude; EP peak potential . =

=

=

=

=

The peak current also de p e nds on the amplitude o f the voltage pulse, �B i • ( F igure 2. 1 8 ) and p pulse duration, tri· However, not only does the p ea k height increase with the p ulse a mp l i tude so a lso does the peak wi d th , thus decreasing the resolution which is undesirable. For small values of �EP1, the peak half-width, th at is the width at i i/2, is 90.4/ n mV but for larger p ul s e amplitudes it approaches the pu lse amplitude, � EP1• The peak p ote n tia l is also dependent on the pulse amplitude, being sh ifted in a positive direction for a reduc t i o n process (�£ 1 i s n e g a t iv e for 11 a reduction). on the

­

=

46

Introduction to Voltam metric Analysis

I

Cd

50 nA

2 1

- 0.3 - 0 .4 Figure 2 . 1 8

- 0 .5 - 0 . 6 - 0 . 7 - 0. 8

E [V] (vs. Ag/AgCI , 3 M KCI) I nfluence of the pulse ampl itude o n the differentia l pu lse polarograms of Pb(l l) and Cd(ll) 1 0 mV, (2) MP 1 -2 5 mV and (1 0 mg L-1 ) i n acetic acid solution at pH 2 .8. (1 ) MP 1

(3) MP1

= -

=

=

-75 mV.

Determinations by DPP or DPV are in general ten times more sensitive than determinations with NPP with the limit of determination for the differential pulse methods being about 1 0-8 mol L- I for a reversible process. For an irreversible process it falls off only slightly to 5 x 1 0-8 mol L- 1 • The loss of sensitivity due to irreversibility is thus smaller for differential pulse methods than it is for other pulse methods. On the other hand, catalytic and other complications of the electrode process which lead to non-linear iP versus c. plots in DCP have a similar effect in both the normal and differential pulse methods. A review of developments in p ulse voltammetry is given in reference 2 1 .

2 . 5 ALTERNATING CURRENT POLAROGRAPHY AND VOLTAMMETRY When the slow linear potential scan ( analogue instruments) or staircase potential ( digital instruments) applied to a SDME or DME is modulated with a small amplitude sinusoidal voltage (!'.E_ = 5-2 0 mV) of low frequency ( 5-500 Hz) then the technique is called alternating wrrent polarography, ACP. After filtering out the d.c. component of the current, the alternating current, i_, is plotted against the d.c. voltage, £d e> to give the peak-shaped a.c. polarogram as shown in Figure 2 . 1 9. Figure 2 . 1 9 shows diagrammatically why the p l o t of i_ versus Edc is peak- shaped with a maximum value ( iJ P at £ 1 12, the d.c. half-wave potential. ( With a DME the peak will have the oscillation s due to the drop growth and fal l . ) In digital instruments, /'.E_ is applied towards the end of each voltage step ( Figure 2 . 20 ) . I n a.c. polarography, the concentration profile at the electrode surface established b y the d.c. potential is modulated by the small a.c. voltage. For a simple reaction the surface concentrations of the reduced and oxidised forms of an an alyte are the sam e at £ 1 12 • These concentrations fluctu­ ate in response to the a.c. voltage modulation-in the positive half-cycle the oxidised form increases and in the negative h alf-cycle it is the reduced form which increases. When Edc £1 12, =

Techniques

47

i_

i_

- Ed. c. Figure 2 . 1 9

Waveform a nd response in a.c. polarography. !::!. £_ peak t o peak ampl itude o f t h e a .c. modulation voltage; i_ a.c. current; EP_ d.c. potentia l of the a . c. current peak; b 1 12 peak half-width {when ;_ = V2).

these concentration changes are gre a t e st so t h a t i_ ;which refle c ts these c h ange s , has i ts m aximum

value at this p o t en t i al .

If a so lid electrode or HMDE is u s ed instead of the DME, the resultant a.c. vo lt am mogra m has essentially the same s h a p e as the a.c. p o l aro gra m obtained with the DME but without the oscilla­ tions due to drop gr o wt h and fall. An alternative way of prese n ti n g the information that may be obtained by mo dula t in g the d.c. potential wi t h a small a.c. vo lt age is to plot th e cell i mp e d a n ce as a function of p o t e n t ial . This latter approach is val uable when i nfo r m atio n about the kinetics of the electrode process is being sought b u t as it i s experimentally tedious it is not used a s an an a lyt ­ ical tool. Very good accounts of ACP wh i ch give a full discussion o f this and related t e c hn i qu es are presented el sewh e r e [ 22 , 23, 24, 25 ] . Because an electrochemical cell is a c o m p l e x, non -linear component in an electric circuit, the current response obtained in ACP has, in ad dit i on to the fundamental harmonic current, iw of th e same frequency, m, as the a pp l i e d a . c . v o lt a ge , c o m po n e n t s at fre q u e n c i es that are integral multi­ ples of the fundamental fr e qu e n cy ( i2ro, i3ro, ) as well as at zero fre q u e n cy (d. c. current) . These • • •

48

I ntroduction to Voltammetric Analysis

-.I 1 I I

A

tm

I 1+-

I I

a

-.:

'

D rop life

t

: +I

Meas u ring

E

I

Starting

- - - - - -

potential

I

I I I

+-

p e riod

b

�Estep

- - ·

t

c

Figure 2 . 20

- E dc Variation of electrode su rface area with time and current measuring program (a), potential-time profile (b ) and the resulting a.c. polarogram (c) for a.c. polarography using a staircase voltage with the SMDE. � = current sampling period, acm a .c. voltage modulation with a peak to peak potential of b.E_; � Estep and t,1•P the voltage step and step duration of the staircase voltage; EP_ d. c. potential of the a .c . polarographic peak. =

=

=

components c a n be s e p a r ated by mean s of tuned ci rcuits in the measurin g equipment. Funda­ menta l harmonic ACP is the most common techni que used, although phase selective second har­ monic ( i20) ACP is gaining wider use as suitable instrumentation becomes m o re re ad i l y available. The theory of ACP [ 2 3 ] is d eveloped usi ng the same a pp r o a c h as for DCP with the m odifica­ t io n that the potential ap pl i e d to the working electrode contains an a.c. term:

(2 .39) where

oo

is the frequen cy of the a.c. component of the applied voltage in rad s- 1 ( oo = 2rcfwhere fis expressed in Hz { cycle s-1 } ) .

Tech n iques

49

reversible electrode process, the peak po t ent i a l of t h e fundamental harmonic a.c. polarogra m is identical with th e h a lf- wave p o t en t i a l of the d. c. p ol a r o gr a m ( EP = I! 1 2 ) . The fu n 1 damental harmonic peak current, ( ioo) , is give n by r 4 1 : p For a

­

( 2 .40) where

the area of the e l ect ro de , D. is the diffusion coefficient of the analyte, and the other symbols h ave their usual meaning. A is

The width of the peak at i00 = ( i00 )/ 2, the peak half-width, b 1 12, is 90/ n mV at 25°C when m V a n d a plot of ( i,)p ve r s u s w 112 is linear th ro ugh the origin. These are tw o of the c r i te ria wh i c h characterise a reversible electrode process. Since b 12 90/ n mV a n d ( i00) P i s propor­ 1 tional t o n2, the more electrons involved in t h e electrode reaction, the higher and sharper is the peak in the ACP o r ACV. The continual ch a rgi n g and discharging of the do ub le l ayer b y th e a p pl ied a l tern at ing vo l t a ge results in a high c apa c it i ve c urrent component i_, in ACP. This l i m i t s t h e se n si t i vi t y of t h e tech­ nique to ab o ut 1 0-5 mo l L-1 • F ro m equation 2.40 it is seen that increasing the frequency w i ll increase the m agn it ud e of ( i00)P s ince the l at t er increases with w 112 • It m i gh t be expected then that increasing the fre qu e n cy would increase the se n s i t iv i ty of A C P . Unfortunately, the chargi ng cur­ rent i_c, wh ich is given by

dE_ < 8/ n

­

=

( 2 .4 1 )

a faster rate wit h w ( to the first power) t han does the faradaic current ( i00) P ( to the half so that the faradaic to charging current ratio decreases with increasing frequen cy, leadin g t o a decr eas e i n sen si t ivity. It i s for this reason that in pr a ctice ACP measurements are usually made at frequencies of about 5 0 Hz ( w 3 14. 1 6 ra d s I ) The se n si tivity can be improved by u sing one of two variations on the basic ACP t ec h n i q u e . In the first, phase selective a. c. polarography, PSACP, a dva n t age is taken of the fact that when an alter­ nating cu r re n t flows t hro u gh a res i s to r , the alternating vo l t age drop across the resistor ( E_ = i_R) is in phase with the current. However, when a n al t e r n at i ng current flows through a c a pa c i to r , E_ is no l on ge r in phase with i_ but lags behind i_ by 90° (rt/2 radian) . The d i ffe r e n c e in phase between the c u r r e n t and voltage i n a n a.c. circuit is called t h e phase angle, 8 (90° i n the above case). In an el ectro c h e m i c al cel l the fa ra d ai c curren·t, i_, is found to lead the a . c . vo lt ag e by 45° (1tl4 radian) or less, while the cha rgi ng current, i_c, leads AE_ by 90°. By using phase-selective detection of th e current and s el ect in g only the i n - p h as e (8 = 0) c om p on e n t of (, the c a p a c i t ive com p on ent b ec omes i nsi g nifican t while the faradaic component is only re du ced at most by a factor of 1 1,[2 . The net result is a m a rk e d increase in sensitivity over the b a s i c ACP by some two , orders of mag ni t u d e to about 1 o-7 mol L_ _ The se c ond va ri a t i on is to use t h e p h a se sel ective s eco n d harmonic a.c. r es po n s e . The m agni ­ tude of the second h a rm o n ic alternating current, i2 w is given by [ 23 ) :

increases at

power),

-

=

i zw

{ =

n3 F A c. ( 2 wD. )

l l2

A £ sinh

.

G) (

sin 2 w t -

�)} ( 2 .42 )

50

Introduction

to

Voltammetric Analysis a

b

t

ip -

� l p :;C I .

Figure

2.2 1

a

l p

.,

l

p

Cu rrent-potential profile and eva l uat io n of phase selective second harmonic a.c. polarograms; (a) for horizontal and ( b) sloping residual currents.

j ( nFI R ] ) ( Edc - E'1 12 ) and the o th er sym b ols have the same s ignifi ca n c e as in e qu a tion 2.40.

where For

.,

I

=

reversi b le process there arc two peaks in th e s e c ond h a r m o nic p o l aro gra m which o cc u r at 34/ n mY and the peak current is given by

E ' 1 12 ±

. ( lzwl p

=

3 ,.,3

112

2

{ n t' A c. ( 2 ro D. ) L1 E_ } 2 ....2 41 .57 R 1

( 2 .43 )

In p h a se selective second harmonic ACP it is t h e total peak current, il'l' = ( i + + iPJ , that is p u s ed for a n a lyt i c a l purposes ( Figure 2 . 2 1 ) . L a rger a m p l itu d es of L1 E_ in the range 25-30 mY, are used because of the de pende nce of ( i2ro)P on L1E_ 2 • Unfortunately th e gain in sensitivity is to s o m e extent countered by t h e peak broadening associated with the l ar ge r values of � E_ . The use of the s ec o n d instead of t h e fundamental harmonic further reduces i_, so that the main s o u rce of noise in th e signal output is often instrumental in o r i gi n . A lt h o ugh ( �o) is m uch smaller than ( iro)p, the p iF to ic rat i o is very favourable a n d the detection limit is lowered to the range 1 0- R to 1 0-7 mo l L- I . Detailed analysis o f the case for a quasi- reversible charge t ran s fer reaction, that is a reaction which is reversible in DCP b u t not in ACP corresponding to reactions wit h k,ed val ues ( e q u atio n 1 .3 ) i n the range 1 0- 2 to 1 em s- 1 , shows that fo r the fundamental harmonic p eak current [ 23 ] :

( iro)p C irolp.rcf( a, kred' Da , ro ) , wh e r e C irol p,re• i s t he value of ( iw)r for the reversible case (equation 2 . 40 ) and F( a, k,ed, D., ro) is a complex function of the four p ara m e t e rs , is less than u n ity and decreases as k,.d decreases; (ii) the p e ak p otenti a l va ri e s wi th ro fo r all values of a other t h an a = 0.5; ( iii) ( i0,) P is no lo nge r a l i ne a r function of ro 1 12 ; and ( iv) the peak is b ro aden e d and no longer symmetrical except when a = 0 . 5 .

(i)

=

Slight changes i n solution composition s uc h as changing the c o ncentr at io n of the supporting electrolyte or the presence of tr a c e s of organic surfactants may have an effect on k,.d an d a, wi th a c o n seq u e n t marked effect on the s h a p e and height of the a. c. p e ak . Therefore the a p p lic atio n of a.c. methods to the determination of a nalyte s whose electrode reactions a re q u asi - r ever s i b l e must be undertaken wi th ca ution. This, when c o n s i d e red with the lower s e n s it ivi ty obtained with

Techniques

51

qu as i revers i b l e systems, means that ACP and ACV are not r e c o m me nde d fo r d e t erm i n in g th e conce n tra tion of anal yt es whose electrode reactions are quasi-reversible. For those a n a lytes whose el e ct ro d e reactions are irreversible, DCP or other te ch n iq ue s should be used sin ce the ACP or ACV s i gn a l is so small that it is of no a n al yt ica l use. This s e le c t ivity with a.c. m ethod s is s i m i l ar to t h at of SWV a nd may be exp lo ited in practice. An a nalyte which is reve rs ib ly reduced or oxidised can be determined in the presence of a large excess of ano t he r com­ ponent in the test s ol uti o n which i s involved in an irreversible process a t the p ot e n t ia l at wh i c h the analyte re a ct s since the anal yt e giv es a good an a lyt i cal s i gn a l in ACP while t h e i r r evers ib ly reduced component giv e s virtually no a . c . current at a l l . The i n fl u e n ce of oxygen (irreversibly reduced) in the c e l l solution is n owhere ne a r as c rit i cal as i t is in DCP an d DCV a l th o u gh for b e s t results it i s still recommended that so l u ti o n s be deaerated p r i or to making the a.c. measurement. The current in ACP and ACV is more strongly i n fl u e n ce d by t he components of the t e st solu­ tion such as the s u p p o r ti n g ele c t ro lyt e solvent and surfactants, than is the case in the other polarographic metho ds. The effect of s urfactants becomes m o re noticeable as th e number of elec­ trons i nvo l ve d in the electrode reaction increases. For exa m pl e even the minute traces of surfa c tants that are present in de i o ni s e d water can interfere with t h e determination of Bi(lll ) , ln(III) and Sb ( I II ) . -

,

,

,

­

-

2.5.1 Tensa m m etry Tensammetry is a vari an t of ACP (or ACV if a fixed area el e c t ro d e is used ) . In this t e ch n iq ue [ 24 ] , the cap ac it ive current component, i_c, whose value is d et er m ine d by the double l aye r cap a c i t ance (equation 2.4 1 ) is measured ( preferably at a phase angle of 90° ) i nste a d of the faradaic a l ter n a ti n g current, i.p The ad so r p t io n or d e sorp t i o n of s u rfacta nt molecules at the electrode interface causes changes in the d ouble l ayer c apa c i tance and le ad s to two almost sym m et ri ca l c u r re n t p e aks in th e a.c. p o l ar og r a m at th e potentials where the c omp o u nd is adsorbed and desorbed. S uch peaks are referred to as tensam metric peaks and the co mp o u n d is a ds o rbe d in the p o t e n t i a l re g io n between the peaks. An a dso rb e d surfactant m o l e c u l e replaces the small s o l ve n t molecules on t h e surface thereby i ncreasing the width of the do u b l e l aye r and lowering the double l aye r capacitance. As a result, at p ote n t i al s where the c o mp o u nd is adsorbed t h e ca p a cit ive or base cu rre n t is s m al ler than the base current i n a s o l u ti on of the s up p o r ti n g el e ct ro lyt e a lo n e ( F igur e 2.22 ) . A t concentrations o f s u rfac t an t b el ow th at re qu i r e d t o produce a m o n o l ayer on t h e surface, the he ight o f t h e p e a ks d e c re as e s wi t h d e cr e as i n g concentration and the pe aks move closer together ( E+ more negative and E_ m o re positive ) . The depr e s s i o n of the base c u rrent A4,asc> also decreases with de cr ea si ng concentration of t h e surfactant. In favou ra b l e cases, A4,asc may be us ed to d ete rm in e the concentration of the s u r fa cta n t [ 26 ] . The s hift in the p o t entia l of a peak in an a.c. polarogram wi t h the co n cen t r at i on of surfactant in dic � te s that it is a tensammetric peak and i s one of the features t ha t d i stin g u i s h it from a fa r ad a i c peak, the p o te n t i a l of which is independent of analyte concentration. Ten sa mm et r i c pe a ks are obtained in the a.c. p o l a r o gr a m s of both electroactivc and e le c t ro i n active s u rfa c tan t s Th ree main types of peaks are ob s er ved : ( i) those caused by the a dso rp t i o n or desorption of a n electroinactive species, ( ii ) t h o s e in which an ad s or b e d species with an asym m et ­ ric sh a pe u ndergoe s a re or ie n t a ti on on the surface, and (iii) t h o se in which the s orp t ion process is ac co m p a n ie d by an e l e ct r ode reaction of the surfactant, t h at is they ar e mixed tensammetric and faradaic peaks. These can be distinguished by s t u dyi n g the d.c. polarogram obt a i ne d in the ,

­

.

-

same solution.

When t h ere is no wave i n the d.c. polarogram at the p o te nt i a l of the a. c. p o l a ro g raphi c peak, the a.c. p eak is p urely tensammetric and arises either from the a d s o rptio n or desorption of a species (base l i ne depressed on one side of the p e ak) or fr o m the re-orientation of an adsorbed species (base line depressed on b o t h sides of the peak), which is not clc c t roa c t iv e at that po t e ntial . On the o t h e r hand, when a peak in the a.c. p o l a ro g r a m occurs at the same pot e n t i a l as a wave in

52

I ntroduction to

Vo lta m m etric

Ana lysis

E

�

I�

Potential rahge of adsorption -E

0

Figure 2 . 2 2

[V]

- 1 .0

Tensammogram: a.c. polarogram o f the supporting electrolyte alone (-----) a n d after t h e addition of a surfactant showing the resultant adsorption-desorption or tensammetric peaks and the depression of the basel ine current over the potential region where the surfactant is adsorbed. i_, capacitive current; E+ the anodic and E_ the cathodic potential at which the surfactant is adsorbed or desorbed.

=

=

=

the d.c. pola rogra m and ( i ) th e base current in the a.c. pola rogram is de p r essed on only on e side o f t he pe a k then a c h a rge transfer reaction is ass o c i a te d wit h t h e s o rpti o n process; (ii) there is no d ep r ess i o n of the base c urre nt in t h e a.c. polarogram, th e n there is no s orp t i o n process occurring and the peak is pure l y faradaic in n at ure and (iii) th e base current is depressed on both sides of th e p e a k , t h e n t h e peak i s faradaic and both t h e reduced and oxidised forms of the surfactant are adsorbed. An alyte s for which the electrode process is irreversible and hence are n ot expe c ted to give any s i g n i fi c an t fa ra d a i c p e a k in th e a.c. p olarogram, may gi v e rise to a peak when the irreversible charge transfer process is accompanied by the adsorption of either the oxidi s ed or reduced form of the analyte. The plot of ( U p ve r su s c. has t he c h a r a cte ri sti c s h ap e o f a n a d s o rp t ion isotherm wit h a l i m i t i ng value of ( j_) r wh en the adsorbate reaches a m o n ol aye r coverage of the electrode surface. Such systems are useful for analytical p u rp oses at analyte concentrations wh e r e the ( i_ ) P v ers u s c, plot i s approximately li n e a r th at i s at concentrations below t h a t re qu i re d t o gi ve approx­ i m a t e ly half-monolayer coverage of th e adsorbate on the e l e ctro de Th e m a in app l i c at i on of t e n s am m e t ry is to the characterisation of adsorption phenomena which may i n ter fe re with the vo l ta mm et ri c determination of an a n a lyt e (§ 1 .4) an d fo r the analy­ sis of some el ectro inactive su rfa c tan ts The detection limit in these cases is abou t 1 0-6 mol L- 1 and is dep en d e n t on drop time. The sensitivi ty can be improved by u sing a stat i on a ry e l ec t r o de . ,

,

,

.

.

2.6 (HRONO POTENTIOM ETRY In voltammetry, th e current flowing t h rou g h the working e le c tro de is m e asu r e d as a function of the a p pl ie d vo lt a ge and the rate at which that voltage varies with ti m e . In chronopotentiometry, CP, the cu r re n t is the e lectri ca l variable that is c o n t ro l l e d by m e a ns of a galvanostat wh i l e the variation of the w o rki ng e l ectro d e p ot e nti a l wi th t i m e i s m o n it o r e d C h r o nop o t e nti o metry has also been referred to as galvanostatic voltammetry an d therefore can be groupe d with the m ethods d es cri b e d in this chapter. Th e t h e ore ti c al basis of chronopotentiometry was developed ea r ly in the 20th .

Tech niques

53

Constant current source (Galvanostat)

Working electrode

Recorder

Figure

2.23

century

Simple experimental arrangement for chronopotentiometric measurements.

[ 27, 28] but it was not until the late forties that Ruis [ 29 ] used it as an a nalytical tool and

that Gierst and Juliard [ 30 ] showed it to be a useful procedure for studying electrode kinetics.

When a controlled current is passed through a cell between the working and auxiliary elec­ trodes, initially the potential of the working electrode wi ll change rapidly as charge builds up a t the electrode-solution interface until a potential is reached at which t h e electrolysis of a n electro­ active component of th e solution (e.g. the analyte) begins. I f the analyte is present in solution in its oxidised form, aox' then with the onset of electrolysis it will be reduced and its concentration at

the electrode surface, ( c,) ox, will decrease. At the same time electrode reaction products will form an d ( c,\ect will increase from its initial value of zero . This will cause the potential of the electrode to change, but at a relatively slow rate u ntil ( cJ ox becomes effectively zero. At this point th e poten­ tial of the working electrode will again change rapidly until it reaches a value where the electrolysis of another component of the cell solution, for example a second analyte or an ion of the support­ ing electrolyte, commences. The basic experimental arrangement for obtaining chronopotentio­ grams is shown in Figure 2.23. When the current is controlled at a constant value by means of a galvanostat, the potential of the working electrode is found to vary with time as shown in Figure 2.24. The time between the steeply rising portions of the potential-time plot, that is the period over which ( c.)ox changes from ( c. )ox to zero, is called the transition time, 't , which for a diffusion controlled process is given by the Sand equation [ 27 ] : 't

where

l /2

1/?

=

{ n FA ( 7t D0x) - c c. ) ox }

( 2.44)

2i

i is the constant current in amperes, ( c.)ux is the cell concentration o f the analyte in mol em

Dox

is the diffusion coefficient of the analyte in cm2 A is the electrode area in cm 2 •

s-1 ,

\

and

54

Introduction to Voltammetric Ana lysis

E

Time Figure 2 . 24

Chronopotentiometric (potential-time) curve for a reversible electrode process . time; Ett4 potential when t t/4. =

=

t

= transition

When the electrode process is reversible ( equation 2. 1 0 } , the surface concentrations of a0, and { ( c,)ox and ( c,) '"..J will control the potential of the working electrode in accordance with the Nernst equation ( equation 2. 1 1 ) . Expressing ( c, ) 0, and ( c,) ,.d in terms of time using equation 2 .44 [ 5 , 2 8 ] leads to the equation which describes the shape of the reversible chronopotentiogram obtained at constant current ( Figure 2 .24), namely: a,.d

(2.45)

where

E,,4

=

E

o

oxtre d

+

RT ln nF

{ (D"d)1 '2 (fox )} Dox

(2 .46)

J;cd

is the potential of the working electrode when t = 't/4 and is identical wi th the reversible polaro­ graphic half-wave potential I! 1 2• 1 Equation 2.44 shows that the square root of the transition time, which is proportional to the concentration of the analyte, is the analytically significant parameter. From equation 2 .45 it is seen that a plot of E.vE versus ln{ (t/ t) 1 1 2 - 1 } will be a straight line with a slope of 59.2/n mV at 25°C for a reversible electrode process. The shape of the chronopotentiogram for an irreversible process is determined by the rate of the charge transfer reaction. In this case equilibrium between aox and a,cd on the electrode surface cannot be maintained and so the Nernst equation no longer applies. Instead the rate expressions (equations 1 .3 and 1 .4 ) are used to obtain the relationship between E.., E and time, namely: EwE

=

Eo oxtrcd

+

RT ln anF

{nFA(c.)0,k,cd } i

.

+

RT in anF

{(•)1 12 1 } t

-

( 2 .47)

Techniques

55

In this case, a plot of Ewr versus ln { ( 't/ t) 1 12 - l } enables the transfer coefficient, a, and the rate constant, kred• to be evaluated from the slope and intercept, respectively. As is the case for d.c. polarography, an irreversible wave is shifted to more negative potentials for a reduction and more positive potentials for an oxidation, and is more drawn out along the potential axis than is a reversible wave. Eventually at a sufficiently high potential, the electrode reaction becomes fast enough for diffusion control to be achieved. This means that the transition time, 't, like the limit­ ing diffusion current, id, in DCP, is not affected by the degree of reversibility of the electrode process. On the other hand, when two analytes are present in solution the transition time of the second analyte is enhanced by the presence of the first analyte. This arises because when the second ana­ lyte is being reduced, the first analyte is still diffusing to the electrode surface and being reduced, so that only part of the applied current is available to reduce the second analyte. This is in contrast to the case for DCP where the limiting currents of the two analytes are simply additive ( § 2 . 1 . 5 ) . The transition time of the second analyte i s related t o i t s concentration b y the expression: ( t 1 + t 2 ) 1/2

_

l/2

t1

=

n

Z FA { ( D ) 1t

ox

'

�

} I/2 ( cJox2 . 2t

( 2.48)

where the subscripts l and 2 refer to the analyte that appears first and second, respectively, in the chronopotentiogram. If ( c.)ox l = ( c3)0x2; ( D0x) 1 = ( D.,.) 2 and n 1 n2 ; then t2 3'tp Thus the consumption of part of the current by Ox 1 when Ox is being reduced results in a marked increase in the transition time 2 for the second analyte over that which would be observed if it alone were in solution. This enhancement effect has been used to increase the sensitivity of CP analyses. If instead of a constant current a time-varying current is applied to the cell, equation 2.44 no longer describes the relationship between transition time and analyte concentration. For th e gen­ eral case of a steadily increasing current: =

=

( 2.49 )

where � is the current-time proportionality constant. The transition time is given by ,._('{ + •

1/2)

=

k

(y) Ca

( 2 . 50)

where k(y) is a constant which depends on y.

When y = 1 /2 , a linear relationship between the transition time and the analyte concentration is obtained [ 31 ] .

The main problem with chronopotentiometry has been the difficulty o f accurately measuring the transition times, particularly the short times obtained at low analyte concentrations. More accurate transition times may be obtained if the first derivative dE/dt or, better still, the second derivative, d 2 E!d r , is plotted as a function of time rather than plotting the potential of the elec­ trode directly against time [ 32 ] . The beginning and end of the transition arc indicated by peaks in the first derivative plot and by the times at which the second derivative passes through zero (crosses the time axis ) as it changes from a positive to a negative value. Figure 2.25 illustrates the advantage of the second derivative plot compared to the simple chronopotentiometric E versus t plot in the analysis of a mixture of copper, cadm ium and zinc. Because it is easy to electronically detect the time at which d2 E/d r changes from a positive to a negative value, the second derivative tech nique can be readily automated to just record the transition times. It is also the best way to

56

I ntroduction to Voltammetric Analysis

E -

[V]

1 8 .

- 1 .4 - 1 .0 - 0.6 - 0.2

0

------ Zero l i n e

I

2

3

1

1

Fig ure 2 . 2 5

:+-- 0.32 s �:I 0.,06� S 1

�0.26 s -.:

t [s]

4 Position of i nflection poi nts

N orma l and second derivative chronopotentiog rams for a m i xt u re of 1 x 1 o-3 mol l-1 Cu(ll), 1 x 1 0-4 mol l- 1 Cd ( l l ) and 5 x 1 0-4 mol l_ , Zn(ll) i n 0 . 5 m o l l- 1 KN03 recorded at a current density of 0.858 rnA cm-2 [32] .

a n a lys e t h e chronopotentiometric signal when s m a ll amounts o f an analyte are t o b e determined in the p rese nc e o f l arg e amounts of other electroactive species. C hro n opo ten ti o m etry , d i re c t or derivative, is not an i mp o rta nt a n a lyt ic a l t e c h n i que . Detec­ t i o n limits are in the range 1 0-5 to 1 0-4 mol L-1 for si m p l e c h ronopotetiometric analysis and about 1 0-7 mol L- 1 a t best for d er i v ative chronopotentiometric a n alys i s . However c h r o n o p o t e n ti om etry is i mportant as the basis of t h e very sensitive stripping t e c h n i q ue s , derivative and differential s tr i ppi n g c h ro n o p o tenti om e t ry and of po t e nt iom e tr i c stripping analysis, used in trace and ultra­ trace analysis (see ch apter 3 ) . Referen ces 1

2 3

4 5

6

7 8

L. Meites, Po la rograp h ic Tec h niques, 2nd edn, Interscience,

J. Heyrovsky and J. Kuta, Principles

New York, 1 96 1 . of Polarography, Academic Press, New York,

1 966. D. Ilkovic, Co llection of Czechoslovak Che m ica l Com munications, 1 934, 6, 498. A.J. Bard and L . R. Faulkner, Electrochemical Methods, Jo h n Wi ley, New York, 1 980. A . M . Bond, Modern Polarographic Methods in Analytical Chemis try, Marcel Dekker, New York, 1 91!0. l.M. Kolthoff and J.J. Lingane, Polarography, 2nd edn, ln tersci cncc, New York, 1 95 2 . J. Kuta and I. Smoler, Progress in Polarography, Vol. 1 , P. Zurnan and I . M . Kolthoff, editors, Interscicncc, New York, 1 962, p. 4 1 . J. Heyrovsky and D. Ilkovic, Collectio n of Czechoslovak Chemical Comnumica tions, 1 935, 7, 198.

Techniques 9 !0 11 12 13 14 15 16

17

19 20 21 22 23 24 18

25 26 27

28

29 30 31 32

57

) . Tomes, Collection of Czechoslovak Chemical Com m un im tions, 1 9 37 , 9 , 1 2. D .I. Sawyer and J.L. Roberts, Experimel'ltal Electrochemistry for Chemisls, Wiley, New Yor k , 1 9 7 4. E. Wahlin and A. Bresle, A c ta Chem ica Scarrdinavica, 1 956, 1 0 , 935. Metrohm VA Processor 693 Instruction manual, 1 995. ) . E . B . Randles, Faraday Society 1 ransactions, 1 9 4 8 , 4 4 , 3 2 7 . A. Sevcik, Collectiorr of Czech os l o va k Ch emical Commun ica tions, 1 948, 1 3 , 3 4 9 . P.T. Kissinger and W.R. Heineman, Laboratory 1 edmiques i n Hlectroa nalytical Chemistry, Marcel D e kke r, New York, 1 984. G . C . Barker and A.W. Gardner, Fresen ius Z Analytisclze Ch e m ie, 1 960, 1 73 , 79. G.C. Barker and l.L. Jenki ns, Analyst, 1 95 2 , 77, 685. J.G. Osteryoung and R.A. Ostcryoung, Analytical Chemistry, 1 985, 57, l O l A. G.C. l1arker and A .W. Gardner, Analyst, 1 9 8 2 , 1 1 7, 1 8 1 1 . E.}. Zuchowski, M . Wojciechowski and ). Osteryoung, Analytica Chim ica Acta, 1 986, 1 83, 47. ).G. Ostcryoung and M.M. Schrei ner, C.R. C. Critical Reviews i n Analytical Ch e m is t ry, Supp. 1 , 1 988, 19, 1 . B. B re yer and H.H. Bauer, Alternating Current Polarography and Jensammet1y, lnterscience, New York, 1 96 3 . D. E. Smith, Electroanalytical Chemistry, Vol. 1 , A . } . Bard, ed i to r, Marcel Dekker, New York, 1 966, p. 1 . M. Sluyters- Rehbach and J . H . Sluyters, Electrocmalytical Ch e m is t ry, Vol. 4, A.J. Ba rd, editor, Marcel Dekker, New York, 1 970, p. l . D. E. Smith, C R. C. Critical Reviews in Analytical Chem istry, 1 97 1 , 2, 247. S. Sander and G. Henze, Electroanalysis, 1 997, 9, 243 . H.J.S. Sand, Philosophical Magazine, 1 90 1 , I , 45. Z. Karaoglanoft� Z. Elektrochemie, 1 9 06 , 1 2 , 5 . A. Rius, J. Llopis a n d S . Polo, Annales de Ia Sociedad Espanola de Fisica y Quim ica, 1 949, 4 5 8 , 469 . L. G ierst and A.L. Juliard, ( a ) Proceedings of I he Second Mee t i ng of the In ternational Com mittee on Electrochemical Thermodynamics and Kinetics, Tamburini, Milan , 1 9 5 0 , p l 1 7, p279; ( b ) f. Physiwl Ch em ist ry, 1 9 53, 57, 70 1 .

G. Henze and R. Neeb, Fresenius Z. A n a ly tis ch e Chem ie, 1 98 2 , 3 1 0,

R.W. Murray and C.N. Reill ey, }. Electroanalytical Chemistry, 1 962, 3 , 64.

I l l.

3

Stri p p i n g a n a lysis

3 . 1 ELECTROCH EMICAL STRIPPING TECHN IQUES Electrochemi cal stripping techniques are among the most sensitive of instrumental analytical techniques. Because of their high sensitivity and selectivity they are important in trace analysis and speciation studies. With detection limits generally in the 1 0- 1 1 to 1 0-9 mol L- 1 range and in a few cases as low as 1 o-1 2 mol L- I , they are three or four orders of m agnitude more sensitive than the simple polarographic methods. The high sensitivity and selectivity of stripping techniques arises because two distinct controlled electrochemical steps are involved. In the first step the analyte, initially present in the test solution, is transferred to the electrode s urface in one of two ways namely: ( i ) by electrolytic oxidation or red uction which results in the analyte forming a deposit on the working electrode ( when the analyte is a metal ion, its reduction onto a mercury electrode may produce an amalgam rather than a deposit) , or (ii) by the adsorp­ tion of the analyte directly or in combination with other reagents onto the electrode surface. This first step, which is a pre-concentration, enrichment or accu mulation step, is carried out while the working electrode is held at a constant potential. Constant area mercury ( hanging drop or thin film ) , carbon, noble metal or modified electrodes m ay be used. The transfer of the analyte to the electrode surface is usual ly enhanced by stirring the solution or in some cases by rotating the elec­ trode. In the accumulation process a fraction of the analyte is transferred from the test solution matrix and forms a phase on or in the electrode in which its concentration is orders of magnitude higher than it is in the test solution. On completion of the accumulation step, a rest period of 1 5 to 30 s is observed so that the test solution can come to rest before the second step is commenced. In the second step, the amount of analyte accumulated at the working electrode is determined by monitoring the electrical response produced as this accumulated material is either oxidised or reduced from the electrode. This process is referred to as the stripping step since the material accu­ m ulated duri ng the first step is stripped from th e electrode and normally passes back into solu­ tion. The stripping is carried out in one of three ways namely: (i) varying the potential on the working electrode and monitoring the current response (voltammetry), (ii ) passing a constant current through the cell and m easuring the potential that the working electrode develops as a function of time ( chronopotentiometry), or ( iii) chemically and monitoring the potential of the working electrode during the stripping process ( potentiometry). In the voltammetric methods the potential applied to the working electrode in the stripping step may be in the form of a linear scan, stai rcase, a.c. modulated linear scan or staircase, square wave modulated linear scan or stair­ case, or a pulsed linear scan or staircase voltage. Because the different accumulation and determination procedures may be combined in a variety of ways, a whole family of electrochemical stripping techniques h as been developed many of which are listed in Table 3 . 1 A comprehensive account of stripping analysis h as been presented by Wang [ 1 ) and Brainina [ 2 ] . In discussing these techniques it is convenient to group them into three main categories based on the stripping process with sub-categories determined by the accu­ mulation process.

Stripping analysis Table 3 . 1

Classification of electrochemical stripping techniques.

Stripping step

- - ·· · · · · -··· ·

Accumulation step

Measure

Te chniq u e

Electrolytic reduction

i vs E

Anodic Stri pping Voltam metry

------

Linear or stai rcase anodic vo lta ge

59

sc an Adsorp ti o n

Pulsed anodic voltage scan

Ele c t roly t i c red uct io n

Ads orptio n Li near or s t airc a s e

El ectrolytic oxidation

ca t h o dic vo lta ge sca n Adsorp t i o n

i vs E

� i vs E � i vs E j VS f.

i vs E

Adsor ptive Anodic Stripping Voltamm etry Differential Pulse Anodic S t r i p p i n g

Voltammetry Differential Pulse Ads o rpt ive Anodic Stripping Voltammetry Cathodic Stripping Vol ta m m e t ry

Adsorptive Cathodic Stripping Voltamm etry

Pulsed c a t h od i c vo ltage

Electrolytic ox i d a tio n

scan

Adsorption

Constant anodic c u r re nt

Constant cathodic current

reduction*

Ele ct rolyt i c oxi d a t i o n

Adso r p t i o n

Constant current*

Chemi cal ox i dat i o n

Electrolytic reductio n

or

El ect ro lyti c reduction or oxida t i on

�i vs H � i vs E H vs t

Anodic Chronopotentiometric S t ri p p i n g Analysis ( ACPS)

d t/ d E vs E

Differential ACPS

E vs t

Cathodic Chronopotentiometric Stripping Analysis ( CCPS)

dE/d t vs t

Derivative CCPS

d t/ dE vs E

Differential CCPS

E vs t

Ad s o r p tive C h ro n o p ote n t io m e t r ic St r i p p i n g Analysis (AdCPS)

d E/d t vs t

D e r ivat ive Ad C P S

d t/d E vs E

Differential AdCPS

E vs

t

E vs t dE/dt vs t

reductive stripping often

Differential Pulse Adsorptive C ath o d ic Stripping Vo l ta mm c t ry

De r i vative ACPS

d t/ d E vs E

' Oxidative and

Volt a mm e tr y

dE/d t vs t

dE!d t vs t

Ad s o rp t io n

Differential Pulse C a t ho d ic Stripping

d t/d E vs E

not distingu ished in the techn ique name.

Potentiometric St rippi ng A n alysis Derivative Potentiometric Stripping A n alysis

Differential Potentiometric S tri p p i n g

An alys is

Adsorptive Potentiometric Stripping A n alys i s ( AdPSA ) Der ivative AdPSA Di ffe r e nt i a l Ad PSA

60

Introduction to Voltammetric Analysis

3 . 2 VOLTAMMETRIC STRIPPING 3 . 2 . 1 E lectrolytic accu mulation 3.2. 1. 1 Anodic voltammetric stripping Zbinden [ 3 ] had demonstrated in 1 93 1 th a t small amounts of copper could be qu a n tifi e d by plat­ ing the copper onto a plati n u m electrode and then m e a s u ri ng the current that flowed during the electrochemical stripping of the copper from this electrode. However it was not until the late 1 950s that t h e high sensitivity of s t ri p p i n g analysis and its po t en t i a l for the a na lysis of trace m et a ls at the s u b p a rt per billion level was fully recogn ised [ 4 , 5 ] . During the 1 960s and 1 970s the theo­ retical principles of stri pp i ng voltammetry were established [ 6 , 7] and t h e meth od was success­ fully applied to the an alysi s of metals at the trace and ultra-trace level [ 8 , 9] and to the environmentally i m por tant pro blem of trace metal sp e ciat i o n in natural waters [ 1 0- 1 2 ] . The two pro ce ss e s involved when the a n alyt e i s accumulated by electrolytic reduction and d e te rm in ed by voltammetric o xid a t i on are summarised in e q uat i on 3 . 1 : -

M e "+ + ne- + (Hg)

Acc umulation

Determination

(3. 1 )

Since the determination step is the inverse o f the ac c u mu l atio n processes, the term Inverse Voltammetry was at one time used to describe the stripping technique [ 1 3 ) . After a variation on this technique was d ev elo p e d in wh ich the en riched analyte was determined by a reduction rather than an oxi da ti on step, th e above technique became referred to by th e more clearly descriptive name anodic stripping volta m metry or its ac ro ny m ASV. In those cases where the analyt e accumu­ lated on the electrode is measured by a re d u ct io n step, the technique is r efer re d to as cathodic stripping voltammetry or CSV (§ 3 . 2 . 1 . 2 ) . The steps a n d re s u l tant current-potential profile for a n a nalysis b y ASV are depicted i n Figure 3. 1 . The l e ft side of the diagram represe n ts the accumulation step while the right side depicts the determination step. D u ring the time interval , a, referred to as the accu m ula tion time, tacc ' the ana­ ,

lyte is transported to the working electrode surface by convection and diffusion and d ep o s i t e d on the electrode by electrolysis at a constant potential , Eacc' The convective transport is achieved by st irri ng the sol ution or ro t a t in g the electrode at a con stant rate so that a constant di ffu si o n layer thicknes s is m a in t ained (see § 1 .4. 1 ) . During ac cu m ula t i on , the electrolysis is not exhau st ive and on ly a sm al l fraction of the analyte is transferred fr o m the test solution to th e working e lectro d e Typically, when the test so lu t i o n volume is 25 mL only about 0.25% of the analyte is transferred to the electrode after 5 minutes a c c um u l a ti o n [ 6 ) . With smaller sample volumes, a higher propor­ tion of the analyte is accum ulated for a given tacc· In o rder to obtain reliable analytical results with high precision it is essential that the same fraction of a n al yt e be transferred to the electrode in each experiment. Therefore it is n e c e s s a ry to c a re fu lly control all those factors which affect the transport of the anal yt e to and its depositi on on the w o rk i ng electrode. In addition to holding the temperature and electrode potential c o ns tant , s p e c i a l attention must be paid to th e reproducibi l ­ i ty of t""' the test solution volume, t h e el ectrode position, t h e surface area of the electrode, and the mechanical c on d iti o n s , such as the shape, size and p o si tio n ing of the stirrer, and the solution stir­ ring speed or electrode rotation rate. The time interval, b, a ft er stopp ing the sti rring or rota tion, must also be c a r efu l l y controlled. D u r ing this ti me called the rest period, the w or ki n g electrode is held at the accu m ul at i o n poten­ tial, the motion of the solution ceases and the cathodic current drops to a sm all value as the con­ tribution from the forced co nvecti o n falls to zero. In an a lyses where a m etal amalgam is p rod u ced in a mercury drop electrode, the rest p e r i o d also allows time for t he metal concentration to become u n i form t hr o u gh o u t the drop. At th e end of the rest peri od, a linear or staircase anodic voltage scan is applied to the working el ectrode from an i n i t i a l value o f Eacc' thus i ni t i a ti n g the determ ina tion or stripping step, c, .

,

Stripping analysis

..--- a � Figure 3 . 1

b

61

+-- c � . +-- d --

Potential-current-time relationshi ps dunng an anodic stripping voltammetric analysis: (a) accumulation time, (b) rest period, (c) determination or stripping step, (d) anodic dissolution of mercury.

The recorded voltammogram exhibits a peak as a result of the oxidation and transfer of the enriched analyte from the working electrode back into the cell solution. Ideally, this transfer should be complete but, particularly with an HMDE, it may be incomplete because the metal dis­ solved in the amalgam may not have completely diffused from the centre of the drop during the time of the potential scan. This is indicated by 'peak tailing'. The peak is characterised by the peak potential, Ep, and the peak current, ip , which are determined by the nature and concentration of the analyte, respectively. The rise in the current in the fourth section, d, is due to the anodic dis­ solution of the mercury working electrode. Following the introduction of the hanging mercury drop electrode [ 4 ] , anodic stripping volta­ mmetry was applied to the trace analysis of those metals which dissolve in mercury to form amal­ gams, such as antimony, bismuth, cadmium, copper, gallium, indium, lead, manganese, thallium, tin and zinc. In addition to the mercury drop electrode, the mercury thin film electrode, MTFE, which has a much higher surface area to volume of mercury ratio than the HMDE, was also devel­ oped for the determination of the amalgam-forming metals. The thin film of mercury is plated onto an inert supporting material for which purpose glassy carbon is particularly well suited. The mercury is either plated onto the glassy carbon prior to the accumulation of the analyte or, more commonly, generated in situ to produce the MTFE. In the latter method a small amount of mer­ cury( I I ) nitrate is added to the test solution and the mercury film (thickness ca. 50- 1 500 nm) is formed on the GCE surface simultaneously with the deposition of the analyte. In order to deter­ mine mercury itself and the more noble metals, such as Ag and Pt, by ASV it is necessary to use an inert solid electrode such as a carbon electrode. As is the case with most polarographic and voltammetric analyses, it is necessary to remove oxygen from the cell solution in stripping voltammetry. The reduction of the dissolved oxygen produces a current which affects the background current and produces hydroxide ions at the elec­ trode surface. These may react with analyte ions in the diffusion layer. Furthermore, if oxygen is present in the test solution, it may chemically oxidise the amalgam in competition with the elec­ trochemical oxidation in the stripping step. This can lead to a decrease in the peak current in the anodic stripping voltammogram and unreliable results.

62

Introduction to Voltammetric Analysis

Linear Sweep Voltammogram

-E Anodic Stripping Voltammogram

Figure

Comparison o f t h e linear sweep voltammogram with t h e anodic stripping voltammogram o f a n analyte.

3.2

The linear sweep voltammogram for the reduction of an analyte in solution and the anodic stripping voltammogram for the oxidation of the reduced analyte back into solution are shown in juxtaposition in Figure 3 . 2 . The shape of the stripping voltammogram depends on the nature of the electrode reaction and on the surface area to volume ratio of the electrode. The sharpness of the peaks increases with the number of electrons involved (Table 3 . 2 ) and with the reversibility of the electrode process and depends on the voltage waveform used to strip the analyte from the electrode. By using smaller mercury drops or lower voltage scan rates, sharper peaks are obtained so that a better separation of closely lying peaks becomes possible. Even better resolution of the current peaks results from using a MTFE rather than an HMDE because of the sharper and more symmetrical current peaks obtained with the former electrode as a consequence of its larger sur­ face area to volume ratio. Better peak resolution and symmetry means that analytes with smaller differences in their peak potentials can be successfully resolved in a single run. The accumulation potential, Eacc' used in the second step may be determined from the linear sweep voltammogram (Figure 3 . 2 ) or from the DP or DC polarogram or voltammogram of the analyte in the same supporting electrolyte as that to be used for the analysis. The value of Eacc for the case of an electrode reaction where n = 1 should be at least 0.2 V (for n = 2 at least 0. 1 V) more negative than the half-wave potential or peak potential for the reduction of the analyte, that is the value of Eacc should be in the potential region of the diffusion limited current ( Figure 1 .5 ) . Ta b l e

3.2

Peak widths at half peak height, b 112, for different ASV techniques [ 1 4 ] . HMDE

TFME Metal ion in 0. 1 M HCl

Linear scan b112 ( mV)

Differential pulse b112 ( mV)

Fundamental a. c . b 112 ( mV)

Second harmonic a. c. b112 ( mV)

Tt

74

86

94

84

Pb2+

34

44

56

44

33

44

38

In3+

25

Theoretical basis

The development of the theory is treated in detail elsewhere [ 1 , 6, 1 5 ] and only a summary is pre­ sented here. For those metals which form amalgams with mercury, the concentration of the metal in the mercury working electrode is governed by Faraday's law and given by

Stripping analysis

63

(3.2)

where

carna l is the concentration of the reduced analyte, Me0(Hg), in the mercury electrode, i.,, is the current which flows during the electrolytic accumulation of the analyte, t.,, is the time of accumulation, v is the volume of the mercury working electrode ( HMDE or MTFE ) , Hg n i s the number of electrons p e r mole o f electrode reaction, and F is the Faraday constant.

For an HMDE with a drop radius, rd, v H g (4/3 )7tr/, and for a MTFE of thickness, l, and sur­ face area, A£> vH g Arf. It should be noted that the MTFE obtained by depositing a thin film of mercury on a glassy carbon substrate is really a series of micro-droplets rather than a true contin­ uous film (Figure 4 . 1 5 ) [ 1 ] . Such a deposit however behaves as a thin film. During the accumulation step when the solution is stirred, the analyte is transported to the diffusion layer by forced convection and then diffuses through the diffusion layer to the electrode surface. For the HMDE in a stirred solution, the accumulation current is given by [ 1 6 ] : =

=

(3.3)

and for a MTFE in a stirred solution or for a rotating MTFE by l ace =

.

where

k2 n FA f D

ro

213 1 12

vk

1 /6

c.

( 3 .4)

k1 and kz are constants, D is the diffusion coefficient of the analyte, rd the radius of the mercury drop ( HMDE), Ar is the surface area of the mercury film (TFME ) , ro i s the rate o f rotation o f the electrode or of stirring the solution, v k is the kinematic viscosity of the solution, c. is the concentration of the analyte in the cell solution, and the other symbols have their usual meaning.

The second term in the brackets of equation 3.3 is the sphericity correction which allows for the effects of the curvature of the electrode. Since this term is only significant when working with micro-electrodes, it can be ignored when using normal-sized mercury drop electrodes ( rd 0.2-0.5 mm ) . O n combining equation 3.2 with 3.3, o r with 3 .4, it can b e seen that by using a constant stirring rate, the concentration of the enriched analyte in the amalgam is proportional to both the accumu­ lation time and the concentration of the analyte in the cell solution provided that the latter is not sig­ nificantly depleted during the accumulation period. This is the case under the experimental conditions normally employed in ASV. When using a mercury drop electrode, only a few tenths of one percent of the analyte is deposited out of a 20 mL test solution after 1 5 to 20 minutes electrolysis. With micro-cells this is not the case and the product ( i.,J.,,) in equation 3.2 must be replaced with the integral Ji.,,( t)dt. In the case of micro-cells with volumes < 1 00 j.!L all of the analyte in the sample may be deposited after 1 0 to 1 5 minutes electrolysis. A high efficiency of deposition is an advantage when carrying out continuous voltammetric measurements in flow-through cells. The flow channel in such cells is designed with a very small cross-section area so that complete electrolysis of the ana­ lyte may be achieved in the time it takes for the sample solution to pass across the electrode surface. =

64

Introduction to Voltam metric Analysis

Wh e n workin g with a MTFE, the GCE disc o n t o which the thin mercury film is plated m ay be rotated at constant sp ee d instead of s ti rrin g the solution about a stat i o n a ry electrode. Such an e l e c t ro d e , referred to as a rota ting disc electrode, or RDE, re s u lts in an im pro ved rate o f transport of the analyte to the electrode surface and gi ves greater reproducibility than that obtained with a s t at io n ary electrode in a stirred sol u ti on . In an an alo gous way to linear sweep voltammetry (LSV) and cyclic voltammetry (CV) ( cf. e q u at i on 2 . 30 ) , the p e a k current, iP, in anodic stri p pin g v olt am m et ry using a l i n e ar pot e nt ial scan a n d an H M DE is gi ve n to a good approximation by the Randles-Sevcik e qu at i on [ 1 7, 1 8 ] , the dif­ fere n c e being t h at the solution parameters in e quation 2.30 (for LSV and CV) are rep lace d in ASV b y the corresponding parameters for the enriched analyte i n the amalgam to give (3.5) where

v

is the v o l t age scan rate in V s - I , k3 c ontain s all t he relevant numerical and physical constants and the mass transfer terms ( stirri ng rate, vi sc os i ty) , Dam al is the diffusion coefficient of the metal in the am a l g a m, and th e other sym b o l s have their usual meaning.

No t in g that from equations 3 . 2 and 3 . 3 or 3.4, carnal is proportional to both tacc an d ca and that for the HMDE, AHg k4 rd2 , equation 3 . 5 leads to the following expression for t he s tr i p pi n g p eak current when using a n HMDE ( ignoring th e s pheri c i ty c orr ec t i on ) : =

( 3 .6a)

and when u s i n g a MTfE [ 1 9 ] : ( 3 .6b) where

k5 and � are constants.

From these equations it can b e seen that for both electrodes the peak current i s proportional to tacc ca , so that the concentration r ange over which iP versus c. is linear may be extended s im p l y by varying tac c. It follo ws from this that th e detection l im it for a given determination, which de pend s on the lowest value of iP t ha t can be meas ured above th e noise level, wi ll a l so d epe n d on tacc I t should be noted that the peak current should vary w i t h v 1 12 and rd fo r the HMDE but with the first p o wer o f v a n d the area of the film, and be i n de p e n de n t of t he film thickness fo r the MTFE. Comparison of mercury drop and film electrodes

tive than l!112

With the HMDE, th e peak potential is independent of t he scan rate and is 28.5/ n mV more posi­ for a reversible pro c es s at 2 5°C i n ASV and 28.5/ n m V more negative in CSV (cf. LSV in § 2.3 ) . On the other hand, for the MTFE, the p eak potential varies with the scan rate and the film thickness, and the d i ffe re n c e between EP and F! 1 1 2 is a p p roxim a t el y four times that for the HMDE. Ano t her s i gn ifi c a n t difference between t h e d rop and film is t ha t the value of iP d e p e nd s on the concentration of the metal in the ama l ga m for t h e HMDE but on th e amount of m et a l in t h e amalgam for the MTFE. The differences in the performance of the mercury d ro p and thin film e l e c t ro d e s when us e d in the determination of trace metals by ASV arise m a i n ly fro m th e l a rge difference in their surface area to vo l u me ratios [ 1 4, 1 5 ] . Typically a MTFE has a s u rfac e area s o m e ten times th a t of an H M D E or SMDE, while the volume of mercury in t he former is less th a n 0.5% of t h at in the latter

Stripping ana lysis

65

two electrodes. This makes the MTFE in h e r en tly more sensitive but it s la rger surface area means that the blank ( ch a rging c u rrent ) s i gn al is larger a n d it is more prone to interference from surface active i m p ur i ti es in the sample. The mercury d ro p electrodes (HMDE or SMDE) h av e some disadvantages c o m p a red to the MTFE:

The accumulation efficiency is l o w because of the s m a ll s urfac e a r e a . The stripp ing p e a k s are broadened as a result of the lar ger volume of mercury (the the­ oretical peak half-width, b1 12 , 1 08/n mV for t h e HMDE c ompared to ( 3 8-50 ) / n mV for the MTFE [ 1 ] ) . This peak bro ade ning arises because of the finite time required for t he metal to diffuse from the i n te rior of th e drop. L on g accumulation times, which allow the metal to diffuse up into the mercury column, result in further peak broadening. (iii) In order to maintain drop s t ab i l ity only low solution stirring rates ( and hence lower accumulation rates) can be used. (iv) The rest period prio r to s tripping must be ke pt at about 30 seconds since it takes this time for the deposited m e ta l to reach a uniform concentration t hrou gh out t h e d ro p co m p a r ed to a few seconds to achieve u n ifo r mi ty in a thin mercury film.

(i) (ii)

=

Lon g e r accumulation times may be used to counter (i) and ( iii ) , however, as alre ad y noted, this leads to enhanced peak b road e n i n g and extends the time required for each analysis. P e a k broaden ing, as a result of the lon ger accumulation times r e qu ir e d to obtain a measurable peak current in the analys i s of ultra-trace metal ion concentrations, is one of the factors that deter­ mines the limit of detection in ASV. While the M T F E requires sh o rte r accumulation t i mes because of its large surface area, gives sh a rp e r , m ore easily resolved s tri pp i n g peaks because of the small volume of mercury, and is stable at h ig h st irri n g rates, there are a few problems that arise from the fact that only a sm a l l amount of mercury is present in a film electrode.

Erratic behaviour i s observed with very thin films, h o we v e r films thicker t h a n 1 00 nm g ive reproducible behaviour ( de scrib e d by eq u ation 3 .6b ) and th e p e ak currents observed for a given amount of metal in th e a m alg a m are found to be independent of film th ic k n ess for l 1 00 to 2 0 00 nm [ 2 0 ] for both linear and pulsed voltage stripping t e ch niques . (ii) Metals with a low solubility in mercury may quickly reach sat u ra t i o n concentration during the a cc u m u la tion step. The excess m et al is d e p os i ted on or th rou g h the mercury film thereby c re ati ng a second phase on th e electrode and a lte r ing t h e p h ysi c al and elec­ trochemical properties of t h e electrode. The pro bl e m metal may come from t h e reduc­ tion of a metal ion of no analytical inte rest in the s amp le matrix or be the reduced analyte itself. For ex a mple , it ha s been c alculated that for a MTF E , 6 m m dia. and 250 nm thick, being rotated at 60 rps in a 25 mL solution co n t ain i n g 6 ppm Cu2+ , t h e mer­ cury film will become saturated in c opp e r after only 2 m in utes electrolysis [ 2 1 ] . (iii) Sub strates o n wh ic h the mercury film is st ab l e and which do n o t contaminate the mer­ cury are limited. The m o st suitable is glassy carbon, however while thin m e rc u ry films on glassy carbon are q u it e stable in n e u t ral and alkaline s o l u tions they are much less stable in acid solutions, failing after a fe w hours in s ol uti o n s at pH 1-2 [ 22 ] . (iv) Several metals when dissolved t o geth e r in mercury form inter- metallic comp ou n d s in the amalgam. This problem b eco m es more pronounced as the metal concentrations in the amalgam increase a nd s o affe cts anal yses u s i ng the MTFE to a m uc h greater extent than analys e s u si n g a drop electrode. Inter-metallic c o m p ou n d formation may occur in the amalgam between the m etal s from two a nalytes or an analyte and a reducible matrix

(i)

=

66

Introduction to Voltammetric Analysis

component. It leads to do ubl e pea ks changes in peak pot e n t ial s or the comple t e disap­ pearance of a peak from the stri pp i ng volta m m o gram Metal combinations which have been reported to form int e r me tallic co m pound s in mercury include Co with Zn; Cu wi t h Cd, Ga, I n , Mn, Ni, Sb, Sn, Tl a nd Zn; F e with Mn; and Ni with Mn, Ga, Sb, Sn and Zn. Platinum also forms inter-metallic compounds with many other elements in m er­ cury and for this reason is not suitable as a substrate for the MTFE. ,

.

-

The determination limits attainable with anodic stripping voltamm e try using the HMDE gen­ the range 0.5 to 1 �g L- 1 • This c an be i m p rove d by up to a fac to r of 1 0 when a MTFE formed on a glassy carbon or graphite substrate is used. More sensitive and reproducible mea­ surements can be obtained when the usual graphite or glassy carbon substrate is replac ed by the re ce ntly developed ultra-trace g r a p hite el ectrode of Metro h m (§ 4.5.2). The surface of t h i s elec­ trode, which displays micro-electrode-like properties, can be c o ated wit h m e rc u ry dropl ets and used to a ccumulate a nalytes from rapidly stirred solutions. In g e n e ral more reproducible results are obtained with hanging d r o p electrodes at the h igher c o ncen t r at i o n s while at very low concen­ trations more precise result s are obtained when a mercury film el e c t rode is used. In anodic stripping voltammet ry the differential pulse mode (DPASV) is p refer re d for strip­ ping the analyte from the electrode. Under identical exp erim ental conditions (supporting electro­ lyte, Eacc' tacc a n d c. c ons t a n t ) a h i g h e r s e n s iti vi ty is obtained us i n g DPASV than that obtained with linear scan stripping ( LSASV ) . T h e a dvantage of using LSASV is that sur fac e active substances cause less interference. On the other hand, interference from i rreversible processes can be dimin­ i s hed by us i n g an a.c. m odu l ated waveform for the s trippi n g process (ACASV). When a staircase waveform is used instead of a linear (DC) scan to stri p the metal from a MTFE, the faradaic cur­ rent p e a k i s s i gn i fic a ntly reduced for thin films with l = 0.25- 1 .5 �m. The decrease in p e ak current amounts to ca. 28% for .1.Estcp 2 mV and to ca. 50% fo r .1.Estep = 1 0 m V [ 20, 23] The p eak current decreases with ste p h e igh t (.1.E51,P ) at constant scan rate (.1.Estep/.1. tstcp ) but increases with .1.Estep at constant .1. t,1•P ' that is with increasing .1.E,1./.1. t,1ep · However the staircase waveform discriminates against the c h arg i n g current ( 24 ] , so t h a t t h e ba c kg ro u n d current is much less than and not ste e ply rising as is the case when a linear scan is used for the stripping step. The n et result is an i m prove m ent in sensitivity when a staircase waveform is used. The sensitivity of staircase strip­ p i ng is s i m il a r to that of di ffe re n tia l pu l se s tr i p p i n g, h owever much h ighe r scan rates can be used with staircase waveforms resulting in much reduced analysis times. The stripping vo l tammo gram s obtained for the determ ination of zi n c i n aci d so l u t i o n shown in Figu r e 3 . 3 illustrate the differences in the data obtained by using different stripping vo lt age waveforms [ 25 ] . The rise in the b ase current produced by the i rreversible reduction of the H+ ion (begins ca. - 1 .0 V) is much lower in the AC-voltammogram than in t h e DP-voltammogram. By far the greatest effect however, was on the LS (DC) -voltammogram where t h e formation of the p ea ks was s evere ly h i n de re d e rally lie in

,

=

.

Resolution of current peaks in ASV

In trace an alysis it is desi rab l e that the treatment of the s a m p le prior to th e quantitative measure­ ment step be kep t to a minimum. It is p referable if the t echn ique used to obtain the q uan ti tat ive signal is able also to differentiate between the various analytes in a mixture so that th e s a mple does not have to be s ubj e cte d to a s epa rati o n p rocedu re which m ay h ave a po o r recovery efficiency or may introduce contamination. A number of experi m en tal strategies h ave b e en develop ed in ASV which utilise both the c hem i c a l and electrochemical p rop e rties of the analyt es to facilitate the res­ olution of their stripping peaks so that all desired co mp o nent s of the s am pl e may be a nalyse d in a single run with the m in im um of pre- treatment. Since the accumulation ele ctr o lys i s is c a rr ied out potentiostatically, i t is p o s s i b l e ( i n p ri n c i p l e ) to adjust the a cc u mula t i o n potential so that a nal yte s that have stripping p eaks at si m ila r p ot en -

Stripping ana lysis DC

a

0.6 0.7 0.8 0.9 1 .0

1 .1

1 .2 1 . 3

AC

,.1 I I I I I I I I I I I I

b

:

•

I I I I I I ' I ' ' '

_ _ __

Figure

3.3

I I I I I I I I

-

E [V] (vs. SCE)

' I I I I • I ' I I I I ' I I

I

I

0.7

67

' I I I I • I ' I I ,'

I

I I

/

,.,. , '

... _ __ ,

0.8

0.9

1 .0

1 .1

1 .2

- E [V] (vs. SCE)

Comparison of t h e anodic stripping voltammog rams obtained for the dete rm i n ati on of zinc in 0.1 mol l-1 HCI solution using an H M D E and th ree d i fferent stri pping techniques: linear scan (DC}, differentia l p u l se (DP} and an a.c. modulated l i near scan (AC). face = -1 . 2 5 V (vs SC E); t,,, 3 m i n . ( a ) 1 00 IJ9 L-1 Z n 2 + a n d (b) 1 0 j.Jg L-1 Zn 2 + . =

tials m a y b e s eparat e d duri ng the a ccumula ti o n step. Simultaneous det e r m i n at io n s r eq ui r e a dif­ fe r ence in the peak p o t en t ia l s of �EP > 1 00 mV for good resolution. The s it u ati o n is illustrated in Figure 3.4 for the determination of P h , Cd and Zn wh e r e it can be seen that by the ap pro pr i ate choice of Eacc' separation of the ana lyt e s can be ach i eve d . The resulting stripp i n g vol t am mo gra ms sh ow pe a ks for Ph, for P h a n d Cd, or fo r all th r ee metals t o gether depending on the p o t e ntia l selected for the accumulation step. Another way to analyse mixtures of metal ions with poorly resolved stripping p e aks is to st op the poten t i al sweep at a suitable value during the course of the st rippi n g step. While the electrode is held at this co n s t a n t po ten tial , those m etal s with more n egative stri p p i n g potentials are stripped out of th e a m a l gam . On co n t i n u i ng the potential sweep, the metals with the more positive peak p ote n t i a l s can be analysed without interference from the more easily oxidised metals, even when the fo rm e r are p r e s en t at much lower concentrations i n the test solution than the latter.

68

Introduction to Volta mmetric Analysis

a

- 0.4

- 0.8

- 1 .2

E

[V] (vs. SCE)

b

Figure

3.4

Se l e ctivity i n anodic stripping voltammetry by the choice of the accumulation potential: (a) d. c. polarogram of lead, cadmium and zinc (each 1 0-3 mol L- 1 ) in 0 . 1 mol L- 1 KCI, (b) a n odi c stripping voltammograms of lead, cad mium and zinc ( ea c h 10-6 m o l L-1 ) i n 0. 1 mol L-1 KCI after accumulation at various potentials, (i) E'.,,. stripping vo l ta mmogr am of lead alone, (ii) E2ac" stripping voltammogram of lead and cadmium, (iii) E\cc , st i p p in g voltammogram of lead, cadmium and zinc.

For those metals that form inter-metallic compounds in the a m a l ga m , t h e p eak s h i ft i n g and elimination caused by the inter-metallic co m po un d formation can be t u r n ed to an advantage. For example, the presence of copper markedly interferes wi t h the ASV determination of zinc. The problem can be o ve rc ome by the a ddition of Ga3 + to the s olu t io n and co-depositing gallium with th e zinc and copper. The Cu-Ga c omp o un d is much more stable than the Cu-Zn c o m p o u n d so that the copper combines wi th th e g alli u m , freeing the zinc which can now be determined free fro m interference [ 26 ] . The selective determination of lead or thallium in the p r e s e nce of b i s mu th or indium i s a ch i e v e d us i n g t h e same p r incipl e by c o - d e po s it i n g copper or gold on the w o rkin g electrode w i th the an alytes [ 27, 28 ] . Metal- ion peaks that lie close together in the s t rip ping vo lt a mmogr am may also b e separated from each other by complexing the metal ions in solution. There are two possibilities. In the first case , an appropriate c o m p lexin g agent, wh i c h shifts the half-wave and stripping p e ak potentials of one of the metal io ns to a more neg at ive value, is added to the sample so l u tio n. Both metal ions t h e n can be j o int ly accumulated at the appropriate more negative pot e n ti a l and give separate peaks in the stripping voltammogram. Alternatively, by using a less n e ga tive accumulation poten­ tial only the metal ion which has n o t reacted with the complexing agent may be accumulated.

Stripping analysis

Sn2 + Tt

2+ Pb

1

69

\ lil T Tl

Sn

Pb

2 Tl

II

3

Tl

Sn

Pb

I

Sn

Pb

4

- 0.4

- 0.5

- 0 .6

- 0. 7

- 0. 8

- 0. 9

- 1 .0

- 1 .1

E [V] ( vs. SCE ) Figure 3 . 5

The effect of changing the supporting electrolyte composition on the separation of the potentials of the anodic stripping voltammetric peaks. Supporting electrolytes: (1 ) 1 . 0 mol L"1 HCI; (2) 1 .0 mol L-1 HCI + 2 . 0 mol L-1 ethylenediamine; (3) 1 .0 mol L- 1 HCI + 2 .0 mol L- 1 N a O H + 0.2 mol L-1 ethylenediamine tetraacetic acid; (4) 1 . 0 mol L-1 HCI + 2 .0 mol L- 1 NaOH + 0.2 mol L- 1 sodium potassium tartrate.

A n o t her s e p aratio n t e c h n i qu e which makes use of c o mplexa tio n is t h e so-called solution or mediu m excha nge method. Th e p r i n c iple of this method is that after the electrolytic a cc u m u l a tion step, the cell s o l uti o n is r e p l ace d by a second solution. Often, in order to o b t a i n the best s e p a rat i o n of the desired a n a lytes fro m the sample matrix, the electrolysis m u s t be c a r ri e d out using a s up p o rt i n g e lect r olyt e so l u tio n which is un s ui table fo r o b ta i n i n g goo d p e ak separations in the s tri p p in g step . By changing the cell solution to o n e c o n ta i n i n g an a pp rop ri a te complexing agent t he d es ire d se pa ra t i o n can be achieved. This is illustrated by the data presented in Figure 3.5 [29] . T h e a c c um ul a t i o n of tin, lead and thal l i um is ca r ri e d out in 1 mol L- 1 h yd roc h l ori c acid solution. However, the peak p o ten t i a ls in t h e stripping vo l t ammogram of these three ions in 1 mol L- 1 hydrochloric acid are too close to ge t h e r and unsuitable for the simultaneous determina­ tion of each of t he th ree ions ( Figu re 3.5, l i ne 1 ) . When, after the electrolysis, t h e hydroch loric acid solution is re plac e d with a solution which contains either ethylenediamine, EDT A, or tartrate as complexing agent, the p e ak shifts as indicated in F i g u re 3 . 5 ( l i n e s 2, 3 and 4) are o b t a i n e d . In the si m pl est case, the solution exchange is a c hi eve d by the rap i d exchange ( < 5 s) of the el ec tr olys i s cell with another vessel c o n t a in ing the deaerated s ec o n d s u p p ort i n g electrolyte. This method of solution exch ange, however, yields unsatisfactory results with base metals such as z i nc because on contact with the air the amalgams of th e s e metals can be ra p i d l y oxidised by atmosph eric o xyge n . A quick, complete and con tinu o u s solution exchange can be achieved best wi t h a tl ow -th ro ugh c e ll ( ch a p t e r 5 ) . Alternatively, t h e supporti n g electrolyte may be modified by sim p l y a d d ing the appropriate oxygen-free reage n ts to the cell s o l u t i on used for the accumulation step. For example, in th e ASV det e rm in at i o n of l ea d and thallium the metals are best accumulated from 0. 1 mol L- 1 HCl. However in this electrolyte the stripping peaks are too clos e fo r goo d reso­ lution. By changing the pH o f the cell solution to 1 3 with deaerated 5 m ol L-1 NaOH, well s e p a ­ ra t e d p e aks are obtained in the st r i p p i n g voltammogram. exchange

70

Introduction to Voltammetric Analysis

In summary, a n odi c st ripp i n g volt a m me t r y u s in g mercury electrodes can be u se d for the trace analysis of antimony, bismuth, cadmium , c opp er , galliu m , germanium, indium, lead, manganese, thallium, tin and zinc especially in a range of water samples ( d ri nki n g , surface, sea, i n d ustr i al effluent, sewage) and biological fluids. Many of these s a mp l e s will c o ntain organic material which will adsorb onto the w o rki n g electrode and interfere with the ASV determination. In these cases it is necessary to d es tro y t hi s or g an i c material by U . V. i r ra dia t i o n or oxidative microwave diges tion in the presen ce of H202 b e fo re proceeding wi th the ASV determination. For samples with a high organic c o n t ent such as sludge or tissue material, wet acid digestion u si ng aqua-regia will be n ec­ essary (chapter 4). ASV in c o mb i n a ti on with U.V. d i ges tion is also used to determine the concen­ trations of the various forms in which a t o xic heavy metal may be present in a water sample (metal ion s peciation ) ( c hapter 6). 3.Z. T . Z Cathodic vo/tammetric stripping Ca th o d ic s t r ip p i ng vo l t a m m et ry is used for t h e indirect determination of inorganic and organic anions which form sp ar in gly soluble s a l ts wi th Hg(I) or Ag(I ) . When a mercury (or silver) elec­ trode is anodically polarised, Hg2 2 + (or A g +) are produced and react with the analyte to form the sparingly soluble Hg(I ) { or Ag( I ) } sal t which a c c umu ates on the electrode surface. The accumu­ lation potential d e pen d s on t h e supporting electrolyte, the solubility p rod uc t of the Hg(I ) or Ag( I) sal t , and the co n cen tra t ion of t h e an a lyte in th e test solution. For mercury electrodes E.cc is usually in the range +0.4 V to -0.2 V ve r su s Ag/AgCl, 3 M KCI) . In the d ete rm inat on step, a cathodic p ote n tial scan is a pp l i e d to r educe the H g ( I ) o r Ag( I ) in t h e sparingly s olu ble salt accumulated on the elect ro d e surface which gives rise to the voltammetric current peak. CSV can b e used for the determination of trace levels of halides, pseudo-halides or sulfide ( m e rcu ry [30] or s i lve r [ 3 1 ] electrodes) and a number of o xom e ta l a t e ions such as vanadate, ch ro ma t e tungstate and m o yb date and orga ni c anions ( mercury electrodes) . The analytical pro­ cess is summarised in the fo llowing eq u a t ion s ( for a mercury elec trod e ) .

l

(

,

i

l

l

Hg �+

Accum ulation:

+

2Hg � Hg;+ + 2e­ 2A- � Hg 2 A 2

(3.7)

.!

Determination:

The current peak height d u e to the electrolytic d iss o l ut ion of the insoluble l ayer is propor­ tio n a l to the amount of anion accumulated on the electrode surface, which under c o nst a n t accu­ mulation and stripping c o nd i t i on s is in turn pro p o rt i o na l to t h e concentration of the analyte in s o lution The peak potential is determined m a i nl y by the solubility product of the acc u m u l at ed salts. The differences in s o l ub i l i t i es between the Hg(I) halides and b etwee n Hg(l) ch ro mate , molybdate, tungstate and vanadate are not ve ry large so that the strippi n g peaks occur a t similar potentials and the r eso luti on of adjacent p eak s i s limited [ 32 ] . This in direct p r oc e d u re can b e used to d et e rmin e a number o f organic substances which are capable of forming in s ol uble Hg( I I ) compounds on an anodically polarised mercury electrode. These are mainly sulfur-containing s ub s tanc es such as thiols, t h i o am ides thiobarbiturates a n d dithiocarbamates and some s ulfo n i c acids [ 3 3 ] . Al th o u gh there is still s o m e uncertainty about the m echanisms involved, it is generally believed that the cat h o d i c s tri pp i ng t e chn i q u e for t h e deter­ mination of thiols is based on the reaction: .

,

accu m u lation

2RSH + Hg

cathodic stripping

(3.8)

Stripping analysis

- 0 . 77

V

71

(As)

a

- 0. 1 8

- 0 .04

+ 0.02

Figure

3.6

V

V

V

b

(Cu) (As) (Hg2CI2)

The differential p ulse cathodic (a) and anodic (b) stripping voltammograms of arsenic. The cel l solution was 0 . 1 mol L-1 HCI containing 2 x 1 0- 3 mol L-1 Cu2 + and 1 0 j.Jg l-1 As. Voltage scan rate 1 00 mV s- \ face = -Q. 5 5 V ; tacc 1 min. =

while for primary thioamides the accu mulation rea ction is [ 3 4 ] : RC ( = S ) NH 2 +

Hg --� HgS + RCN

+

2H+ + 2 e-

(3.9)

and the cathodic stripping peak arises from the red uction o f t h e HgS layer o n t h e mercury elec­ trode. Secondary thioami des also give rise to a layer of HgS o n oxidative accumulation but the ter­ ti ary compounds appear not to accum ulate oxidatio n products on the electrode and so can not be determined by CSV. A typical application is the determination of thiourea, H2N ( C=S) NH2, in

1 mol L_, N aOH solution, which can readily be determ ined at the 1 jlg L-1 level using differential 2 7 , chapter 7 ) . A n umber o f organic compo unds which d o not contain sulfur such a s various flavins [ 3 3 ] , phenylhydrazine, uracil derivatives and n u cleic acid bases [36] also form well developed cathodic pulse cathodic stripping voltammetry [ 3 5 ] ( see Application

stripping peaks after oxidative accumulation. Many

of these compounds arc known to form in sol ­

uble mercury compounds and the electrode processes involved in th eir analysis by stripping vol­

tamm etry can be summarised in an analogous way to that for the thio compounds. However, there is evidence that in some cases a dsorption of the analyte onto the electrode surface plays an importan t if not domi nant role in the accumulation step. Metal ions such as a rsenic ( III ) , selenium and tellurium ( Mea" ' ) may be determ i n ed by strip­ ping voltammetry after adding a second metal , such as copper

(Mcb"'+ ) , to the test solution and

72

Introduction to Voltammetric Analysis

two metals onto the surface of the HMDE. Th e copper acts as a co - d ep osition agen t a nd fa c il i tates th e d eposition of th e a n alyte , Mea, on the electrode surface as an inter- metal­ lic com po u n d . T h e ana lyt e may t h e n be s t rip p e d from the e l ectr o d e either b y oxidation (ASV) or by further reduction (CSV) t o an anionic s p ec ie s accordin g to th e fo l l owi ng r eact i on s ch em e : c o- ele ct ro lys i n g the

A cc u m u l ation :

St r ipp i n g: Arsenic, selenium and tellurium are th ree such elements which m ay be determined by cat h o d ic stripping volt a m met ry a fte r h aving b een re d uct i ve l y co-deposited with copper [ 3 7 ] . A characteristic of the cathodic stri pp i n g vo l t a m m o grams of these three elements is that o n ly a si n gl e c urre n t peak which arises from the further red uc t io n of th e depo sited a n alyt e to As3-, Se22 or Te - , respectively, is o bs erve d . In this case t h e d ete r mi n ati on b y CSV is more selective than by ASV, s inc e in th e a n od ic di s so lut i o n addi ti on a l cu rrent signal s are obtained: these arise from t h e oxidation of the copper an d po ssib l y also of mercury [ 3 7 ] . Typ i c al stri pp i n g voltammograms for the de t er m in atio n of arsenic by both ASV and CSV after de p os i t i o n from a co pp er - con tain in g solution are shown in F i g u re 3 .6. The determination limits for the CSV a n a lys i s of As, Se or Te in th e presence of copper were fo u n d to be 0.5 fig L-1 fo r As a nd 0.2 flg L- for Se and Te. Note that o nl y As (Ill) is det e rmi n e d since under t h e s e exp er i m en t a l conditions As(V) is not electroactive. The small amount of arsenic present in water s amples i s m ain ly in the +5 oxida t io n state as a result of o xi dati on by oxygen . T h i s must fi rst be reduced to As(III ) with hydrazine or sodium s ul fi te before pr o c ee di n g with the voltammetric determination [ 38 ] . Recently it has been found tha t if the test s ol ut i on contains D-Mannitol, As(V) can be reduced ele ct rolyti c all y to As( O). This is the basis of a m eth od for determining As(V) in the presence of As (III) by cathodic st rip p i n g vol ta mm e t ry [39] (chapter 6 ) .

1

3 . 2 . 2 Adsorptive accumu lation A wide range o f orga n ic m o l ecul es a n d m etal c hel a te co m p l exe s are surface active and can be accumulated on the surface of a mercury e l e c trod e by adsorp tio n processes rather than by a c h a rge t ra nsfe r ( o x i d a t i o n o r r e du c t i o n ) process. When the adsorbed compounds can be oxi­ dised or reduced, they may be e s t i m a t e d by stripping t h e m from th e el ectrode surface using linear sweep, staircase or differential pulse voltammetry. T h e technique, referred to a s adsorptive stripping volta m m etry, AdSV, is a powerful method for deter m in i n g ultra-trace levels of a v a ri ety of m e t al ions ( as their chelate complexes) , organo-metallic and organic compounds [ 1 , 40-43 ] . It has grea t ly exte n d ed t h e a ppl i ca b i l i ty of st r ippin g voltammetry to the a n a lysis of any el e m ent o r co mp o u n d that ca n be adsorbed onto an electrode surface and which can be reduced or oxi­ dised within t h e potential range a vail abl e wit h th e p a rt i cul a r working electrode-supporting el e c ­ trolyte and solvent combi nation used in the an a lys i s . This te chni q ue is particularly important in environmental studies for the determ i nati on o f trace and ultra-trace levels of m eta l ions that do not form a m alg am s with mercury or are n o t re a d ily d epo sit ed as the e l em e nt on a m e rc u ry elec­ trode. Adsorptive accumulation has also been used with t e n s amm etri c s t ri ppi n g for the d et e rmi­ nation of some n o n - e l ectroactive but su rfa ce active orga nic c o m p ounds . Two ap p ro a c he s have been u se d to effect the adsorption of metal ions onto the el e ctro d e sur­ fac e as the metal chelate. The simplest is to add an excess of a s u ita b l e comp lexi ng agen t t o th e t e st so l u t i o n p r i o r to the accumulation step. This is t h e most common ap pr o ac h and is used with m er­ cury, gold or p l at i n um working electrodes. A s ele c t i o n of frequently used c o mpl ex i ng agents is listed in Table 3 . 3 . An alte rna t ive approach is to m od i fy the s urfac e of the working electrode wit h

Stripping analysis

73

Comp l exing agents used for t h e determi nation of metal ions by a d sorptive stripping voltanunetry.

Table 3 . 3

Complexing agent

Elements

Reduction of the central metal ion

1 ,2-dihydroxybenzene (catechol)

1 ,3- b utanedionedioxime ( dimcthylglyoximc)

8- hydroxyquinoline ( oxi ne) 2-hydroxy-2,4,6-cycloheptatriene (tropolone) 2 , 5 -dichl oro - 3 , 6 - di hydroxy - I ,4-benzoquinone ( chloranilic acid)

U,

Cu, Fe, V, Ge, Sb, Sn, As

Co, Ni, l'd Mo, Cu, Cd, Pb, U

Mo, Sn

U, Mo, Sn, V, Sb

N-n itroso- N-phenylhydroxylamine ( cupferron )

U, Mo, Tl

(OCP)

Cc, La, Pr

Reduction of the ligand

o-cresolphthalexone

5-sulfo-2-hydrobenzeneazo- 2 -naphthol ( soloch rome violet RS - SVRS )

AI, Fe, Ga, Ti, Y, Zr, V, Tl , Mg, alkali

1 ,2 - d i hydroxyanth raquinone-3 -sulfonic acid ( alizarin red S)

AI

Chrom azurol B ( M B9, mordant blue 9 )

Th, U

and alkaline earth metals

Catalytic hydrogen evolu tion

Pt

Formazone

the complexing agent. The metal ion is then accum ulated by reaction with this modified surface. Electrode surfaces have been modified by physi cally coating the electrode wi t h a layer of an insol­ uble complexing agent (carbon electrodes), by chemically attaching a complexing agent to the electrode surface (platinum or carbon electrodes) , by incorporating a suitable complexing reagent into the polymer matrix of a polymer coated electrode, or by mixing the complexing agent into the matrix of a carbon paste electrode. Such electrodes often exhibit a high degree o f selectivity for a particular analyte. A metal-ion analyte is adsorbed onto the electrode as its complex at a fixed potential as for electrolytic accumulation and determined voltammetrically by the reduction or oxidation either of the central metal ion or of the ligand complexed with the metal ion . Organic analytes can be adsorbed in an analogous way and determined via the oxi dation or red uction ( as appropriate) of their electro active functional groups. The magnitude of the peak current, iP, obtained in th e linear scan voltammogram is deter­ mined by the accumulation time, tacc' the amount of the adsorbed species on the electrode surface, 2 r.d A (where r.d m ol em- is t h e surface concentration of adsorbate and A cm2 is the electrode area), and on the scan rate, v, V s- 1 . Furthermore, r.d is proportional to the concentration of ana­ lyte in the test solution, c., provided that the surface coverage is small (less than 50% ) . Assuming for the sake of simplicity that the electrode processes are diffusion controlled, then for a spherical electrode of radius, r, th e peak current in the stripping voltammogram is given by [ 44, 45 ) :

(3. 10)

where

k

=

n2 P v 4RT

For mercury drop electrodes, for which r i s approximately 0 . 5 m m, the first term i n equation 3. 1 0, the sphericity correction, is small for t"'" < SO s and has only a slight effect. Thus, iP is propor-

74

Introduction to Voltammetric Analysis

ti o na ! t o

the square root of tacc · This is not the case whe n 1 0 11m) are used as the first term then becomes the dominant term and iP bec o mes p rop ort ional to the first power of tacc S i nce the accumulation process involves the for­ mation of a film of adsorbed a na lyte on the e lectr od e surface, the surface concentration of analyte will reach an e q uilibrium value, (r.d)e ' determined by the so l uti o n conc e ntrat i o n , c If the solu­ q tion c on centra t i o n of the ana lyt e i s s u fficiently h igh , the val ue of r.d wil l be t h at for the s u rface be i n g saturated ( c o m p le tely covered) with the analyte, (r.d)sat· Correspondingly, the peak current should increase l ine a rly with tac c 1 12 fo r a gi ve n value of c. unt il the time is reached when r.d has attained its equilibri um value o r the su r fa c e becomes saturated. At this time, iP reaches its maxi­ mum value, ip( max)' which now remains con stant with acc u m u lation time and is given by c.

and approximately prop o rtional to

micro - e le ct rode s ( r -

•.

ip(m ax )

=

kA rad (eq )

(3. 1 1 )

If th e increase of i with c. at constant tacc is measured, a similar situation is observed. The peak P current increases l inearl y with increasi ng c. un t il the concentration that produces complete coverage in the chosen tacc is reached, at which point ip(max) ( corresponding to cr.d)sat) is rea ched and no longer increases with further increases in c• . Knowing the stripping voltage scan rat e and the value of ip (max)' rad(rnax) may be calc ulated and used to determine the surface area occupied by each an alyte molecule. This may g ive useful information about the structure of the layer of a n alyte adsorbed onto the electrode surface particularly when c. is such that surface saturation is achi eved [ 44 J . 112 Th e li n e a r re l a t i o n sh ip between ip and c. at constant tacc ( and b etwe e n ip and tacc for constant c.) fo r all val ues up to ip(max) i s an ideal situation. In practic e most iP versus c. calib rat ion curves have a shape similar to an ad s orption isotherm, that is they are e ss e ntially linear at the lower con­ centration ranges but become non-linear ( decreasing slope) at higher c o nc e ntrations as full s ur ­ face c over a ge of the electrode by the adsorbed analyte is approached ( see Figure 3. 7). Al though these c u rve s are u su al l y qu ite reproducible under fixed accumulation conditions, it is pre fe rab l e to carry out a n a l ys e s usin g the linear region of the ir versus c. pl o t . D e p a r t ure s from li nea ri ty observed at the high er analyte concentrations are overcome by us in g shorter accumulation times, by dilution of the sample solution or b y either using lower s ti rr in g rates or not stirring the solu­ tion at all du r i ng the accumulation step ( i . e . allowing the diffusion laye r thickness to increase) 1 • All these procedures either s i ngly o r i n combination lower the electrode surface coverage o f the adsorbed analyte from a given sample so that measurements can b e made in the linear range of the i versus c. p l o t . I n general, in AdSV short accumulation times in the range 20-60 s are used P I for the det e r m i nat i on of analytes in th e 0. 1 to 1 0 11g L- range. Too lo n g an accumu lation tim e n ot only re sults in a non-linear peak current-concentration p lo t but also affects the p e a k p o te n t i al s . This is illustrated by the voltammograms obtained in the presence of dim ethylglyo xime [47] as comp l exin g agent for the AdSV d eterm i nati o n of nickel and cobalt at different concentrations, usi n g different accumulation times. When a 30 s acc um ulat io n time is used t he p ea k p o tent i a l s are co nst ant and linear iP versus c p lo t s are obtained. H oweve r when a 1 20 s accumulation time is used, the nickel peak moves to more n ega tiv e po t en ti al s and iP for the cobal t peak only increases slightly on the addition of standard amounts of each m e tal io n (Fig ure 3 . 8 ) . Th e a dso rp tio n of neutral m olecules i s governed b y the adsorption energy o f the surface active molecule at the e l e ct r o d e-s o l u tion interface. In electroanalytical chemistry, it is the s p e cific adsorption of the analyte wh i ch takes p l a ce within the compact part of the do uble l aye r (see Figure 1 .6, § 1 .4.3 ) that is imp o rtant . The adsorption process involves the re placem ent of solvent In many ca ses the determination can be carried out a l higher concentrations in the und iluted solution a nd without accumulation using l i near sweep voltamrnctry at an HMDE or d ifferenti al pulse polarography a t a DME or SMDE ( chapter 2 ) .

Stripping analysis i

1 00

75

[nA] c

80 60

b a 0

Figure 3.7

20

40

60

1 00

80

1 20

1 40

1 60

1 80 t

[s]

Relationship between accumulation time, analyte concentration and peak cu rrent for the determination of 5-fluorouracil in a supporting electrolyte of pH 7.8. 5-fl uorouracil concentration a - 5 x 1 0 7 M ; b - 1 0-6 M ; c - 5 x 1 0-6M [46). -

1 20 nA

I

Accumulation t ime: 30 s

Co

1 20 nA

I

Accumulation ti m e : 1 20 s

Ni

- 0 . 85

Figure 3.8

E

[V]

- 1 . 2 35

- 0 . 85

E

[V]

- 1 . 235

The effect of different accumulation times on the adsorptive stripping volta mmograms of cobalt and nickel u sing a H M DE. Sample solution: 2 0 119 L-1 Ni and 20 11g L-1 Co in 0.1 M N H3-NH4CI (pH 9.3) + 5 x 1 0 -4 M dimethylglyoxime. Standard additions: each 200 ng Ni plus 200 ng Co.

molecules on the electrode surface by analyte molecules. The amount of adsorbed analyte that is required to fully cover the electrode surface to give a monolayer coverage depends on the size of the adsorbed molecules and their orientation with respect to the surface. The extent of adsorption of the analyte and, in th e case of asymmetric molecules, its orientation, is related to a range of fac­ tors such as the solubility of the analyte ( low solubility often enhances adsorption ) , the hydropho­ bic nature of the analyte, electrostatic interactions between ionic analytes and the charged

76

I ntroduction to Voltammetric Analysis

el e c t r o d e ,

di p ol e interactions between polar po rt i on s of the analyte and the electric field o f the double layer, and t h e ch emisorption of certain con s tit u e n t atoms of the a n a lyt e onto the surface of the electrode mater i a l . The s ign an d magn it u de of the c h arge on t h e electrode surface, which is r e l at ed to th e el ect ro de p o t e n t i a l b y eq u a t i o n 1 . 1 4a, d e term i nes the mag nitude of the a d s o r p t ion energy a n d degree of ads orpti o n of the analyte. This h a s been the s u bj ec t of s t udy by m a n y w ork­ ers [48, 49 ] . In ge ner a l , neutral molecules tend to be a d so rb e d at p o t e n ti a ls in the re g io n of the p ot en tia l of zero charge, Em, ( see § 2 . 1 . 5 ) , that is when t h e el e ctrode c h ar ge is s m a l l and the electric field i n the double layer is r ela t iv e ly weak. Anionic molecules and those con tai ning high electron d en s i ty centres such as a r om a t ic rin g s , are general ly ad so rb e d at potentials wh ere the electrode is p o s i t ively c h a rge d while cationic molecules are more likely to be adsorbed at p o te n ­ ti al s mor e negat i v e than Ezc· P o l a r centres in a m o lec u l e also infl uence the p o te nt ia l range over wh ich a molecule is adsorbed and in many cases the o rie n tat i on the adsorbed mo l ecule adopt s at a p a r t i c ul a r po te n tial [ 50, 5 1 ] . When m et al ions are be i n g accumulated as th e i r complexes, the metal-ion c omp l exes nor­ ma lly d is p l ace th e co m pl exi n g ag ent , HL, fro m the surface of the ele c t ro de , that is ML11 is m ore strongly adsorbed than H L. There are several po ss i b le mechanisms by wh ich a m e t a l may be accu­ m ul ated on the el e c tro de surface as its complex: 1

The anal yte forms an uncharged co m p l ex with the ligand L- in solution

and the c o m p l ex is adsorbed onto the electrode surface. M Ln(soln) � ML n(ad
The complexin g agent is adsorbed onto the elect r o d e s u rfa c e

H L(soln) � H L (a c.lsorhed)

and the c o m p l ex is formed

M ;�:In l

3

on

the e le ct ro de surface.

+ nH L (ads o rbedl �

n H tsoln l

When the a n alyt e M "+ does not form a s urface active compound with t h e com plex ing agent but c a n be o xi di s ed or reduced to a n oxidation state

which docs form

a

c o m pl ex either in s o l u t i o n

l M(n±m + 111 )HL (soln or adsurl>ed ) ( so l n ) + + ( n 4

ML n(adsorhed l +

or

..___ ML

�

on the el e ctro d e s u rface .

( n±m )( adsurbed )

The analyte M "+ e xists in a form in so l u t i o n which so must first be re d uced to the metal

pl exi n g a g e n t

M (,+soln l

+

ne-

..___ �

M (oHg)

Of

+ ( 11 +

does not

M "(su rface)

rn

)H +

re a d ily react with the com ­

Stripping analysis

77

and then re-oxidised to a valence state which forms a complex on the electrode surface

Mo

+ mHL
�

ML,,adsorbcdi + mH + + m e-

( m may or may not be the same as n ) . A n important application of adsorptive stripping voltammetry i s t o t h e determination of trace and ultra-trace metal ions in environmental aquatic samples [ 5 2 ] . A few examples are given in detail in chapter 6 and a selection of applications is listed in Appendix l . The detection limits listed are in the middle to lower ng L- L range. These must be taken as indications o n ly as they depend on the experimental conditions used, especially the accumulation time used. AdSV is thus one of the most sensitive of instrumental analytical techniques. Care must be taken throughout the whole analytical procedure-from the initial sampling to the final current measurement-to avoid contamination and losses if the accuracy of the results is to be assured. This is particularly true when determining omnipresent metals such as iron and lead. The adsorption of metal complexes on the electrode surface is, with few exceptions, subject to interference from other surface active species that may be present in the sample and which com ­ pete with the metal complex for surface adsorption sites. Unless the sample has a very low organic content such a s is the case with many sea water, drinking water and the cleaner ground water sam­ ples, it wi ll be necessary to pre-treat the sample in order to destroy the organic matter. Therefore all waste water, seepage, river water and other contaminated water samples must be pre-treated using oxidative microwave digestion or U.V. photolysis. A lso, all organic matter must be destroyed in samples being analysed for metals such as Pt and Ti using methods based on catalytic effects as these methods a re particularly sensitive to surfactants. The exceptions are systems in which the metal complex is adsorbed in a different potential region to that of the impurity surfac­ tants in the sample or is much more strongly adsorbed than these surfactants. One of the few examples of such behaviour is found in th e determination of uranium in con­ taminated river water without pre-treatment using chloranilic acid ( 2 ,5-dichloro-3,6-dihydroxy1 ,4-benzoquinone) as complexing agent. In figure 3.9, the stripping voltammograms of the same sample of water from the Elbe river using catechol ( a ) and chloranilic acid ( b) as complexing agents [ 53] are sh own. In the former there is non-specific adsorption of electroactive surfactant impurities competing with the adsorption of the uranium-catechol complex. While it was neces­ sary to usc a potential of -Q. I V (versus Ag/AgCl, 3 M KCl) to accumulate the catechol complex, it had been established from an a.c. polarographic study ( Figure 3. 1 0) that the c hloranilic acid complex could be accumulated at potentials in the range +0.05 to + 0.2 V. This potential range is outside that in which the orga nic material in water samples is normally adsorbed , that is at poten­ tials more negative than 0 V, and so only the urani um-chlorani lic acid complex i s adsorbed . Sim­ ilar behaviour is observed with the uranium-cupferron complex which can also he accumulated at potentials between + 0 . 05 V and +0.2 V and so is little int1uenced by the presence of extraneous organic matter in the sample [ 5 5 ] . Unusually h igh sen sitivities are observed with detection limits i n the n g L-1 range when the stripping of the adsorbed metal complex involves a catalytic reaction . Examp les include the deter­ mi nation of Pt as the formazonc complex [ 5 6 J and the determination of Ti as its mandelic acid complex in the presence of chlorate [57] . The determ i nation of Cr(VI) as the Cr ( I I I ) diethylene­ triam inepentaacetic complex in the presence of nitrate [58 J with and without initial oxidation of the sample enables both Cr(VI ) and Cr( I I I ) to be determined in the sample ( see chapter 6 ) . Other examples of speciation analyses using AdSV i nclude the determination of Sb( T I I ) in the presence of Sb(V) in the presence of chloranilic acid [ 5 9 ] and the determination of tributyltin and its

78

Introduction to Voltam metric A n a lysis a

b

2 nA

� OH

HO

�OH

- 0. 1

Figure 3 . 9

- 0.2

- 0. 3

E

- 0. 4

[V] (vs. Ag/AgCI, 3

- 0.5

- 0. 6

- 0.7

M

Cl

- 0. 8

E

KCI)

J)c 0

I

[V] (vs. Ag/AgCI, 3 0

- 0. 1 5

0

M

I

CI

OH

KCI)

Determination o f u ranium in water from t h e E l b e river without a n y sample pre-treatment: (a) in the presence of 1 x 1 0-4 M catechol at pH 4.5; (b) in the presence of 2 . 5 x 1 0-4 M chloranilic acid at pH 2.7.

i [nA] 300 Adsorption peaks c

EP

=

+ 0. 08

V

200

1 00

0.2 0.1 0 - 0. 1 - 0.2 E [V] (vs. Ag/AgCI, 3 M KCI)

Figure

3.1 0

Tensammetric curves for the uraniu m-
Stripping analysis

a

SW

I

40

79

nA

b

I

DP

50

nA

- 0.4

Figure

3.1 1

- 0 .6

E

- 0 .8

[V] (vs. Ag/AgCI,

3 M

KCI)

Comparison of the stripping voltammograms of riboflavin (vitamin 82) in 0.01 M NaOH using (a) square-wave mode (scan rate 80 mV s-1 ); (b) differentia l pulse mode (scan rate 1 0 mV s-1); and (c) l inear sweep mode. E lectrode, MTF disc.

metabolites dibutyltin and m o n obut yl ti n i n a d m i xt u re us in g t ro p olo n c as c o mp l exi n g a ge nt (F igu re 7 . 1 4 ) [ 60 ] . In most cases, the current peaks measured in AdSV of metal c o m pl e xe s arise from the reduction of th e central metal atom. There are examples h o w eve r in vo l v i n g metal ions that are not, or only with diffi c u l ty reduced in aqueous media, which may be det e r m i n e d by me asu r i n g the c u rre n t that is p r o d u c e d by the r e duc t i o n of the ligand rather than the m e t a l ion in the complex. Exa m p l e s are listed in Table 3 . 3 . N o r m a ll y s i n gle an a lyt e s are determined by AdSV. However, wh e n the reduction p o t e n t i a l s o f m etal c o m pl ex es with a par t i c u l ar complexing a g e n t are s uffici ently different from each o th e r multi-elem ent ana l yses are p o ss ib l e [ 6 1 ] . Alterna­ t ivel y by u si n g two or m o r e c o mp l e x i n g agents t o g eth er it is p o ssi ble to de t erm i n e several ele­ m en t s simultaneously [ 62 ) . Ad s o rp t ive stri p p i n g voltam metry is a l s o a very sensitive method for the de te r m in ation of ultra-trace l evel s of a ran ge of organic molecules which c o n t a i n electroactive functional groups. The d eterm i n ation limits a re of t he order of 1 0- 10 M a n d lower. In p rac ti ce mercury electrodes are used for the a ccumulation and determination of o rg a nic c o mp o u n ds with r ed ucibl e functional groups, whil e those with oxidisable fu n c ti o n al groups are accumulated and determined u s i n g noble m etal or ca rb on electrodes. Dyestuffs, insecticides, phytopharmaceuticals, pharmaceuticals, biological c o m p o un ds vitamins, hormones, nitro-compounds, benzodiazepi nes and numerous other c o m p o un ds m ay all be determin ed by AdSV. Examples are given in ch a p te r 7 and lis t e d i n ap p en di c es 4 a n d 5 . Str i p p i n g voltammograms are us ually recorded u s i n g t h e differential pul s e mode. With adsorptive accumul ation, the a n al yte is pres en t on the surface of the electrode and does not dif­ fuse into the electrode as is the case wi th the re d u c t ive accumulation of metals on t h e HMDE when the analyte fo rm s an amalgam . Hence in th e stripping p r o c ess the analyte is i mm e d i a t e ly ,

,

,

,

,

,

80

Introduction to Voltammetric Analysis

and totally available and, since diffusion from inside the electrode is no longer a factor, faster volt­ age sweep rates can be used. The current peak is then proportional to the sweep rate ( cf. thin film electrodes ) . By using square-wave voltammetry at high frequency to strip the adsorbed molecules from the electrode surface increased ir: to ( ratios ( an d hence higher sensitivities) a re obtained [ 61 J compared to the response obtain ed when using a differential pulse or linear voltage scan. This is illustrated in Figure 3 . 1 1 where the stripping voltammograms obtained for the determina­ tion of riboflavin using a mercury thin film electrode and different stripping voltage waveforms arc compared [ 63 ] .

3.3 CHRONOPOTENTIOMETRIC STRIPPING ANALYSIS

As is the case with voltammetric stripping methods, the sensitivity of chronopotentiometric anal­ ysis (§ 2.6) is markedly increased wh en the analyte is first accumulated on or in the working elec­ trode and then stripped (dissolved) from the electrode by means of a controlled current. In chro twpo tentiornet r i c stripping analysis, CPS, the analyte is first accumulated either electrolytically or by adsorption as for stripping voltamm etry. A constant current is then passed through the cell to strip the analyte from the surface and the change in potential of the working electrode during the stripping process is recorded as a fu nction of time. In essence, the technique is the inverse of the normal chronopotentiometric experiment ( m aterial being removed from the electrode rather than being deposited on it) . The time required to completely strip the deposited analyte from the working electrode, the transition time, 't, depends on the concentration of the analyte and so is the analytically usefu l parameter. It is analogous to the peak current, iP, that is measured in a stripping voltammogram. From the comparison of the two responses depicted in Figure 3 . 1 2, it can be seen that while Erw the quarter-transition time of the stripping chronopotentiogram, is not identical to Ef 1 12 as is the case in normal chronopotentiometry, it may be used to identify the species being stripped, and so corresponds to, but is not equal to, the peak potential, EP, of the stripping voltammogram. In CPS, the most commonly used electrodes are the HMDE and the TFME. The transition times using these electrodes for the CPS of metals which form amalgams wi th mercury arc given approximately by [ 64 ] : ( n Fcamal rd )

3 jo

( 3. 1 2 )

for the HMDE, and for the TM FE by 't TFME

where

=

-. ""'-­

( n F cama l l ) Jo

(3. 13)

camal

is the amalgam concentration after accumulation ( given by equation 3.2), " is the radius of the drop, l is the thickness of the mercury film on the electrode surface, and j0 is the current density during the stripping process. '

Since, under constant accumulation conditions ( fixed accumulation time, stirring speed and electrode size) carna l is proportional to the analyte concentration, c. , (equations 3 . 2 , 3.3 and 3.4 ) , it follows from equations 3 . 1 2 and 3 . 1 3 that the chronopotentiometric stripping transition time is also directly proportional to '� for fixed accumulation conditions. This should be contrasted with the case in the direct chronopotentiometric determination of an analyte where it is the square root of the transition time that is proportional to the analyte concentration in accordance with

Stripping analysis

_, /

i

[JlA]

ASV

"

"

81

/ //1

L-- �ip

E

[V]

CPS

t

Figure

3.12

[s]

Comparison of the traces obtained for the anodic stripping voltammetric (ASV) and the chronopotentiometric stripping analysis (CSA) of a metal which amalgamates with mercury. Note EP � E,14

the Sand equati on ( equation 2.44 ) . The difference arises because the transition time in the strip­ ping method is the time required to completely strip the analyte from the electrode surface, whi le in the direct method, where only a small fraction of the analyte in solution is reduced or oxidised, it is the time required for the concentration of the diffusing analyte to reach zero at the electro­ lyte-electrode interface and diffusion control to be established in the cell solution. Another important difference between CPS and chronopotentiometric analysis, CPA, occurs when several electroactive species are present. In CPS, the entire amount of each species is stripped off the electrode before the working electrode potential shifts to that at which the next species starts to pass back into solution . This means that in CPS the transition times of the various species are not affected by the presence of other species which are more easily stripped from the electrode. On the other hand, in CPA, once the transition time for the first species has elapsed and its surface concentration becomes zero, that is diffusion control is attained, the electrode potential shifts to where the next species begins to react b ut the first species continues to diffuse to the elec­ trode surface and consume some of the current. This means that only a portion of the current is available for the electrolysis of the second and subsequent species so th at their transition times a re enhanced by the presence of the first species in CPA ( § 2 . 6 ) . The compl ete removal of o n e arialyte from the electrode before the stripping o f a second ana­ lyte commences results in an improved selectivity for CPS compared to voltammetric stripping techniques where the time required to completely remove the first analyte, especially when the HMDE is used, leads to tailing of the peak and overlap with the next peak when these are close together ( § 3 . 2 . l . l ) . This is pa rticularly so when the first an alyte stripped from the electro de is

82

Introduction to Voltammetric Analysis

much higher concentration than the se c o nd For exa mple lead may be dete r m in ed in a 500-fold excess of cadmium by CPS whereas o n ly a ten-fold excess of cadm ium i nterferes with the ASV determination of lead, and mercury m ay be determined in the p rese nce of a ten fo ld excess of bismuth by CPS even though their st ri pp ing potentials are only 40 mV apart [ 65 ] . In CPS, the important factors which affect the determination limits are the time o f the elec­ trolysis in the accumulation st ep as is the case for voltammetric s t rip p ing and the current density during t h e stripping step. As the current density decreases, the transition time increases ( equa­ ti ons 3 . 1 2 and 3 . 1 3 ) and so the determination becomes more sensitive. However, the difficulty in accurately determining short transition times and the increase in noise (proportion of the applied current that is required to charge the working electrode ) as the current is decreased means that the l i m i t s of detection for CPS are about I o-s mol L- t [ 66] . present at a

.

,

-

3 . 3 . 1 Data presentation The stripping chronopotentiogram o bta in ed for t h e determination of cadmium, thallium and lead in admixture using a TF M E pr ep a r ed in situ is shown in F i gure 3 . 1 3 a [ 67 ] . This data is pre­ sented as is n orma l in chronopotentiometry, namely as a plot of p o t entia l versus time. The transi­ tion times required for the quantitative measurement of the concentrations of the analytes are the times between the inflection points that mark the onset and completion of the electrolysis of the different analytes ( 1 , 2, 3 a nd 4 ) . I n s p ec tion of curve a reveals the difficulty of identifying these inflection points between the stripping waves of the different analytes , particularly when the strip­ ping potentials are close. More accurate transition times can be obtained by making use of the fact that, as in CPA (§ 2.6) , when t 0 and t 't , the plot of E versus t is steepest so that t h e first derivative, d£/dt, will 2 be a maximum and the second derivative, d E/d r will be zero . The second derivative plot, d 2 E/d r versus time, i s pre fe rred for measuring the tra n sit ion times required t o strip the analytes in a multi -component sample. This is particularly so if the stripping potentials of the analytes are c l os e together, or if one analyte i s present in a m uch lower c o n ce n trat i o n than the others. As Figure 3 . 1 3 b s h ows , the transition times are readily obtained from the times where d 2 E/d r crosses the time axis. Processing of the potential-time data so that it is presented in t he form of t h e first or second derivative p l o t is a simple operation for modern microprocessor controlled equipment, as is the detection of the time at which the value of the second derivative changes from positive to negative on c rossin g the time axis. When the data obtained from the constant current strippin g process is presented either as d E/dt versus time or as d2 E/d r ve r su s time, the technique is referred to as Deriva tive Chronop oten tiometric Stripping Analysis, DerCPS. Details of the equipment used for DerCPS which also automatically determines the transition times is to be found in reference 67. Wh e n measuring short transition times or determinin g an aly tes with closely adjacent transi­ tion ti mes, the results obtained by the automatic treatment of the data are found to be more reproducible th a n those obtained by the manual analysis of E versus t plots. An alternative way of processing the primary potential-time data is to present it as a plot of dt!dE as a funct io n of potential [68 ] . This way of presenting the data has led to a s i gn ifi ca nt improvement i n the selectivity and a marked improvement in the sensitivity o f CPS. In order to distinguish this method of presenting the data from derivative CPS the name Differen tial Ch rono ­ poten tiometric S tripping A nalysis ( DifCPS) h a s been suggested fo r the t ec hni qu e in which the data is presented in the form of d t/dE as a function of the po t entia l of the working electrode as the ana­ lyte is being stripped with a c onst a nt current [68 ] . In F i g u re 3 . 1 4 t h e differential CPS plot obtained for the analysis of Co a n d Ni after adso r p ti ve accum ulation is compared with the normal chronopotentiogram . Garai et al. [ 6 8 ] have deve l op ed the theo r eti c al basis of differential chronopotentiometric stripping analysis, the essential features of which are summarised below. The differential function d t/d£ is given by =

=

Stripping ana lysis

E

83

[V]

- 1 .0

- 0.8

- 0. 6

- 0.4

- 0 .2

0

b

Ze ro l i n e

I I" I I I I I

Figure 3 . 1 3

1

I I I ..., :1( )I I I •I I 0.83 s 0 . 33 s I I I I �.23. s I I

2

I

3

t

[s]

I

4

Simu ltaneous determination of Cd(l l) (5 x 1 o-7 mol L-1 ), Tl(l) {2 x 1 o-7 mol L-1 ) and Pb(l l) (3 x 1 0-7 mol L-1 ) by stripping chronopotentiometry using a TFME. Cell solution 0.05 mol L- 1 HCI + 0.5 mol L- 1 NaCI c o ntaini n g 1 00 mg L-1 Hg{l l ) . Ele ct ro l ysi s time: 1 0 m i n ., stripping current 1 0 IJA. Curve a - potential vs time plot. Cu rve b - second derivative, d2 £/d t , vs time plot [67] .

(3.1 4)

which

has a maximum

value ( 6 - 4 Ji ) nPr RT

=

0

.

343 n Ft

The potential at which thi s maximum occurs for a TFME is

RT

3 ( . 1 5)

84

I ntroduction to Volta mmetric Analysis a

E [V] (vs. SCE)

b

dt dE

- 1 .1 - 1 .0

[sV -1 ]

Co Ni

- 0.9

t--i 1s

- 0.5 - 1 .1

Figure 3. 1 4

E [V] (vs. SCE)

- 0. 9

Stripping chronopotentiogram - f vs t (a) and the d ifferential chronopotentiogram d tld f vs E (b) for 1 IJgl-1 each of cobalt and nickel accumulated as their dimethylglyoxime complexes at •

-0.5V (vs SCE) for 65 s fr o m pH 9.2 buffer and stripped into 5M CaCI2 using a current of 1 IJA. Working electrode MTFE [69] .

Emax

=

P

+

RT{ tmax• nF

1n ( 2 l( D1t )

- 1 12

112 1 [2 + 1] 1 } ) + 2 ln --- - 2 ln 't 2

0.414't.

(3.16) =

0

Furthermore, d t/dE should be zero at t and and the time a t which i t occurs, t 't, that is at the onset and completion of the s t r ipp i n g process, respectively. From the analytical viewpoint the most i m porta nt feature is that the maximum value of d t/d E is di r e c tly proportional to the transition time and hence to the concentration of the an al yt e ( equa­ tions and Other fe atu r es are: (i) The baseline arises as a result of the el ec t ro d e charging process and is close to zero; ( ii) Emax is affected by 't ( e q u ati o n and hence by the analyte concentration. This change in amounts to approxi mately 1 5 mV for each ten-fold change in the concentration of an analyte for which n = 2. (iii) The peak half-width is equal to 66. 1 / n mV and should be c o m p a red with 90/ n mV for the half-width o f p eaks in fundamental a.c. polarography and in differential pulse polarography. That is, the peaks obtained in differential CPS are m uch sharper than those obtained in the other techniques and so a better resolution of analytes with similar st r i pp ing p o tenti als is achieved with chronopotentiometric str ip p in g . In contrast to voltammetric stripping, the determination of t ra ce elements in aqueous samples by chronopotentiometric st ri pp i n g does not re qu ire pre-treatment of the sample with U.V. irra­ d i at io n or m icr owave d igesti o n in all cases, since the presence of o r gan ic material only sl i gh tly interferes with a CPS det e r m i nat i o n . It is o n ly water sam ples with a very high o r ga n i c content that may re q ui r e any pre-treatment, and in m ost cases simple d ilu ti o n of the s am pl e is all that is required. While the above d i s c u ssion has been focused on the constant current stripping of metals that have been accumulated by ele c tro lyti c reduction at a m e rc u ry electrode, the same pr i n ci p le s apply to system s which have been accum ulated by adsorption and are st ri pp e d from the working elec=

3.2, 3.4, 3.12

3.13).

=

Emax

3.16)

Stripping ana lysis

-

1

J

'

85

Potentiostat

Potentiam eter

Pt

Jf

Auxi l ia ry electrode

......_

�

Wo rki n g electrode

i

Reco rd e r

Reference

Figure 3 . 1 5

e lectrode

Bas i c experi menta l arrangement for potentiometric stri pping analysis [72] .

trode by either an oxidation or reduction pro c e ss , or to sys t e m s which use electrodes o t h e r than mercury electrodes. The major p ri n c ipl e in all v ar ia t io n s is that both the transition time and th e h eight of the d t/ d E peak are p r o p o rt i o na l to th e concentration of the a na lyt e p r ovi d ed constant accumulation and stripping c o nd itio n s are maintained. The reader should consult the l iterature for the specific mathematical re la ti o n sh i p s that apply to the electrode-accumulation-stripping

pa rt i c ul a r a n a l ysis . In order to obtain the h i ghest po ss i b l e sensitivity in CPS a n d to avoid s y s t e ma t ic errors, the test solution must not contain any oxidising impurities such as oxygen and Hg( I I ) . Therefore it is necessary to p rep ar e m ercury th i n film e l ect ro de s ex-situ, to remove as c o m ple te ly as possible the oxygen d i ss o l ve d in the te s t solution and take all precautions to prevent t h e d e - o xyge n a t e d test solution being exposed to atm o s ph e ric oxygen. These p r e c a u t io ns are not appl i c a b le if a c h e m i c al rather than electrochem ical method i s used to strip the e n ri c h ed clements from t he electrode. regime to be used for a

3.4 POTENTIO M ETRIC STRIPPING ANALYSIS

When chemical sp ecie s present in excess in the cell s o l ut i o n are used to strip t h e accumulated ana­ lyte from the electrode, the method i s referred to as potentiometric stripping analysis, PSA. In PSA, the working electrode i s u s ual ly the TMFE. The most common oxidants used for the stripping step are either Hg2+ ( added to prepare the TMFE in situ) or the o xy gen no rm a l ly present in an air­ equilibrated test sol utio n . Other oxid ant s that have been used incl ude KMn04 and Au ( I I I ) . Th e re are a few reports of reductive st r i p pi n g using s o di um or zinc ama l ga m s or hydroquinone as the c h e m ic a l r ed uc tan t s . Chemical s t ripp i ng a n a l ys i s , the fo re - run n e r of PSA, was first described by B r uc ke n st e i n [ 70 ] who used t h e t im e re qui re d to strip a given q ua nt ity of silver from a p latin u m electrode as a m e th o d of de t e r m in i ng low concen trations of strong oxidants. The chemical equivalent to the constant current flow in CPS is the con stant flux of o xid a n t to the electrode whic h is proportional

86

Introduction to Voltammetric Analysis

E [V]

- 1 .2 Zn

- 0. 1 t [s) E [V] - 1 .2

Pb

Cu

- 0. 1

dt/dE [s v-1 ] Fi gure 3 . 1 6

Compa rison of the potentiogra m { f vs t) a nd the differentia l potentiogram {d t/d E) for the determ ination of zinc, cadmium lead and copper [74] .

to the oxidant concentration when diffusion control operates. J agner and co-workers [ 7 1 , 72 ] developed the technique under the name potentiometric stripping analysis and introduced differ­ ential PSA ( d t!dE versus E output) [ 73, 74 ] as a powerful method for the analysis of a range of

metals at the ultra-trace leve l . Beca use chemical stripping is used in PSA only simple equipment is required as a constant current source ( galvanostat) is not needed. A basic arrangement for PSA is outli ned in Figure 3. 1 5 . It comprises a potentiostat, a high impedance potentiometer (e.g. a pH meter) and a fast recorder ( either an oscilloscope or a transient recorder) . The E versus t trace can be evaluated by the graphical method ( see figure 2 . 2 4 ) .

Table 3.4

Summary of stri p p i n g techniques.

Principle

I Reduc tion to t h e metal

a

F o r m at i o n of amalgam

b Formation of a metal film

Reaction sequence

Enrichment

Determination

Examples

M" ' + ne- -+ M " ( Hg)

M0(Hg) · - • M "+ + n e

Cd, Pb, Tl

M".,. + m.'- -+ M0

IJ Electrochemical formation of an insoluble

a

d e po s i t

-

By oxidation

Ill a

Electrochem ical formation of i n soluble

salts with rea ge n t a dd it i o n

ny oxidation

b Ily reduct ion

IV Fo r m at i o n of an

oxidation of the electrode m a t e r ia l

V Formation of an i n s o l uble d ep o si t by co­ electrolysis with a n added metal

M "' + OH- -+ M ( OH ) ,

M ( O H ) ., -+ M1 " + 1 i + + e +

M ".,. -+ M 1 "+ t i + + eM 1 ".,. 1 1 .,. + ( n + l ) L- -+ M L : ,+ l i ( n+ l )+ + - --+ M II+ e

M ".,. + nL- -+ ML,

+ M E" -+ M E " + ne­

M �:"+

+

X' ... -+ M EX

J\.1 " + + Mb"'+ + .

( 11 + m ) e- -+ M.,0l\1b0

VI Adsorptive a c cu m u l at i o n

a

Directly from solution

b E n r i ch me n t by complex formation

Au , Ag, H g (C electrode)

ne-

M (OH ) ., .,.

M

i n soluble deposit with

+

J\.1".,. -+ lv11 " ' 1 i + e

M 1 " ' 1 1 + ( n + l ) OH- -+ M ( O H ) 1 ,.,. n (n l l ) l - -+ " . M M + e

b By reduction

M" -+ J\.1 '"

An ( soln: ---+ An : dl·rlnldl·)

M ".,.

+

nHL -+ M L,;clw > + nH-

'>+ 1 + e -+ M + ( n + ! ) OW

nOH

" ML, ,. 1 1 + e- -+ M' + ( n + l ) L-

ML, -+ M 1 " ' 1 " + e- + nLM EX + n e- -+ M E " + x"-

M.,"Mh"

An 1 c1"'"

+ xe- -+ M,x-

+

+

Fe( JJJ)

Co + 1 -nitroso-2-naphthol

Sn + d i cthyldithiophosphatc

c o m p o und s ( M E" = Ag or Hg

Mh"

ne

Oxidation or reduction of either the met<1l or the l i ga n d

M n , Tl, Cc, Pb

51-, cr, CN-, Br-, o rga nic SH­

ne- -+ An ',_ or

A nlele.:t ) -+ An"� +

(Hg electrode)

electrode)

Se, Te, As(M, ) , C u (Mb)

�

-6· "t:l

c om p o un d s

Many electroactive organic Most metal ions with a variety of complexin g a g ents

:;·

<.Q OJ ::::J "'

� v; ·

�

88

Introduction to Voltammetric Ana lysis

The th eory of PSA i s similar to th a t for CPS except t ha t th e c u rre nt t e rm i n equa t io n 3 . 1 3 is replaced by the equi va le n t term for the flux of oxidant to t he electrode surface [ 7 5 ] . Accumulation may be either electrolytic or adsorptive as des c r i b e d above or i n combination where the analyte (insulin or flavin adenine d i nuc l eotide ) is electrolytically r e d u c ed and the re d u ced form adsorbed o n t o the mercury electrode. The adsorbed, reduced a n al yte is then stripped potentiometrically using oxygen as the o x i d an t [ 76 ] . In PSA, the proc e d u r e n ormally adopted for the determination of t ra ce metal ions is to first add Hg(II) to th e test solution and electrochemically generate th e mercury film a n d accumulate the elements to b e determined on a glassy c arb o n electrode by electrolysis at a co n st a nt p o ten t ial . The current circuit is then disconnected a n d t h e d ep o s i t e d elements chemically st ri pp e d from the el e c tro d e by the oxi d ant present in the solution (Hg (II ) ) . The de p o s ite d an alyt e s a re s tri p p ed in the order of their e le c tr o de pote nt i als and the variation of the p o t e n t i a l of the wo rki ng electrode wi th time is recorded, fro m which the transition times are obtained, or the heights o f the peaks in the d t/d E versus E p l o t determined as for d iffe ren ti al CPS ( Figure 3 . 1 6) . I n a s im i la r way , an od i ­ cally produced deposits can be stri p p e d by suitable chemical reductants.

REFERENCES 1 2 3

5

4 6

7 8 9 10 12

11

13

15

14

16

17 18 19 20 21 22

23 24

25 26

27 28 29

30 31

32

j. Wang, Stripping Analysis, V. C . H . , florida, 1 985. Kh.Z. Brainina and F.. Neyman, Electroanalyt ical Stripping Methods, j. Wiley and Sons, 1 993. C. Zbinden, Bulletin de Societe Ch imie Biologique, 1 93 1 , 1 3 , 3 5 . W. Kern ula and Z. Kublick, Analytica Ch imica A cta, 1 9 58, 1 8, 1 04. R.D. De Mars and I. Shain, Analytical Chemistry, 1 95 7 , 29, 1 8 2 5 . E. Barendrecht, Electroanalytical Chem istry, Vol . 2, A .J. Bard, editor, Marcel Decker, New York, 1 967, p. 53. Kh.Z. Brainina, Tala n ta, 1 97 1 , 1 8 , 5 1 3 . L. Duic, S. Szechter a n d S. Srinivasan, ]. Electroanalytical Chemistry, 1 973, 4 1 , 89. A.M. Bond, T.A. O'Donnell, A.B. Wa ug h and A.J.W. Mclaughlin, Analytical Chemistry, 1 9 7 0 , 4 2, 1 1 68. J . Wang, Environmen tal Science a n d Tech nology, 1 982, 1 6 , 1 04A. H.W. N iirnberg, Fresenius Z. A n a lytische Chemie, 1 983, 3 1 6, 557. G.E. Hatley and T.M . Florence, Talanta, 1 977, 24, 1 5 1 . R . Neeb, Angewandte Chem ie, 1 9 6 2 , 74, 203; Inverse Po larograph ie und Voltammetrie, Verlag Chem ic, Weinheim, 1 969. G.E. I:latley and T.M . Florence, ]. Electroanalytical Chemistry, 1 974, 55, 2 3 . A.M. Bond, Modern Polarographic Methods in Analytica l Chem istry, Marcel Dekker, New York, 1 980, Ch. 9 . V.G. Levich, Acta Physicoc lz emica , USSR, 1 9 4 2 , 1 7, 257. j.E.B. R a n dles , Faraday Society Transactions, 1 948, 44, 3 2 7 . A. Sevcik, Collection of Czechoslovak Chem ical Co m m u nications, 1 948, 1 3, 349 . W.T. d e Vries a n d E. van Dalen, /. Electroanalytical Chemistry, 1 967, 14, 3 1 5. U. E is ner, J.A. Tu rner and R.A. O st e r yo u n g , A nalytical Chemistry, 1 976, 48, 1 608. T.R. Copeland a n d R . K. Skogerboe, Analytical Chemistry, 1 974, 46, 1 2 57A. R.G. Clem, Analytical Chem istry, 1 9 7 5 , 47, 1 7 78. J.H. Christie and R.A. Osteryoung, Analytical Chemistry, 1 976, 4 8 , 869. J.H. Christie and P.J. Lingane, ]. Electroat�alytical Chemislry, 1 96 5 , 10, 1 76 . F. \Vhadat a n d R. N e eb , Fresen ius 7.. Analytische Chemie, 1 977, 288, 3 2 . T. R. Copeland, R.A. Osteryoung and R. K. Skogerboe, Analytical Chemistry, 1 974, 46, 2093. J . Wang, P.A. M . Farias a n d D. - B . Luo, Analytical Chemistry, 1 984, 56, 2 3 79. H . - 1. Haase, Electrochemische Stripping Analyse, VCH, Weinhcirn, 1 996. G. Henze and R. Neeb, Elektrochemische Analytik, Springer Verlag, Berlin, 1 986. G. Colovos, G . S . Wilson and J.L. Moyers, Analyl ica/ Ch emistry, 1 974, 46, 1 0 5 1 . I . Shane and S . Pe rone , A nalytical Chemis try, 1 9 6 1 , 33, 3 2 5 . R. Geyer, G. Henze and J. Henze, Wissenschajiliche Z. der Tech n ischen Hochsch u le Leuna/Merseburg, 1 966,

' [ M . Florence, ]. Elect roa na lytical Chemistry, 1 979, 97, 2 1 9. I.E. Davidson and W.F. Smyth, Analytica Ch im ica Acta, 1 983, 1 47, 5 3 . M . R. Smyth and J . Ostcryoung, Analyl ical Chemistry, 1 977, 49, 23 1 0 . E. Palacek, J. Ostcryo u ng and R.A. Osteryoung, !\ t� a ly ti ca l Che m istry, 1 9 82, 54, ! 389. 8, 98.

33 34

35 36

Stripping a n a lysis

37 38

40

39

42 41

43

89

G. Henze, M ik roch i m i ca Acta, 1 98 1 , 2, 343.

G . Henze, A.P. J o s h i a n d R. Neeb, Fresenius Z. Analytisclze Chemie, 1 980, 300, 267.

U. G re ul a c h and G. Henze, A n a lyt ica Ch imica Acta, 1 995, 306, 3 1 7.

C . M . G . van den B e r g, An a lytica Chim ica A c ta, 1 99 1 , 250, 265.

R . Ka lvod a , Instrumenta tion i n A n aly t ica l Ch em is try, J . Zyka, edi t o r, Ellis I Iorwood, New York, 1 994, Ch. 3 . J. \Va ng, Am e r i can Labomtory, 1 98 5 , 1 7, 4 1 . L. Novotny, Instru mental Metho ds in Analytical Chemistry, /. Zyka, e dit o r, State Publishing House, Prague, K. Kano, T. Konse, N . Nish i m ur a and T. Kub o t a , Bulletin of the Chem ica l Society of/apan, 1 984, 57, 2 3 8 3 . 1 989.

44 45

A.J. Bard and L . lt Faulkner, Electrochem ical Methods, Fzmda memals and App lica t io ns, J. Wiley, New York,

A.J. Miranda Ordi eres, M .J. G a rc i a Gutierrez, A. Cost a Garcia, P. Tunon B la n co and W.F. Smyth, Arzalyst, 1 980.

46

1 9R7, 1 1 2, 243.

47

48

49

Metrohm Application Bulletin No 2 3 1 / 1 e.

j. Lipkowski and P. N. Ross, Adsorption of Molecules at Metal Elect rodes, VCH, New York,

B.B. Dam askin, O.A. Petri\ a nd V.V. Batrakov, Adsorp tion of Orga n ic Co mpo un ds on Electrodes, Plenum, New 1 99 2 .

F.G . Thomas, Cl. Buess- Herman and L. G ie rs t , /. Electroanalytical Chemistry, 1 986, 2 1 4, 597.

York, 1 9 7 1 .

50

51 52

53 54 55

56 57 58 59 60 61 62 63

64

65 66

67 68 69 70 71 72

73

75

74 76

F. G . Thomas, ]. Electroanalytical Chemis try, 1 993, 349, 8. C.M.G. van den Berg, Analyst, 1 98 9 , 1 1 4, 1 5 2 7 . S. San d er a n d G . Henze, Fresen i l1s' f . Analytical Chemis t ry, 1 994, 349, 654. S. Sander and G . I I en ze , Electroa 1zalysis, 1 996, 8, 2 5 3 . S. S a n d er, Verhalten u n d Hirzfluss adsorbierbarer Molekule bei der voltammetrischen Sp urerzanalyse von Metal/en, Dissertation , University of Kaiserslautern, 1 995. Me t roh m Application Bull etin No. 220/2 d. K. Yokoi and C.M.G. van d e n Berg, A n a ly t i ca Cfl imica Acta, 1 99 1 , 245 , 1 67 . J. Golimowski, P. Valenta a n d H.W. Niirnberg, Frese n i u s Z. Analytische Ch em i e, 1 98 5 , 322, 3 1 5. W. Wagner, S. Sa n de r and G. Henze, Fres e n i u s ' /. Analy ti ca l Clzemie, 1 996, 354, I I . J. Schwarz, G. Henze and F.G . Thom a s , Freserzius' f. A na lytica l Chemistry, 1 995, 352, 474, 479. C.M .G. van den Berg, Analytica Chim ica Acta, 1 9 9 1 , 250, 265. C. Colombo and C.M.G. van den Berg, A na ly tica Clz imica Acta, 1 997, 337, 29. A. Eco nomou and P.R. Fielder, Electroanalysis, 1 99 5 , 7, 447. M . S . Zakharov, M . S . Pnev and V.I . Bakanov, Elek t ro khi nz iya, 1 97 1 , 7, 6 1 1 . F. Vyd ra and L. Luong, /. Electroanalytiwl Chem is try, 1 974, 54, 447. R. Neeb, Fresen ius Z. Analytische Chemie, 1 962, 1 90, 98. G. Henze, Frese ni us Z A n a lytische Clzemie, 1 98 3 , 3 1 5 , 43 8 . T. Garai, z . Nagy, L Meszaros, L. Bartalits, C . L oc at el li and P. Fagi oli, Electroarzalysis, 1 9 92 , 4, 899. H. Eski lsson, C. Haral dsson and D. Jagner, Arzalytica Chi m im Acta, 1 985, 1 7 5 , 79. S. Bruckenstein and j.W. Bixler, A n a lyti ca l Chemistry, 1 965, 37, 786. D. J a g n e r and A . Graneli, A nalytica Chi m ica Acta, 1 976, 83, 1 9 .

R. Bergland, D. Ja g n e r and L Renman, A n a ly t i ca Chi m ica Acta, 1 9 86, 1 88, 1 3 7 . A. Graneli, D. Jagner and M. Joscfson, Analytical Chemistry, 1 980, 52, 22 2 0 . T. G ar a i , L Meszaros, L. Barta! its, C . Lo c a t el l i and F. Fagioli, Electroarzalysis, 1 99 1 , 3, 955. M.J. Honeychurch a n d M.J. Ri dd, Electroanalysis, 1 996, 8, 49, 362. D . Jagner, Analytical Chemistry, 1 9 78, 5 0 , 1 924.

Practica l aspects

B e for e p e rfo r m i n g a polarograp h i c or voltam metric

d et e r m i nat i o n t h e ana l yt e must be p resent in

solution in a suitable chemical fo rm and fr e e fr o m in t e rfe r i n g c o m p o n e nts . Sol i d s a m p l es there­

fore must be di s sol v e d , and, like aqueous s amp l es , freed fro m interferents; the d i rect d etermina­

tion of the analyte b e i n g o n ly possible in a limited number of c a s es . The preliminary treatment o f t he s am p l e therefore, i s a necessary a n d i m p o r tan t st ep in t h e procedu r e for the polarographic o r v o lt am m e tr i c a nalys is

of most sam p l e s .

4.1 SOURCES OF ERROR In a p o l a r ogra p hi c o r vol t a m m etric pr o c e d u re , the method of s a m p l i n g , th e s a m p l e p repara­ t io n , t h e m e a s u re m e n t co n d i t i on s a n d t e c h n iq u e , and the evaluation of the m easured s i g n a l should be d es c r ibe d . In such a procedu re, each step is a poten t i a l s o u rce of random e r r o r s

and/or systema tic errors.

Syste ma tic o r determ inate errors a r e c o mm on ly unidirectional relative to the t r u e value, often difficult to re c og nis e , alt h o ugh with careful work their source can be i d e n t i fie d a n d t he ir m a gn i t ud e estimated. They m ay be n e a rly consta n t in a series of a n alys e s , for e x amp l e analyt e loss due to ads o rpti on on the voltammetric cell walls or i m p u ri ties in the s u p ­ porti ng electro lyte, or t he errors may be proportional in nature , for examp le a c he m i c a l l y s i m ila r i m p u rity in the o r i g i na l sample wh ich is difficult to s e par ate fro m the analyte. The s i gn i fi c a nc e of c o ns t an t errors increases as th e amount or concentration of the a n a lyt e de c re a s e s . S ys t e m at i c errors are often classified on the basis of their origin, instru mental, personal or procedural, tha t is they o r i gi n a te from the eq uip ment , from the in co m p et enc e of the analyst, or from the properties of the chemical systems involved [ 1 ] . In p o l aro g ra p h i c and voltammetric determina tions the m a in risk of sa m p le contamination arises d urin g sa m ple digestion and/or from the s upporting el ec t ro lyt e . At both points in the p r o ­ cedure l arge am ounts of chem icals are a dd e d to the sample. T h ere fo r e the che m i c a l s and water (or solvent ) used must be of the h i gh e s t p u r i ty, th at is A na l yt i ca l reagen t or S u prapur® g r a d e c h e m i ca l s, hi - d is till e d or Millipore®-pu rified water (hereafter re fe rr e d to as pure water) . Errors may be introduced w h e n p hy s ical and chemical p rocedures such a s extraction, distilla­ tion, sublimation or p r e c i p i t at i o n are used to s ep a ra t e a n d e n r ich trace a n al yt e s from the sam p le m a trix . Such errors a re l ess li k el y to arise i f th e a n a l yt e can be determined by the te c h n i q ue of stripping voltammetry, in which the an a l yte i s electrochemically s e p a rated from the sample solu­ tion and e n r iched on the wo rk i ng electrode su rface in the cell without u sin g l a rge amounts of additional chemicals ( see ch a p t e r 3 ) . A further advantage of this te c h n i qu e is that the separation may be more s e l ect i v e because it is potentiostatically controlled. A frequent s o u r c e o f sys te m at ic errors is contam inated vessels s uc h as volumetric glassware, th e cell, etc. The most e ffec t ive way to reduce these errors is to dean th e gla ss vessels by p l ac in g re p r od u c ib le and s o m e t i me s

Practica l aspects

Figure 4 . 1

91

Apparatus for the vapour cleaning of g lassware with HN03, HCI or H20 [2) : 1 heating mantel, 2 n itric acid, 3 ro u n d bottom flask, 4 soxh let adaptor, 5 vapour c h a m b e r 6 reflux condenser. ,

them in the vapour chamber of a vapour cleaning u nit through which nitric acid is refluxing as illustrated in Figure 4.1. When carefully cleaned vessels are used, errors m a y still arise through a loss of t h e analyte if i t i s adsorbed onto t h e vessel walls from t h e sample solution. T h e adsorption i s variable and depends on factors such as the pH and composition of the sample solution, and on the material and surface condition of the vessels used. While it is not possible to make universally applicable predictions about adsorption behaviour, what has been found in practice is that vessels made of quartz, PTFE (polytetrafluoroethylene) and PFA ( perfluoroalkoxy polymer) have the least ten­ dency to adsorb trace substances from solution and so are the best materials for making the ves­ sels used in trace analysis. Losses may also occur by volatilisation of the analyte du ring digestio n or evaporative concentration of solutions. Systematic errors can produce results which have good rep roducibility, that is good precision, but all have an error in the same direction , that is they have poor accuracy. This is particularly so in trace analysis and the accuracy of the data m ust be checked. The reproducibility of the analyti­ cal results depends on random or indeterminate errors which can be produced in sampling as well as during the preparation and measurement procedure. The greater these errors a rc, the greater the scatter in the values obtained for replicate analyses and the poorer the reproducibility. In ana­ lytical chemistry, the standard deviation, calculated by statistical methods from the experimental data, is used as the measure of the scatter in the results [ 3 J .

92

Introduction to Voltam metric Ana lysis

In the a b se n c e of other errors, random errors p ro du c e results scattered around the true value. When this scatter is large th e cause is u s ua lly personal errors a ri s i n g from the lack of care by the analyst . Through good l ab ora t ory pract i ce and the use of h igh qua l it y instruments gross errors are ra re ly made. The quali ty of modern instruments is s u ch that the m ea su r in g eq u i p m e nt ( i n c l u di ng the working electrode) in general co nt r i b ut es < 2% to the scatter in a vo lt am m et r i c or polaro­ graphic measurement . A further source of errors arises from the temperature dep e n den c e of pol arographic currents. The gre a te s t temperature effects o c c u r when the current is c o n t r olled by the kin et i cs of pre- or po s t - c h e m ic a l reactions associated with the electrode reaction (§ 1 .4 . 2 ) . Currents influenced by a d s o rp t ion process also h a ve a marked temperature dependence. In the case of d i ffus i on con­ trolled c urrents t here is a sm all but significant tem p e r at ure effect. An increase in te m pe ra t u re causes a decrease in the v i s c o sit y of the cell s olution and hence an increase i n the di ffu si o n coeffi­ cient and consequently in the d iffu s io n current (see equation 2 .4 ). T h e temperature coefficient of a p ol arogra ph ic current wi l l depend on t h e nature o f the electrode p roc e s s , lying be twee n + 1 . 5 to + 2 . 2 % per oc for a s im p le diffusion controlled process but b ei n g much larger for a kinetic or c at ­ alyti c current [ 4] . The t e mp era t ure coefficient of the p ea k current in stripping v o lta mme try is of a similar order of m agn it ud e . In t h i s case, when a sta tio n ary m ercu ry d ro p electrode is used, in addition to the effect of temperature on the diffusion coefficients, temperature ch a ng es during the measurement pe ri o d cause a change in the sur fa c e area of the mercury d r o p as a res u lt of the ther­ mal e xpa n s i o n of mercury. It i s therefore important to h o l d the t e m pe ra t u r e constant to better than ± O. I "C du r in g a series of polarographic or v o l ta mmet ri c measurements to obtain good reproducibility. The accuracy of the an a lyt ic a l data s ho ul d be checked e it h e r by c omp a ri ng the polarographic or voltammetric re s u lt s obt a i n e d for the test s a m pl e s with those obtained on th e same samples usin g a re fe re n c e method ( e . g . a to m i c abs o r p tion spectroscopy, inductively coup le d plasma-optical emission spectroscopy, neutron activation analysis ) or by using the same voltam­ metric or p ol aro gr aph ic p ro c e d u re to an a lyse a standard reference sample which is of a similar c o m pos i tion to the test s am ples . A further s i mp l e check is to add successively to the su ppo rting e l e c t ro lyt e solution a l on e in the cell (a blank) either a l i quo t s o f a standard soluti on or w eigh ed amounts of the a nalyte (standard ad d it io ns method) and reco r d the voltammogram or polaro­ gram. A plot of c o n c entra tio n of ana lyt e vers us c u rre n t p eak hei ght should give a straight line which passes through the origin if the measurements are error free.

4.2 SAMPLE PREPARATION

Accurate and reproducible a n a lytica l re s ul ts c a n o n ly be o bta i n e d if the s a mp l i ng and sample p re pa ra t io n have been carried out in a careful and controlled manner. To achieve this, care must be taken to ensure that the sa mp l e taken for a n al ys i s is tru ly representative of the whole of the material b ei n g tested and t h a t , if necessary, the analyte is quantitatively c on v e r te d into a ch e m i c al form s ui ta b l e for the determination. Following the s ampl i ng , some preli minary t r e a tm e nt such as filtration, drying, or s a m p le p res e rvation may be ne ce s s a ry, a n d t h e s a mpl e may need to be stored under c o n d it io n s that prevent sam p l e d et e r i ora t i on ( for exa mp l e acidification or free zi n g) before the diges ti o n and a n a lys i s is carried out. D u r i n g the d i g e s t ion , insoluble substances are converted to soluble forms or matrix constituents destroyed ( for example mineralisation of o rga ni c comp ound s or the decomposition of o rg a n ic material in water s a m p l es ) . The actual t r eatm e nt required d e pend s on the type a nd condition of the sample and must be adj u sted a c c o r d i ngly. The preparation o f water s am pl e s for trace m et a l analysis is re la t i ve ly straightforward. Samples are collected in n i tric acid cleansed p la stic or borosilicate glass flasks which are ri ns e d wi t h t he s amp l e at the c o ll e c ti o n site prior to filling with sa m p le . Suspended m at t er must fi rst be re m o v ed by filtration t h ro ugh 0.45 11m m e m b r an e filters and the s a mp l e stabilised by

Practical aspects

0 1 60

..., .....

n

u VI/ �

,---.

Yi'1 2

1

tv

3

,

"'

n

L

\ IV

�

' !

i2

;

I

4

�

u

3

.__

I

Figure 4 . 2

1r

mm

93

I I

5

U.V. d igestor (Cou rtesy of Metrohm). 1 U .V. lamp - 500 W or 1 000 W, 2 d igestion vessels - 1 2 quartz sample tubes with PTFE locking caps, each having a n effective sample volume of 1 0 ml, 3 water cool ing, 4 ventilator, 5 power cable.

acidification to p H

1-2 with u l tr a - p ur e nitric or s ulfuric acid. I f the sample cannot b e a n al ysed within one week of c o l l ect i on it must be stored in a re fr i ge r a to r or preferably frozen. In a few cases, such as drinking-, ra i n - , some ground- and som e sea-water samples, h eavy metals can be determined d ire c tly i n t he a ci d ifi e d sa m p l e after the addition of the s up p ort i n g e l e c t r o l yt e ( not needed for sea wat e r ) . In most surface wat e r s however, the o rg a ni c matter causes considerable tr o ubl e because o f its surface active and/or metal complexing p r o p e rties . The o r g an i c m atter, which may be o f natural or anthropogenic o rigin, can b e d es t roye d by U.V. p hoto l ysi s or by oxidative microwave d i g es t i o n . The U.V. d igest i on proceeds via a r a dic a l mechanism. Excited oxygen and ·OH r a d icals are p ro du c ed from the wa ter and H202 added to t h e sam p l e. These sp e cies ox i d i s e o rga n i c a ll y bound hydrogen an d carbon to H20 a nd C02, r e sp e c t i vely. The p ho t o lyt i c p ro c e s s p ro ce e ds r ap i dly to virt ual co m pl et io n . The s t ro n ge r the U.V. r a d i a t ion , the lower is t h e amount of org a n i c m a te r i al rem a in in g in the s a mp l e at the end (lower total or ga nic carbon (TOC ) ) . The irradiation i s c arried out i n quartz vessels a t abo ut 90°C. The sample i s ac i d ifie d t o pH 2 with sul fu ri c a c i d , 1 0 flL 30o/o H202 added and the whole irradiated for about 60 min with a 500 W U . V . la mp . If the di ge s t io n is not c o m p l e t e after this time, the i rrad i a t i o n m ust be continued after dilution or th e addition o f more H202• An apparatus for the di ges t i on of up to 12 s a mp l e s is shown in Fig u re 4.2 [5, 6, 7 ] . Ultraviolet photolysis is r e commen de d for sam p l e p r ep arat io n in the determination of h e avy metals in water samples by s t rip p in g voltammetry i n DIN 38 406 P a r t 1 6 a n d is a l s o used for the digestion of beve r ages and urine s a m ples [ 8, 9 j . As an alternative to U.V. p ho tol ys is , oxidative microwave d i g es t i o n can be u sed to de s t roy organic matter in surface waters. The s a mp l e , to wh i c h a suitable oxidant is ad d e d , is p l a ce d in a

94

Introduction to Voltammetric Analysis

Refl ux co ndenser

.....-- Safety shield

Hot p l ate

Fi gure 4 . 3

Digesdahl digestion apparatus produced by Hach.

closed quartz, PTFE or PFA digestion vessel and irradiated in a microwave oven. As the s a mpl e is he a t e d by the microwaves, the p ress ure increases in the vessel and with it the efficiency of the oxi­ dation reaction. An effective oxidant is a m i xtur e of peroxidisulfate and alkali hydroxide such as that marketed by Merck, under the name O xi sol v® (oxidation reagent for use in determining the total N and total P content of water) . Because the oxidation of organic c om p o u n d s with peroxy­ disulfate p ro d uc es protons, the pH of the sampl e changes from 1 1 to 2 du ri n g the course of the microwave digestion. This pH change is advantageous for the d i ges t i on process since the various orga n i c compounds encountered in water samples are oxidised at d i ffe rent pH values [ 1 0 ] . Oxi­ solv® is easy to handle and is prod u c ed from h i gh purity components so that it do e s not intro d uce contamination. The organic material in s u r face waters can be more rapi d ly destroyed by micro­ wave d i g es tion than by U.V. photolysis. Waste water samples with commonly encountered levels of organic mate ria l are completely digested w i th i n 50-60 s. It is only in the analysis of u ltra-trace (ng L-1 to Jlg L-1 ) levels of heavy metals by adsorptive stripping voltammetry, where the sample solution must be e s p e c i a l ly free of organic contaminants, that l on g e r digestion times of 2-3 min are required [ 1 1 ] . Foodstuffs and b i o l ogic al samp les are oxidatively di geste d under acid conditions. This technique, called wet digestion or acid digestion, and the technique of ashing, a r e the oldest of

Practica I aspects

95

Safety valve Safety lid ----+-

G as i n let 320

Reaction vessel

oc

Heati ng bl ock Heating element Tempe ratu re sensor Figure

4.4

High

pressu re

asher (H PA-S) (Courtesy of

Perkin-El mer) .

decomposition methods used for the dissolution of such samples. The procedure is variable and is carried out either in open or closed vessels. Typical digestion agents are a mixture of a solvent plus an oxidant such as H2S04 plus H N 0 3 , or H 2 S0 4 plus 30% H202 [ 1 2 ] . I n simple cases the wet digest i on i s ca rried out i n a quartz Kjeldahl flask o r in a commercial digestion apparatus ( e.g. the Digesdahl digestion apparatus of Hach, USA, Figure 4.3 ) . The disad­ vantage of wet digestion in open vessels is that a relatively large amount of reagent is used and that in spite of refluxing not all volatile compounds are retained. This digestion method is not used for

elements that are volatile or form volatile compounds, such as mercury, arsenic and selen i um. In the wet digestion in a Digesdahl apparatus, 0 . 5-3 g of the dried, ground organic sample is carefully heated to 300°C in the digestion flask with 3-5 mL concentrated H 2 S04, 2 . 5-5 mL 30% Hz02 added dropwise and finally heated to fumes of H2S04• If a dark solution results, a further 2.5 mL Hz02 is added and the digest again heated to fumes of H2S04• The cooled digest is diluted to 100 mL with pure water a nd analysed. For safety reasons, the H2S04 m ust be added to the sample first and then the H202• Wet digestion carried out in closed vessels becomes a pressure digestion. The advantages that this has over an open system are that the consumption of chemicals is minimised, no volatile element loss occurs, and higher digestion temperatures and hence more rapid digestions can be achieved. Very resistant organic materials can be oxidatively dissolved to a residual carbon content of < l lVo in a high pressure asher (HPA ) . The oxidant mainly used for this purpose is nitric acid [ 2 , 1 3 ] . The HPA ( Figure 4.4) plays an important role in sample preparation for the polarographic or voltam metric determination of trace elements in organic samples. The weighed sample and digestion reagent are added to the quartz digestion vessel, the lid closed and the vessel placed in the heating block of the autoclave. The autoclave is scaled, filled with nitrogen to a pressure of ca. 1 30 bar and heated according to a pre-determined, microproces­ sor controlled temperature program to a maximum of 320°C. The pressure of nitrogen in the autoclave counteracts the build up of pressure in the d igestion vessel as a result of the digestion reaction. After cooling, the nitrogen is released and the digestion vessel, which contains the clear digest, is removed from the autoclave and carefully opened to release the gaseous reaction prod­ ucts. The digestion of biological materials is relatively slow with the whole heating and cooling program taking up to 8 hours although with recently developed equipment this time c a n be reduced to about 2-3 hours.

96

I n troduction to Voltam metric Ana lysis

Pressu re measuring system Hydrau lic system Rotor sensor plate

Air duct

Figure 4.5

Construction and arrangement of the pressure vessel in a microwave oven (Courtesy of Perkin­ Elmer) .

The pressure di gest io n using microwave excitation (with frequencies of 2.45 GHz t o h e a t the sample) p r o c ee ds m uch faster than does digestion wit h thermal excitation. The pressure micro ­ wave digestion of various p la nt materials, drinks and geo l ogical s a m p les for heavy metal a nalys is is u sually c o m pleted with i n a few minutes, while the ti m e re q u i red for thermal d i ge s tio n is in the order of hours [ 1 4 ] . As with ther m ally exci t e d p ressur e d i g e s tio n microwave excited press ure digestion re q u ir es oxidising mineral acids to be added to the sample. In practice 65% HN03 is almost excl u si ve ly used [ 1 5- 1 7 ] . The vessels u sed for the pressure d ige s t ion i n the microwave d ige s tio n system o f Perkin- Elmer are made o u t of quartz or fluoropolymers (PFA o r PTFE ) . As a rule organic samples are digested with nitric acid in quartz vessels and inorganic s am ples with HF and then HN03 in fl u oropo lym er vessels. The construction of t h e pressure vessels a n d their arrangement i n a m i c ro w ave oven with pressure cut-off, t emp e ra t u re measurement and i n tegrate d air cooling for the regulation of th e d i ge s t i on conditions is s h own in F igu re 4 . 5 . Cold plasma ashing i s a n o ther d iges t i on te ch n i q u e s uitable for s a m ple p r e p a ratio n for polaro­ graphic and vo l ta m metric analyses. As is the case with dry decomposition, o r g an i c s a m p le s are ashed in an oxygen a t mo sph e re which i n this case is excited by a high frequ e ncy field. The oxi­ dants are short-lived oxygen radicals, which at a temperature below 1 50°C c o m pl ete ly destroy mo st o rg a nic material with i n 2-4 h [ 1 3 ] . I n th e determination o f o rga n ic a nalyt es u si ng am pe ro m et ric o r vol tamm e tric detection, solid ph a se extraction (SPE) is bei n g used more t h a n HPLC as a m e t ho d of sample pre-treatment. C om p ared to the classic m etho d of l iq u i d-l iq u id extract i o n , s u bsta ntia l ly hi gh er and more repro­ duc i b l e enrichment rates a r e obtained with SPE usin g much less s ol v ent Numerous sil ica gels with a wide range of p ol a riti e s are available fo r use as solid pha se ma terial. With these, active sub­ stances and p o l l uta nt s in a variety of samples (serum, pla sm a , urine, tissue, sewage, soil and food) can be extracted . A typica l solid phase extraction scheme would i n c l ud e the following steps: ,

.

(i)

T h e extraction column is

conditioned by r in s i ng it wi th ( a ) the so lven t i n which the

s am p l e is dis s o l ve d ( nor m a lly water) and then with the solvent that will be used for the elution of the enriched a n a lyt e .

Practical aspects

97

( ii )

T h e analyte i s loaded onto the column (extracted from the sample) by sucking the aque­ ous sample through the column with the help of a water vacuum pump. (iii) After loading the column is dried by sucking a stream of nitrogen through it using the water pump. ( iv) The analyte alone is then eluted from the colum n with an appropriate solvent. In this way pesticides, polyaromatic hydrocarbons ( PAHs ) , and so on can first be separated as classes of compounds from aqueous samples, then separated into the individual compounds by HPLC and determined at the ultra - trace level amperometrically [ 1 8 ] .

4.3 SUPPORTI NG ELECTROLYTES The primary role of a supporting electrolyte is to reduce the ohmic or iR voltage drop in the cell to a minimum and to effectively eliminate the contribution of the analyte to the migration cur­ rent. Common supporting electrolytes are the chlorides, sulfates and nitrates of lithium, sodium and potassium, lithium and sodium perchlorate and tetraalkylammonium salts of the general for­ mula NR/X- (R methyl, ethyl, n-butyl and x- Cl-, Br-, I-, Cl04 - ) . In addition acids (HCl, H2S04 ) , bases (LiOH, NaOH, NR4 ' O H - ) and buffer solutions, which at the same time control the pH of the solution, are also used. The concentration of the supporting electrolyte is generally between 0. 1 and 1 .0 M and high purity chemicals and water (or solvent) must be used to prepare these solutions. Supporting electrolytes can be used to change metal ions in the sample to metal-ion com­ plexes with different electrochemi cal properties. In this way a m etal ion may become electro­ chemically inactive or have its half-wave or peak potential shifted to more negative values. With appropriate complexing agents, interfering components can be m asked and overlapping current signals better separated and resolved. Simple inorganic ligands such as eN-, OH-, p-, NH3, and the hal ides and strong complexing agents such as oxalate, citrate, EDT A and tartrate are com­ monly used for these purposes. ( Examples can be found in Table 6. 1 and Figure 6.2.) Also, the shape of the polarographic curve can be improved in those cases where the metal-ion complex, in contrast to the unbound cation, is reversibly reduced. High salt content, which can result from sample digestion or preliminary separation proce­ dures, changes the viscosity of the supporting electrolyte and th e diffusion properties of the ana­ lytes. With increasing salt content, the polarographic current becomes smaller and displaced to more negative potentials, while the peaks in stripping voltammetry are diminish ed and shifted to more positive potentials. Small concentrations of electrochemically inert salts, as are found in natural water samples, are as a rule without influence on the analytical measurements. In the case of stripping voltam­ metric determinations of trace clements in sea water, it is even unnecessary to add supporting electrolyte to the sample. In addition to influencing the peak and half-wave potentials, the supporting electrolyte deter­ mines the useabl e potential range for the polarographic or voltammetric measurements (Table 2 . 1 ). In the cathodic direction the range is limited by the reduction potential of the cation of the supporting electrolyte. I n acid solutions the reduction po tential of the cation, H+, also depends on the hydrogen overpotential of the electrode material, being most negative on mercury. In the anodic direction , the working limit is determined by the potential of the oxidation of water, the oxidation of the anion of the supporting electrolyte, or, in the case of mercury, the oxidation of the mercury electrode, which also depends on the anion of the supporting electrolyte. Before every polarographic or voltammetric measurement, the supporting electrolyte solution must be freed of dissolved oxygen. Air-equilibrated aqueous solutions contain up to 1 0-3 mol l-1 =

=

98

I ntroduction to Voltammetric Analysis

of oxygen which is reduced in two steps. These reductions are p H dependent and p roceed as indi­ cated in the following e quations. In acid solution:

In

02 + 2H+

+ 2e-

--� H202

H z02 + 2 H '

+ 2e-

2H20

alkaline solution: 02

+ 2H 20

H P2

+ 2e+

2 e-

--�

H p2

+

20H-

20H-

The half-wave or peak po t ential of the first step for the reduction to H202 lies between 0 V and -0 .3 V and the second step b e t ween -0.7 V and - 1 . 3 V (versus Ag/AgCI, 3 M KCl ) . The large reduction cu rr ent of these re actions ( ca. 2 11A each step in DCP) m ar k e dly lowers the sen­ sitivity of the polarograp h i c or voltammetric determination. The oxygen is removed by passing high pu rity nit r og e n or argon through the cell solution (deaeration ) for about 5 to 10 min . A rgon is pre ferred because being heavier than air it better forms a b la n ket over the cell solu­ tion to p revent the i n gre s s of atmospheric oxygen. In alkaline soluti on, oxygen can be removed by reduction with Na2S0 3 • The reaction proceeds slowly and often remains i n complete . In acid solutions, ascorbic acid has been used to remove oxygen in the stripping voltammetric determi­ n ation of trace metals. The determination of inorga ni c analytes is almost always carr ied out i n aqueous supporting electrolyte solutions. For many organic or organo-metallic analytes, m ixed aqueous-organic sol­ vent or pure o rganic solvent systems are used. The more commonly us ed solvents are methanol, ethanol, acetonitrile, propylen ecarbonate, dimethylformamide and dimethylsulfoxide. The useful potential range at d i fferent electrodes for supporting electrolytes in selected solvents are shown in figure 4.6. Organic solvents are also used for extracti n g metal ion analytes that have been converted to their chelates or organic a nalyte s from a queous matrices. The determination follows either di re ctly on the extract o r aft er the addition of a suitable polar solvent in order to improve the sol­ ubility of the supporting electrolyte. Extraction voltammctry (or polarography) combi nes the selectivity of the extraction method with that o f voltammetry and may lead to an i mp roved deter­ mination in a n organic solvent. On ex tra ction the analyte is enriched and sep a r ated from inter­ fering sample constituents [ 1 9 ] . '

'

,

4.4 VOLTAMM ETRIC CELLS In modern voltammetric practice the cel ls must be designed to accommodate three electrodes­ working electrode, the re ference electrode and the auxiliary electrode-a nitro gen bubbler and a sti r rer . In order t ha t the reference electrode may r eliably carry out its function of controlling the working electrode potential , it is necessary in every measurement to position the tip of the salt bridge connecting the re fere n c e electrode to the cell solution in the same position with respect to and as close as possible to the working electrode. The reproducible position ing of the electrodes is achieved via the position ing and angle of the electrode insertion ports in the cell lid. The salt bridge tip, often r efer red to as a Lugin capillary, should be rel a tively small a nd not be positioned betw·een the working and auxiliary electrodes. Most manufac t u rers of voltam metric equipment supply c ells with lids conta i n in g insertion ports for three electrodes and gas i nlet and outlet ports, and cell stands ranging in c om plexity from those containing only a stirrer and a system of valves the

Practical aspects

Solvent

E lectrode

Pt

1 M H CI04 1M

1M

Propylen-

carbonate D i m ethylformam ide D i m ethyl-

su lfoxide

N aO H

1 M HC I0 4

Hg

{ { { {

N aO H

1 M HCI04

c

Water

Aceton itrile

99

1M Pt

NaOH

TEAP

Hg

TEAP

Pt

TEAP

Hg

TEAP

Pt

TEAP

Hg

TEAP TEAP

Pt Hg

TEAP

I

+2

I

I

I

I

+1

0

I

-1

I

I

-2

I

I

-3

E [V] (vs. SCE) Polarographically and voltammetrically useful voltage ranges i n various solvents at Hg, C and Pt electrodes . (TEAP = tetraethylammonium perchlorate) .

Figure 4.6

for c o n tr ol l i n g inert gas flow to th o s e with m uch of the electronic ci rcuitry requ ire d

for voltam­

metric experiments. ­

The ve s s el used as the voltammetric cell sh o ul d only be as large as neces s a r y to hold the s a m in o rde r to av o i d long deaeration times. In the s i m p l es t case a gl ass beaker may be us ed but commercial cells ( Figure 4.7) in wh i c h the el e ctro d e s are a rr anged in constant posi t ions with respect to each other are preferable. G e neral l y, the cells are made from gl a s s or p l a s t i c rarely from quartz, and d e s i gn e d to hold from 5 to 20 mL of sample solution. Special thermostat j a cket e d cells and m ic ro cell s for s a m p le volumes of 0.05 to 1 .0 mL are al s o available. Flow-through c ells for measurements on flowing solutions will be d e sc r i b e d in c ha p t e r 5 . ple,

,

,

-

4 . 5 ELECTRODES 4. 5 . 1 Refe rence electrodes of the reference ele ctrode is simply to control the p o t e n tia l of the wo rking electrode. The vo l ta g e of the auxiliary e l ec trod e with respect to the working elec t ro d e is electronically adjusted via the potentiostat to the desired volt age pl us the drop in the s o lut i o n so that the po t e nti al d i ffe r en c e b etwe e n th e wo rki n g and reference electrodes is maintained at the required value. In practice, the tip t h e of salt bridge of the reference electrode c an n o t be placed c lo s e enough to the working elec t r o de in order for the p o t e nt i o st a t to fully compensate for the iR drop [20, 2 1 ] in the cell. Further compensation can be achieved by u si ng positive electronic feedback but this is not necessary in most analytical situations. In co nt ra s t to the two - el e ct r o de technique

The fun c ti o n

iR

1 00

Introduction to Volta mmetric A n a l ysis

F i g u re 4 . 7

/,·· · Stand with voltammetric cell and (from left to right) a Ag/AgCI reference el ectrode, a platinum cou nter electrode, graphite counter electrodes, a rotating disc el ectrode and a mu lti-mode electrode (Courtesy of M etrohm).

no current flows t h rou g h the reference electrode in the three-electrode technique. The desirable characteristics of a reference electrode are described in s ec t i o n 1 .2 . In current polarographic a n d voltammetri c practice the reference electrodes used arc elec­ trodes of the second kind, of which the silver-silver chloride and cal omel e l e ct ro d e s are the most important (Figure 4.8) . E lectro des of the second kind consist of a metal electrode coated with an insoluble salt of that metal in c o n tact with a s ol u t io n containing the anion of the insoluble salt The potential of these electrodes is a fu n c t i o n of th e activity of the anion in solution. In the case o f the silver-silver chloride electrode, the electrode reaction consists o f two steps as follows: .

Ag 1 ' "'�l + e- � Ag ( s l AgCl ( s l Ag+ (aql + Cl-(aq)

Overall reaction AgCI<sl + e- � Ag(sl + Cl - < a > q

The p o t e n t ial of this electrode is given by:

The glass frit at the end of the e l e c t ro d e shaft separates the solution in the salt bridge of the ref­ er e n ce electrode ( KCl solution) from the cell s olu t i o n ( supporting el ec tro l yt e and s a mp l e compo­ nents ) . I n s t e a d of the glass frit, porous pla st i c agar-agar or gelatine plugs ca n be used. For systems sensitive t o halide ions, KN03 solution should replace the KCl so l u tio n in the outer jacket of a jacketed s a l t bridge and for sys te m s containing perchlorate ions, NaCl or NaN03 solution ,

Practical aspects

1 01

a

Hg P t Contact w i re

Silve r wire

Aqueous Aqueous

KC I sol ution

G l ass frit

Figure 4.8

KCI sol ution

AgCI Iaye r

G l ass frit

Calomel (a) and silver-si lver chlo ri de (b) reference electrodes.

should be used in the j a c ke t o f the sa l t b r id ge instead of po tassiu m salts. The salt bridge should not be al lowed to dry out. The potentials ( relative to the standard hydr o g e n electrode) of some imp o rt ant reference el ec ­ t ro d e s are co m p a r e d in F i g u r e 4.9. In addition to the calomel and silver-silver chloride electrodes, the mercury-mercury sulfate and thallium amalgam e l e ctrod e s are sometimes used. The thalamid e le c tro d es a r e imp o rt an t because of their hi gh e r te m p e r a tu r e s t abil ity . They c on sis t of a 40o/o t h a l­ lium a m al ga m covered in a l ayer of s ol i d TlCl in a 3 M or saturated KCl s o l u t i o n . Half-wave or p eak potentials are a lways quo ted relative to the po t e n t i a l of a r efere nce electrode. In the o l d er work the r e fe re n c e electrode used was m ostly the saturated calomel electrode ( SCE) , wh ere a s now, because of e n vi ron m enta l concerns about mercury, one of the silver-silver chloride elec­ trodes ( u s ual ly the 3 M KCl e le c t r o de ) is ge n e r a l ly p referred.

4 . 5 . 2 Working electrodes It is t h e c urrent generated by the electro chemical pro cesses which occur at th e wo rkin g el ectrode that is imp o rt a nt and is re co rd e d in t h e current-voltage c urve . The current flowing t h ro u gh the w o rkin g electrode as a consequence of the charge transfer reaction will flow in or ou t of the cell by way o f the auxiliary electrode. In o r de r that the electrochemical reactions o ccu rr i n g at the auxil­ i a ry el ectrode do not a ffec t the measured c u rren t , the sur fac e area of the a uxi l i ary electrode ( a grap h it e or platinum rod ) must b e re l atively l a rg e s o that the current d e n si ty a t t h i s electrode is sma l l and do es not influence the rate of ch a rge flow. The working e lec trode is thus th e most i m p o rtan t component of the voltammetric c e l L The material from which th i s electrode is constructed, as we l l as its geometry and the condition of its surface, determine t h e course of the electrode reaction, and in pa r t i c ul a r , t he s e n s i t iv i ty and reproducibility of the m e as ure d current. An overview of m a t e r i a ls suitable for t h e construction of working e l e c tro de s is given in Figure 4. 1 0.

1 02

I ntroduction to Voltammetric Analysis

+ 0 . 678

Hg I Hg2S04 • 1 M H2S04

+ 0.7

+ 0 .650

Hg I Hg2S04, sat. K2S04

+ 0.6

+ 0 . 334

Hg I Hg2CI2 , 0. 1 M KCI

+ 0.5

+ 0 . 280

Hg I Hg2Ct2, 1 .0 M KCI (NCE)

0.4

+ 0.241

Hg I Hg2Ct2, sat. KCI (SCE)

+ 0.3

+ 0 . 235

Ag I AgCI,

+ 0 . 208

Ag I AgC I , 3 M KCI

+ 0 . 1 97

Ag I AgCI, sat. KCI

+

+ 0.1

1 M

KCI

Pt I H2 (1 atm) 1 1 M H+ (SHE) Standard hydrogen electrode - 0. 1 - 0.2 - 0.3 - 0.4 - 0.5 - 0.6

- 0 . 567

Tl (Hg) I TI C I , 3 M KCI

- 0 . 577

Tl (Hg) I TI C I , sat. KCI

- 0.7

Reference electrode potentials at 2 5°( relative to the standard hydrogen electrode.

Fig ure 4 . 9

Wo rki ng el ectrodes

Liquid

-------

I

Merc u ry

------- Solid ------

Carbon

------

Meta l

Dropp i n g m e rcury e lectrode

G lassy carbon electrode

Hanging mercu ry drop e lectrode

G raph ite electrode

Gold electrode

Static mercu ry d rop el ectrode

Carbon paste electrode

Silver e lectrode

Plati n u m electrode

Mercu ry thin fi l m electrode

Figure 4. 1 0

Materials used for the construction of polarograph ic and voltammetric worki ng electrodes.

Practical aspects

1 03

An 'ideal' wo rki n g e l e ctrod e s h o uld s ati sfy the fol l o wi n g c o n d i ti o n s : e a s y to handle reproducible surface or easy to re-condition l o n g t e rm stability, that is n o fa ti gu e o r p o i s o n i n g p h e n o m e n a as a result of electrochemical re ac tion s ideal phase i nte rface for t h e electrode reaction la rge useful potential range for anodic and cathodic reactions small base ( residual) current for a high sensitivity.

• • •

-

• •

Most of these r eq u ire m en t s are fulfilled by mercury elec tro de s. Sta t i o n a ry ( h a n ging merc u r y d ro p { HMDE} and mercury thin film { MTFE} electrodes) and n on sta t ionary (dropping mercury { D M E } , static mercury dro p { SMDE} a n d mercury film ro ta t i n g disc { M FRDE} electrodes) mer­ cury e l ect r o d e s a r e th e r e fo re p re ferred for use in voltammetric measurements. With mercury, a well defined, homogenous, strain-free, and clean surface can be obtained whic h , b y simple m a n i p ul at i o n , can be rel i ab ly an d rep ro d uc i b ly renewed. A further adva nt a g e is t he high h ydro gen d i s ch arge overpotential of mercury. This means th at voltammetric and polarographic mea ­ surements can be made over a r elatively large po t en t ial range, which in neutral sodium perchlorate solution extends from +0.4 V to - 1 .8 V (versus SCE) and i n neutral p ot a ssi u m chlo­ ride from 0 V t o - 1 .8 5 V . The d ropp ing mercu ry electrode (DME) introduced b y Hey r ov sky consists of a 4Q-80 !Jill b o r e glass ca p il la ry , 1 0-20 em long con ne ct e d to a me rcu ry reservoir by a fle x i b le tube ( Figure 1 . 2 ) . The d is t an c e b e t w ee n th e cap illa ry tip fr o m which mercury d r o p s re g u l a rly fall ( Figure 4. 1 1 ) a n d the mercury level in the reservoir is about 40-60 em and must be kept constant during the de ter ­ m i n at io n Th e dep en den c e of the cu r rent on the height of the m e rc ury level can be used to di sti n gu is h between diffusion controlled, kinetic a n d catalyt i c currents as wel l as c u rre n t s which are influenced by adsorption p ro c ess e s [ 22 ] . The disadvantages of a fr e e ly dropping mercury elec­ trode are the co n t i n u ous l y changing su rfa c e area, the dependence of the drop life-time on the a p p l i e d p o te n tia l and the limited life tim e of the d r o p 1 T he s e pr o b l e m s were overcome with the in tro duc ti on of t he hanging mercu ry drop electrode ( H M D E) and the static mercury drop electrode ( S M D E ) , th e surface areas of which remain constant duri n g the course of a m e a s u re m en t The h angin g mercu r y drop electrode is an extrusion electrode. By t u rn in g a micro-screw, a defini te amount of m ercury is pressed o ut of the reservoir into the capillary of the electrode out of wh ich emerges a h a n g i ng d ro p o f m e rc u ry ( Fi g u re 4. 1 2 ) . At the end of a d et e r m i nat i o n , the drop is disl od ge d ma nual l y and a new drop p ro d uc e d . With this type of electrode stripping voltammet­ ric determinations can be c a r r i ed out with good reproducibility in the measured values. However it was m a n ual ly o p e r at e d and has now been re p l aced b y the m e c h a n ic a l l y operated static m e rc u ry electrode (SMDE ) . In the S M D E a relatively small bore c a pill a ry is connected to the mercury reservoir. The mer­ cury flow is contro l l ed by a valve w h ich i s o pe n ed for a short time (20-200 ms) to enable the dro p to form. After the valve is shut, the size and hence the s urface area of the d r o p remains constant so that the electrode can be used for stripping vol t am m etry in the same way as the HMDE. The vari­ ation of t h e surface area with tim e for the SMDE is c o m pa re d with the surface growth of t h e DME in F ig u re 4. 1 3 . The SMDE i s widely used for polarographic measurements and because the c u rre nt i s mea­ sured wh ile the electrode su rface area remains constant, the c a p aci t a n c e current is di m i n i s hed as the second term i n e q u a t i on 1 . 1 5 b e c o m e s zero. This g rea tl y improves the iF : ic ratio an d he n c e th e -

­

.

­

-

•

.

A fur t h e r problem with the DME i s the considerable amount of mercury requ ired to keep the capiliMy clea n and the merc u r y fl o wing freely. The toxicity of merc ury means t h a t care must be e xerc ised i n its use-avo i d i n g spills, work­ ing in a well ventilated laboratory and collecting and re-cycling the mercury after use. The HMDE and SMDE requi re much less mercury for thei r routine operation.

1 04

I ntroduction to Voltammetric Analysis

Figure 4. 1 1

The dropping mercury electrode [from reference 2 2 ] .

M i c rometer screw M i c rometer scale 0-ring Steel needle 0-ring M e rc u ry

Figure 4. 1 2

Construction of a hanging mercury drop electrode [2 1 ) .

sensitivity of the current measurement. In the case of tast or sampled polarography, for example, the improvement in sensitivity is such that it is now comparable to that obtained with DP­ voltammetry. The special advantages of the SMDE are in its double function as a stationary and a dropping electrode as well as the ease of synchronising the current measurement process with the drop formation. This electrode has largely replaced the DME in routine analyses.

Practical aspects

� Drop fall

1 05

a

�

CIS

b

�

�

::I UJ Q)

e

u Q)

w �------�--� Time +----- Drop life --+� Fig ure 4 . 1 3

Growth of the electrode surface area with time for (a) the D M E and (b)

t he

SMDE.

The so-called 'multi- mode electrode' of Metrohm, which can function as a SMDE, an HMDE or a DME is pictured in Figure 4. 1 4. In this electrode the mercu ry flow is pneumatically operated and c on t rol le d by a n ee d le valve. A capillary with a 50 f.!m diameter bore is used. Three differe n t sized small mercury drops arc produced with surface areas in the range 0. 1 to 0.6 ( m m )2• The usc of small m erc ury drops in ASV gives sharp peaks similar to those obtained when usi ng a mercury thin fllm electrode. The mercury thin film electrode (MTFE) is mainly used for stripping voltammetry and chrono­ potentiometric stripping analysis. The best base m a te ri a l on which to form the thin mercury film has proved to be glassy carbon or iridi um. Platinum is n o t a good base as it d issolves slightly in mercury. The mercury is electrolyti cally deposited on the b a s e electrode ma t e ri al either i n a se pa­ rate o p e rat io n before the voltammetric determ ination, or more commonly, togeth er with the amalgam forming metal analyte during the enrichment step (in situ p rep a re d MTFE ) . The p r e p a ration of the film from a separate solution prior to the enrichment st e p (ex-situ preparation ) is important in flow-through voltammctry. In the ex-situ method, the mercury is electrolytically de po site d from a 20-50 mg L- 1 Hg2 ' solution at -0 .5 to -0.9 V (versus SCE ) . In c om paris on t o the conventional HMDE ( Figure 4. 1 2 ) higher sensitivity a n d better resolu­ tion of adj ac e n t peaks can be a c h i eve d with the MTFE in ASV determ inations ( § 3.2. 1 . 1 and Table 3. 2 ) . The sensi tivity is i nde p e n d en t of the th i ckn e s s of the film, that is on the amount of mercury deposited, for t h i n films (0. 1-3.0 f.!m) , but decreases as the film thickness increases for the thicker films [ 24 ] . The deposited mercury often forms a layer of mercury droplets ( Figure 4. 1 5 ) rather than completely covering th e electrode base with a smooth fi l m of mercury, p articularly with fllms that are less than 1 f..l l11 thick. In such cases the film electrode has a lower hydrogen overpo­ tential and a narrower voltage range than a pure mercury e l ect ro d e ­

.

1 06

I ntroduction to Volta mmetric Analysis

Slit screw

/ �--....

Screw cap w 0-ring

mithh""-:""---...1..:.._

Spring PT FE-Membrane

Nitrogen ports

Dam p i n g M e rc u ry

rese rvoir

Needle valve

G l ass capillary M e rcury

Figure 4.14

drop

The m u l t i - m od e electrode or MME™ (Courtesy of Metrohm) .

In p ri nc ip l e , film el ec t r o d es can be used for many determinations. D o ubts arise however when standard ad d i t i o n s arc u s e d to eva luate the co n c e n tr a t io n of th e analyte from voltammo­ grams obtained w i t h in situ coated electrodes. U nless the fi l m is completely r em ove d between each measurement, e ach su b s eq uen t enrichment also i nc re a s es the film thickness and l eads to false results. At t h e end of each m e as ur em e n t , the film should be removed by wiping the electrode surface w it h a soft t i ss u e and t h e n d i p p i ng it into concentrated nitric acid. After rinsing with dis­ tilled water, the e l e c t ro d e is ready to be coated a ga i n . In ASV determinations using flow-through cells this p ro b l em docs not arise as the film is ap pli e d ex-situ in t h e s e cells. In a flow-through cell, the film is removed from the electrode base at the e n d of the meas urement by anodic d i ss o l u ti o n in co m b i n at i o n with c h e m ica l oxidation using T/KI, K2Cr07 or KMn04 solution. Solid metal electrodes ma d e of the noble metals such as go ld or p l a t i n u m arc p ri nc i p a l ly used for i nvestigations in the p o t e n t i a l region where mercury is anodically d isso l v ed . An example is the determination of m e rc u ry itself by a n o di c stripping vo l t a m m et ry u s in g a g o ld rotating disc elec­ t rod e ( s e e Ap p l ic at i o n 7 ) . As an alternative to a solid go l d electrode a gold film elect ro de can be used with glassy c ar bo n as the base electrode material. The glassy carbon is p o l i s hed and t rea t e d with nitric acid prior to a film of go l d b e i n g electrolytically p l a t e d onto its surface. Measurements made with p la t i n u m electrodes often display poor reproducibility which is due to v a r i o us surface effects s u c h as the fo r m at i o n of hyd ro g e n , oxygen or platinum oxide layers. As a l re ady men­ tioned, solid p la t inu m should not be used as the base electrode material for a mercury film. Carbon electrodes are i m p o rta n t for analyses in the anodic p o t en t i al region. The el e c t ro d e s are made from carbon in t he form of graphite, carbon p ast e or gl assy carbon with glass y carbon elec­ trod es (GCE) an d carbon paste e l e ct r ode s (CPE) being most co m m o n . G l ass y carbon electrodes used for voltammetric an a l yse s are usually disc electrodes which may be either st a tiona r y or

Practical aspects

Figure 4. 1 5

1 07

Photomicrograph of a mercury fi lm on a g raphite base deposited at +0.85 V from a 0.1 M NaCI04 0.5 M HCI04 + 1 0 J.I M Hg(ll) sol ution (magnification x500) .

+

rotating. They are made by p ressing or cementing a

2 to 5 mm dia meter rod of glassy carbon into Electrical contact is made to one end o f the rod and the other o r 'working end' of the rod is polished to a mirror finish with a s l u r ry of w ate r and a l u minium oxide of vari ­

an inert plastic sheath.

ous grades, the final polish being with a

0.05

J..L m Al20 1-water slurry. The surplus Al203 is rinsed

off with di stilled water. In neutral aqueo us solution, the useful vo ltage range of glassy carbon extends fro m ca. - 1 .2 V to + 1 .3 V ( versus SCE). Glassy carbon is the preferred ma ter i a l fo r the worki n g electrode i n v ol t a m m et r i c flow-through detectors ( see § 5 . 1 ) . Su rface deposits which accumulate during use are removed by polish i n g with a 0.05 J..L m Al203 slurry. Carbon paste e lectrodes are a mixture of spectroscopic g ra d e graphite p o wd er and a wa ter

eth­ in the mix­

inso l uble, viscous organic b i nder of the highest purity s uch as nujol, ! -bromo- naphthalene, ylenena phthalene, trimethylb enzene, silicone oil, etc. The ratio of graphite to binder

ture lies b e t\vee n

2: 1

to

5: l .

both the current d ensity and [ 25, 26 ] . The carbon p as te is p re pa r e d by d iluting the

The com position of the paste i n fl u ences

potential range of the voltammetric signal

binder with acetone or petroleum ether, addin g the graphite p owder and grinding the mixture in a

mortar. After evaporating the v ol a til e solvent, the paste is placed into

which serves as the electrode shaft. paste to be p ressed out

as

A

the end

threaded plunger i n the remainder

of the

glass tube

of the tube enables the

required to renew the electrode s urface. Any excess p aste is removed

with a knife and any surplus paste adhering to the edge of the electrode removed with

a tissue. The

is polished by l i ghtly pressing it on a suitable smooth base ( e.g. p a p er ) . Each time the electrode surface is renewed it m ust be tested and calibrated. Carbo n paste electrodes have a relatively l ow base current and i n comparison to other solid electrodes respond more quickly to voltage changes. These electrodes arc used a lw a ys in the anodic region for the voltammetric determi nation of oxidisable organic compounds s uch as phe­ new surface

nols, aroma tic ami nes or hydroquinones.

4 . 5 . 3 M icro-e lectrodes In general, the carbon and mercury working electrodes co m m on ly used i n voltammetry have sur­ 2 face areas i n the (mm) range. This m eans that the linear dimensions of these electrodes a re an order of magnitude

greater than

the thi ckness o f the diffusion l ayer and the transport of the ana­

lyte to the electrode takes place mainly by linear diffusion.

I ntroduction to Voltammetric Ana l ys i s

1 08

a

�� &§1

b

<

20 11-m

M icro electrode

Macro electrode

0 + 0.2

Figure 4 . 1 6

0.0 - 0.2

E [V]

+

0.2

0 . 0 - 0 . 2 - 0.4 E

[V]

Diffusion to a macroscopic disc electrode (planar diffusion) (a) and to a mi cro-disc electrode (hemispherical diffusion) (b) and the corresponding cyclic voltammograms for each electrode.

However, if t h e lateral dimensions of the electrode are similar to or less than those of t h e diffusion l aye r ( < 0. 1 mm ) , the d i ffu s io n of the analyte to the e l e c tro de is then sp h e r i cal rather than linear. Such e l ec t r o d e s with such small d i m e n s i on s are referred to as m icro-electrodes. Micro­ electrodes are becoming more i m por t a nt as wo rkin g electrodes in voltammetric trace a n a lys is [ 2 7-29] . Al th o u gh miniaturisation of the working e l ec t ro de results in small currents flowing in the el e c t roch e m i ca l processes, the material transport to the electrode s u rface becomes accelerated as a consequence of the higher current density at the electrode s u rface The c h an ge from linear diffusion with a m a c r o - d i s c electrode to sp herical diffusion with a m i c r o d is c electrode pro d uc e s a characteristic c h a n g e in the s h ap e of the cycl i c vol t amm o g ra m s as is shown in Figure 4. 1 6. With the ma c ro - di s c electrode the typical vo lt a m m o g r am ( curve a) fo r a reversible e l ec t r o d e reaction is ob tained in wh ich the p ea ks r e sul t i ng from the forward ( r ed u c t i o n ) and reverse ( oxidation) processes are of equal h e i g ht and se p a rated by 5 7/n mV ( § 2 . 3 ) . H oweve r for the same electrode reaction under the same conditions b u t u s in g a micro-d isc electrode, the vo lt a mm o g ra m shown in cu rve b is obtai ned. This has the characteristic wave- shaped current-voltage plot of a 'steady s ta t e process in wh i ch the curve for the reverse process ( oxidation ) is s u p e rimp os e d on that of the fo rwa rd ( reduction) process. The s te a dy state condition p rev a i l s with micro-electrodes b ec au se the maximum transport vel o c i ty of reactant to and product away from the electrode is very quickly reached. T h is is of great a dva n t a ge in a n a lys e s in tlowi ng systems where the co m p on e n ts in the s u p p o rt i ng ele c t r o lyt e arc ch a n gi n g and the p o tenti a l is b e i ng scanned very r ap idly ( see flo w - th r ou gh stripping v o l t a m m e t ry and electrochemical d etec t i o n after HPLC separation in ch a pt e r 5 ) . .

-

'

,

Practical aspects

The l i m i t ing faradaic current, iF , under stea d y state conditions i s given b y

iF

iF

1 09

[30]

4 nFD, rc. for a micro-disc electrode and

= 2 1t n FDa rca for a m icro - hem i sph e r i ca l e l e ctrod e

have th eir u s u a l m e an i n g ( see ch apter 2 ) a n d the current is n ow time i n d ep e n ­ d e nt . Refer r i n g t o e q u a t i on 2 . 5 s h o ws that when the d im e n s i ons of the electrode are v e r y small the second ( ti me - i ndepend e n t ) term in the brackets will be very m uch l a rger th an the ti me - d epe n ­ where the symbols

dent first term.

( iR drop ) which causes distortion of curve ( § 1 .3 ) is com pen s ate d to a l a rge extent in t h e three-electrode system. When the wo rki ng electrode is a micro-electrode, the current flow is so s m a ll ( i n t h e n an o a m pe r e ran ge ) that the iR drop in the cell i s very small and does not cause s i gn i fi ca n t distortion of the i-E p lo ts , even when l i ttl e or no su p porting e l ectro lyte is present. In most cases, e ss e n ti a lly undistorted i-E plots can be o b ta i ned with a m icr o - ele c t ro d e using a s i mp l e two - electrode cell configuration, even in the ab s e nc e of a s upp o rtin g e l ec t r ol yte . The small area of a m i c ro - e l e ct ro de means t h a t the chargi n g current, i,, which i s pro p o r ti o n a l to the area of the electrode ( equation 1 . 1 5 ) , is also s m all . This leads to two i m p o rt a n t adva nta ges o f m i c r o - el e ct ro de s : With a conventional electrode, the ohmic potential drop

th e current p o te n tia l

(i)

(ii)

I mproved sensitivity. F ro m t h e above equations i t is seen that iF is proportional t o the radius of t h e electrode and since ( i s p r o p o rt i o na l to th e area, that is t o ? , t h e si gn al - t o ­ noise ratio iF/ ic = k! r ( wh e r e k is the p ro po r ti o na l i t y constant ) . Thus as the electrode size decreases, the s i gnal - to - n o is e ratio ( o r sens i tiv i ty ) increases. There is o f course a limit to this b e caus e t h e curr en t s p r od u ced by th e s e electrodes are so small, ins tru me nt al noi se a nd pi ck - u p fr om power m ains and so on become s i gn i fi cant . Very fast response to p o tent ia l changes. The response time of an electrochemical cell is de te rmi n e d by i ts time constant which i s gi ven by the p ro d u ct R C. The res i sta n c e R of a cell with a micr o - d isc electrode is inv e rs ely p rop orti o nal to r [ 30 ] and its cap a c i tanc e is proportional to th e area of the electrode, that is to r, so that the time constant , RC k 'r. Therefore as the electrode size decreases so does the response tim e . =

T h e d e c a y of the c a p a c i t iv e current at a m icro-elec trode is ext r emely fast, which makes these electrodes very valuable for making measurements in the very short time intervals required with the p ulse methods and voltammetric determinations in flow systems. The main dis a d v a n ta ges of micro -electrodes are that special e q u ip m e n t is re q u i red to measure the small cu rrents that t hey generate and, a s mentioned above, they suffer from instrumental and mains no i s e p i ck-u p . Also m a ny m icro-electrodes a r e su pp o r t e d in a n epoxy resin ma trix which may be att a c k e d by o r g a n i c so lve n t s . One way of o ve rcom ing th e prob l em of m e as ur in g the small currents is to jo i n many micro­ electrodes in parallel and so o b t a i n a much l a rge r current o utp ut. An arra n gem e n t with many (up to 10 000) single micro-electrodes is c alled a m icro-electrode array. I f the electrodes in an array are far e n o u gh apart so that the diffusion fields of the individual micro - electrodes do not overlap ( separated by a distance 20r, where r is the lateral dimension of each individual elec­ trode ) , then they act independently of each other. By wiring the N single electrodes in an array to ge the r in pa r a l le l , the output c urre n t will be N times that of a single micro-electrode, that is it can be in t h e flA range instead of in the pA to nA range. Such an array, wh i l e h a vi n g the advan­ t ages of a single micro-electrode, pro d uc es a current whi c h i s l arge e n o ugh to be m e a s ur e d wi th co n v e n t i ona l p o l a ro gra ph i c equ i p m e n t . Note that if t h e individual electrodes in the a r ray a r e

110

I ntrod ucti o n to Voltammetric Ana l ysis

• S p h e re

D i sc

@ Ring

Fig ure

4. 1 7

Band

•

•

•

•

•

I l l If

•

Array

•

Wi re

�

•

•

l nterd i g ital

Typical shapes of micro-electrodes [29] .

s ep ar a t e d by a distance of o n ly 2 r to 3 r, the diffusion fields overlap and such an a rr ay wi l l behave as a macro-electrode. With m i c ro ele c t r o d e arrays it i s p oss i b l e to extend the a p p l i c at io n of a n a lyt ic al v o l t a m metry to t h e direct d e t er m i n ati on of a n alyt e s in n o n - aqu e o us samples wi t ho u t a d d i n g s u pp o r t i n g e l e c ­ t ro l yt e fo r e x a m p l e t h e a n alys i s of additives in p et ro le u m . In st r i pp in g voltammetry, the a ccu mu­ lation of t h e analyte o n t h e el ectrode can be achieved in a very sh o rt t im e wi t h o u t s t i r ri n g the -

,

solution because of t h e higher mass transport.

Various types of micro -electro des are shown in figure 4. 1 7. They arc disc, spherical, band, T h e elec ­ tr o d es ar e m a i n l y m a d e fr o m p l a t i n u m or c a rbo n by va rious t e c h n i q u es Disc and c yl i n d ri c a l (wire) micro -electrodes are constru cted m a n u al ly by me lt i n g o r e mb e d d i n g c a rbon fibr e or fine noble m e ta l micro -wires i n to soft glass or epoxy resin. Band electrodes a n d i n t e rd ig i t ally struc­ tured single and a rray electro des may be constru cted b y photolithogra phic methods by the e t c h ­ i n g t e c h n i q u e or t h e 'lift - o ff ' t ech n i q u e as i l l u st r a t ed i n Figure 4 . 1 8 . wire, r i n g electrodes, an e lect r od e array and an electrode with an i n t e r d ig i tal s t r u ct ur e

.

.

b

a

������=== Lacquer

___.C s�=�::at)'1•••••��t

.______

P hotolithography

Photol ithography

t

t L•••••j

E

• • • • •

Etching

j

t

������9 Remove lacquer

Fig ure

4. 1 8

L

t DDDDD

J

Vaporisation

t

� � �HiH.H� � Lift-off

I

t

D O D D DO

I

Photol ithog raphic methods of constructing m icro-electrodes: (a) etch ing process, (b) 'l ift-off' t e c h n i qu e [28] .

Practical aspects

Figure 4 . 1 9

Photomi crogra ph of the el ectrode surface of a disposable micro-electrode Ecossensor. Single electrode diameter 40 !Jm; magnification 1 : 50.

111

a rray produced by

=

End

view

Reference electrode

i

- Working electrode

Polymer

base

Reference electrode

Figure 4 . 20

Sche mat ic construction of Ecossensors d isposable micro-electrode array. E lectrode size - 4.5 em l o n g ; 1 em wide.

I n s u lating

Epoxy i nsulating

casing Sta i n less steel contact

casing

II&"-7L---+- Carbon fibres em bedded i n

e p o x y res i n Carbon fibres

Figure 4 . 2 1

t

Model of a RAM'M electrode s h owin g carbon fibres attached t o a stainless steel embedded in epoxy resin ( Courtesy of CSI RO, Melbourne, Austral ia).

contact a n d

112

I ntroduction to Voltammetric Analysis

In the etching method the substrate is coated with a layer of metal which is then covered by a photosensitive resistance lacquer layer. The desired micro - electrode structure is produced by exposing the lacquer to light through a mask and subsequent development (photolithography). The exposed metal is then etched out and the lacquer layer removed. In the 'lift-otf technique a metal or carbon layer is sputtered or evaporated onto a lacquer layer that had p reviously been struct ured lithographi cally. After removal of the lacquer layer the metal (or carbon) remains on the uncovered surface region of the substrate [ 3 1 ] . In the laser method of preparing micro-elec­ trodes, the electrode material is covered in a polymer layer into which micro dimen sional holes are then burnt. The preparation of thin- layer micro-electrodes and their present and fu ture sig­ nificance in environ mental analyses is reported in reference 32. Single use or disposable micro- electrode arrays are prepared by the screen printing technique in which modified carbon paste is printed onto a plastic or ceramic base. The graphite is then cov­ ered with a polymer layer, which is so structured that circular gra phite surfaces remain uncovered as shown in Fi gure 4. 1 9. The disposable micro - electrode arrays manufactured by Ecossensors are so constructed that on the plastic support a Ag/AgCl reference electrode and an auxiliary elec­ trode are a rranged beside the working electrode as shown in Figure 4.20. The manufacturing pro­ cess enables different sized arrays to be constructed. For example, an array with 250 working electrodes each of 40 11m diameter ( total surface area ca. 0.30 ( m m ) 2 ) , or with more than 1 000 individual tO 11m diameter electrodes which have a total surface area of ca. 0. 1 ( mm f Such elec­ trodes are relatively low priced but can only be used for a few measurements. Note that Figure 4.20 is a magnified view of the end of th e working electrodes of Figu re 4. 1 9 . Another way of making micro -electrode arrays h a s been developed by CSlRO, Australia [ 33-3 5 ] . Carbon fibres with an average diameter of ? !Jm are randomly set in epoxy resin ( Figure 4.2 1 ) . Each fibre is connected to a stainless steel contact. These random assemblies of micro-disc arrays ( RAM™ electrodes, marketed by ADinstruments-Appendix 6) can be made with any desired total area in the (mm)2 to (cm)2 range and are characterised by the number of surface active carbon fibres (up to 1 000) as determi ned from current measurements. In contrast to the micro -electrodes described above, the ultra-trace graphite electrode manu­ factured by Metrohm has no definite micro-structure. The electrode consists of a porous graphite framework filled with epoxi de resin. The 2 mm diameter electrode is encased in an epoxide resin cylinder. M icroscopic examination shows that the electrochemically active carbon particles on the surface are i rregularly distributed with no definite separation from each other. Moreover, partly projecting carbon parti cles give the ultratrace graphite electrode a rough surface at which hemi­ spherical diffusion behaviour operates. The voltamm ograms obtained with these electrodes show behaviour similar to micro- electrodes with a partial overlapping of the in dividual diffusion fields. The 'thick film ' graphite electrode (ThFGE) [36] is similar to the ultra- trace graphite electrode and likewise shows behaviour typical o f an electrode array. A ThFGE is made by applying a mixture of graphite and epoxy resin in a strip ca. 4 em long and 0.2 em wide onto a PVC b as e ( see figure 4 . 2 2 ) . The electrode is then d ried at 1 00°C and coated with ins ulating lacqu er so th at only the two ends of the graphite-epoxy strip and a strip ca. 3 mm x l mm remain uncovered to pro­ vide electrical contacts and the active electrode surface, respectively. The ThFGE is designed as a disposable el ectrode and available from the U ral State University in Ekaterinburg (37] . After modification of the surface they are preferably used for stripping voltammetric determinations. Scanning electron microscopic and electron beam an alysis have shown that thick film graphite electrodes have rough surfaces. The graphite pa rticles range in size from 20-50 11m and are ran­ domly distributed in the epoxide resin. The random arrangement of the graphite particles on the surface of a thick film electrode is like that of the fibres at the surface o f a RAM™ electrode. Because of their micro -electrode-like properties, the ThFGEs are more efficient i n a number of ways than the HMDE or MTFE. When used in voltammetry they give a better sign al to noise ratio and produce much narrower strippi ng peaks than the HMDE or MTFE, and they offer the

Practical

Figure 4 . 2 2

Photograph of a th ick

film

aspects

113

g raph ite electrode.

possibility o f convection dependent measurements in flow-through systems. Since dissolved oxygen or organic impurities in the sample solution cause little or no interference in voltammet­ ric measurements using Th FGEs there is no need to deaerate the sample solution nor to digest the sample prior to the determination in most cases. This leads to considerable savings in the time and costs of the analysis. 4 . 5 . 4 Mod ified el ectrodes

When the surface of an electrode has been chemically changed or treated in order to change its electrochemical properties or to improve its sensitivity and selectivity in an analytical determina­ tion, the electrode is referred to as a modified electrode. The modification of a voltammetric work­ ing electrode i s done by fixing ( immobilising) reagents on the electrode surface. While glass, platinum, gold and carbon have been used as the base electrode material, in voltammetric applications the base electrode that is modified is usually carbon. The reagent that is used to modify the surface is referred to as the modifier. One of the major reasons for the development of modified electrodes is to use them to replace mercury electrodes ( for environmental reasons) and so facilitate the further development of voltammetry, and in particular stripping voltam metry, as a 'leading edge' technique. A major role of the modifier is to improve the selectivity of the elec­ trode and aid the binding and enrichment of the analyte on the electrode surface. There arc several methods used to m odify electrode surfaces: adsorption of reagents on th e surface; covalent bonding by reaction between the modifier and functional groups on the base electrode surface, for exam ple the anchor groups on graph i te are the carboxyl and car­ bonyl groups that exist on a carbon surface and on platinum the modifier is bound via the -OH groups and oxygen atoms on the Pt surface; (iii) incorporation of the modifier within a gel or an electrically conducting polymer film; ( iv) physically coating the electrode surface with the modifier, such as an enzyme; or ( v) mixing the modifier with carbon paste.

(i) (ii)

Carbon paste electrodes have the advantage that the surface can be quickly renewed by simply removing the used surface layer with a tissue or knife. Organic complexing agents, ion exchang­ ers, solid p h ase extractants and biological materials may be used as modi fi ers . The modi fi er s must

have a low solubility in the analyte solution and not be electroactive in the working potential range. Typical examples of complexing agents used are dimethylglyoxime for the determination of nickel, dithizone for the determination of gold, and crown ether modified carbon paste electrodes for the determination of Hg(ll) and Pb(II ) . Liquid or solid cation and anion exchangers have been used as modifiers in the stripping voltammetric trace analysis of heavy metals and various anions. Biological modifiers may be enzymes or micro -organisms, for example glucose oxidase for the determination of glucose, horse radish peroxi dase for the determination of H202 and algae for the determination of Cu(II) and Au (Ill ) . The determination limits obta i n ed with modified 1 carbon paste electrodes lie in the mg L- 1 to 11g L- range [ 3 8-40 ] . Modified screen printed electrodes may be prepared either b y incorporating the modifier in the carbon containing ink or by fiXing it to the screen printed electrode surface. A mercury film,

114

Introduction to Voltammetric Analysis

Thick film

I

graph ite electrodes

Modification

Manual

(in situ)

E lectrochemical

/ With merc u ry

(deposition from solution o n surface)

� With organic compounds

From insoluble i norganic and o rganic Hg-compounds

Cu

Cd

Zn

Fig ure

� Pb Sn Bi

4.23

Cr

w

Ni

� Mo Se Mn

Cu

Zn

Cd

Pb Sn

From soluble Hg salts (Nation)

Cu Cd

Au compounds From insoluble

Au salts

From soluble (Nation)

Zn

Pb Sn

Determination of trace elements by stripp ing voltammetry using mod ified thick fil m graphite electrodes.

electrolytically deposited on a screen printed electrode, was used to determine the usual mercury­ soluble heavy metals and also uranium as the cupferron complex [ 4 1 ] . Thick film graphite electrodes can be modified in a similar way to screen printed electrodes to give thick film modified graphite electrodes (ThFMGE) . After modifying the surface the electrode can he used for the stripping voltammctric determi nation of a number of heavy metals [42, 43 ] . The different modi fications and possible applications of ThFMGEs to the determination of trace elements by stripping voltammetry are presented in Figure 4.23. Electrodes are modified in situ by the electro -deposition of mercury or the electro- sorption of a complcxi ng agent from the supporting electrolyte onto the electrode surface. Mercury modified ThFMGEs are used for the determination of amalgam-form i ng metals by anodic stripping volta­ mmetry. The determination limits are Cu - 1 jlg L- 1 ; Pb - 0.5 jlg L- 1 ; Cd - 0.3 jlg L- 1 ; Zn - 2 jlg L- I ; Sn - 5 jlg L- 1 and Bi - l jlg L- 1 • Surface active impurities have been found to interfere much less with determinations using mercury modified thick film graphite electrodes than is the case with the ordin ary mercury electrodes. Surface waters and blood samples can be ana lysed for trace metals without sample pre-treatment [44 ] . Complexing agents wh ich form insoluble compounds with the analyte are chosen for the in situ modification with organic compounds. Electrodes so modified arc found to be selective and very sensitive when used for adsorptive stripping voltammetric determi nations. Using the appropriate complexing agent W , Mo, Mn, Cr, Ni and Se have been determined in the middle to upper ng L- 1 range. ThFGEs are modified manually by the application of a solution or an emulsion of the modify­ ing reagent ( commonly a mercury or a gold compound, or an organic complexing agent) onto the electrode surface. The solvent is evaporated and the modifier a ttached to the surface by electro­ chemical conditioning (alterna ting the voltage between +0.3 V and - 1 . 3 V at 1 .0 V s- 1 ) . I n some cases, particularly when soluble salts were used as modifiers, the electrode surface was coated with Nafion™ to protect the modifying layer. The limited interference caused by organic compounds in determinations using ThFMGEs is related to the surface structure of the electrode. When modified with mercury or gold, the metal preferentially forms small droplets or particles on the high spots on the surface which are mainly

Practical aspects

Figure 4.24

i

115

Scanning electron micrograph of a gold-mod ified thick film graph ite electrode [45).

[nA]

Hg

i

a

[nA] 250

b

650

200 600 1 50 550

1 00 500 50 450

0.4 0.5 0.6

Figure 4 . 2 5

E

0 �,_�.-.-.-.-.-.-.-.---o.o 0.8 1 .6 2.4 3.2 4. 0 c [IJ.Q L_, ]

[V)

DP stri pping voltammogram o f mercury obta ined u s i n g a Au-PDC modified thick fi l m graphite electrode. Test solution 0 . 1 M H2S04 + 4 mM HCI + x j.lg L-1 H g( l l) (x 1 , 2, 3). face = -1 .0 V (vs Ag/AgCI, 3 M KCI), t.,, 1 20 s. =

=

from each other ( see Figure 4 . 24 ) . In a voltamm etric exp e r ime n t the grea test cu rrent occurs a t these high p o i n ts and the effective electrode potential, which is surface structure dependent, is more negative at these points than in the surrounding region. The deposition of the analyte is favoured on the h igh spots in contrast to the adsorption of the organ i c compounds, possibly because the p o ten tia l at these p o i nt s is o utside the range for adsorption to occur. Gold mod ified thick film graphite e l ectr o d e s have been used to determine trace levels o f mer­ cury by anodic stripping voltamm etry [ 46] . The electrode was m o d i fi ed by placing 0.25 11g Au ( I I T )

separated d e n si t y

116

I ntroduction to Voltammetric Analysis

Sn

0.5 �

1 Pb

- 0 .75

Figure 4.26

- 0.30

E [V]

DP-voltammog rams for the simultaneous determ ination of tin and lead using a Hg-PDC modified thick film graphite electrode. Supporting electrolyte 4.6 g l-1 N H4CI + 9.6 g L:1 (NH4hC204 + 9.4 g l-1 HCI + 1 0 mg l-1 methylene blue. Calibration runs with 1 0, 20 and 30 j.lg l-1 Sn(IV) and 2, 4 and 6 1Jg l-1 Pb(l l), respectively.

and 0.5 flg pyrrolidinedithiocarbamate each in 5 f.LL solution on the electrode surface. After drying in air the electrode was conditioned electrochemically (SO cycles between - 1 .3 V and +0.3 V at 1 0 V s- 1 ) . Figure 4.25 shows the voltammograms obtained for the determination of Hg(II) by standard additions ( § 4.7) . The d etection limit was found to be 5 ng L- 1 after 5 min accumulation. Organic s ubstances were found to interfere only when they were at much higher concentrations than the analyte, for example by more than a 1 000-fold excess of triton X- 1 00 or humic acid. On the other hand, only a slight excess of Cu(II ) , Se(IV) and Ag( I ) affects the Hg signal [47 ] . Electrodes modified with insoluble mercury complexes, such as Hg- diethyldithiodicarbamate or Hg-pyrrolidinedithiocarbam ate, can be used for the determination of cadmium and lead down to 0.5 mg L- 1 [43 ] and of tin and lead in admixture [48] ( Figure 4.26) but not for the determina­ tion of copper. In the determination of trace metals by adsorptive stripping voltammetry, the tra­ ditional mercury drop or mercury film electrodes can be replaced with modified thick graphite film electrodes, if necessary. They have a distinct advantage in field analyses as the surfactant or natural acid content of aqueous environmental samples does not interfere with the determination

Practical aspects

117

0/A/C

W or k i ng

Compute r or VA Trace Analyser

electrode

Cu rrent amplifier

Figure 4 . 2 7

Block diagram for a PC-controlled polarograph for various polarographic or volta mmetric tech niques (Courtesy of Metrohm).

of trace elements in these samples when the modified electrodes are used . I n general the repro­ ducibility of results obtained with ThFGMEs is good. The life-span of a ThFMGE is variable and depends on the method of preparation. For example, the electrodes modified manually with mercury or gold pyrollidinedithiocarbamate are especially stable and continue to function after one year. Electrodes can be used for many determin ations before they need to be regenerated. The main disadvantage is the need to manually prepare and individually modify the electrodes. The electrodes will become especially valuable for field and routine analyses when they become mechanically made and production costs are reduced, as is the case with screen printed carbon electrodes [ 41 ] .

4.6 I NSTRU M ENTATION AND AUTOMATION

The modern instruments used for voltammetric and polarographic studies are digitally con­ trolled. Instruments range from those with a dedicated microprocessor, video screen and cell stand capable of performing a wide range of voltammetric techniques such as the Metrohm 746 VA Trace analyser or Bioanalytical Systems BAS 1 00 series, to more modular instruments which may have a 'measuring stand' with the necessary software so that the analyst may use a personal computer to program the equipment to carry out the desired voltammetric or polarographic pro­ cedures and collect and process the data. Components of these modular instruments usually may be interfaced with home-made units or un its from various sources. Typical of these latter instru­ ments is AD Instrument's PowerLab® system. Also available is a range of specialised instruments which perform a l imited number of techniques such as Radiometer's TraceLabn• system for potentiometric and chronopotentiometric stripping analyses. The basic elements of a computer-co ntrolled pol arograph are outlined in Figure 4.27. The measuring stand m ay be a single unit ( M etrohm VA 757, Figure 4.28) containing the necessary electronics ( the potentiostat, current-voltage converter, an analogue-digital (A/D ) converter and a digital-analogue ( D/A) converter) ; the controls for the mechan ical components, the stir­ rer and the gas flow valves; the voltametric cell with its three electrodes; and the stirrer. The potentiostat controls the voltage for the d.c. voltage ramp (or now more commonly a stepped or staircase ramp) over ranges to ±4 V, the potential pulses for p ulse and differential pulse polarography-voltammetry and the superimposed modulation voltages for square-wave- and a.c.-polarography o r voltamm etry.

118

Introduction to Voltammetric Analysis

F i gure 4.28

Metrohm 757 VA Com p utrac e ™ with cell and electrodes (Courtesy of Metrohm).

Control Sampling Unit

Figure 4 . 2 9

Waste Supporting electrolyte

u n it

Dosimat

VA Stand

Experimenta l arrangement for automatic

Pump Units

VA Tra ce Analyser

batch voltammetric analysis.

Alternatively the measuring stand may be made up of several sub-units, for example in the A D Instruments system the Powerlab® unit is the interface between the potentiostat and the com­ puter, the whole being controlled by the EchemTM software which can be run on either an IBM compatible or a Macintosh computer. The voltammetric cell is connected to the potentiostat (a range of other manufacturers' potentiostats may be used) . The different arrangements reflect the different purposes for which the respective instruments were designed. The single unit Metrohm 757 VA Computrace™ was designed for routine analyses and method development while the Echem ™-Power lab® system is part of a m ultifunctional data collection and processing system designed for teaching p urposes in a wide range of scientific teaching laboratories-from physiol­ ogy to analyti cal chemistry-using a number of instrumental techniques such as spectroscopy, chromatography, voltamm etry and potentiometry. The design of other manufacturers' instru­ ments also reflect the m arket to which they are directed. A list of manufacturers of voltammet­ ric-polarographic equipment is given in Appendix 7. The automation of stripping voltammetric analysis can be based on either the batch technique or a flow- through technique (chapter 5 ) . In the batch technique the sample is inserted into the supporting electrolyte flow and automatically conveyed to the cell. The analysis is then carried out under stationary conditions. A typical experimental arrangement is shown in Figure 4.29. The sampling unit and the pumps are controlled by the control unit. The deaeration and stirring of the cell solution, the accumulation potential and time, the rest time and the stripping potential

Practical aspects

119

Fill sample loop

Pass the sample th rough the supporting electrolyte i n the cel l

Record and evaluate the voltammograms

Store data in P C

Pump out the solution from the cell

C lean the cell and electrodes

Figure 4.30

Flow d i a g ra m of the program for automatic batch voltammetric analysis.

program are controlled by the Trace Analyser while the two - o r three-fold standard addition sequence is controlled by the Dosimat™. The data is processed hy the Trace Analyser and can then be stored in a compute r conn ected to the an a lyser . A flow diagram of the op e ration sequence is given in Figure 4.30. The processor can be pro grammed to omit the enrichment step when the current s i gnal measured in a test run is above a pre-determined value ( corresponding to an ana­ lyte concentration above the range for stripping volt a mm etry) . The determ ination mode thus changes automatically to differential pulse polarography and the standard additi o n dosa ge is also increased [49 ] . A wholly a ut o mat ed system with a motorised b urette , a samp le ch a nger and a flow-through cell ( chapter 5) is shown in Figure 4.3 1 . In addition to prepared samples , standard solutions may be loaded i nto the sam p le changer. With the help of the motorised burette it is p o ssib l e to carry out standard additions (§ 4.7 ) . The enrichment time is determined by the flow rat e and sample volume set on the Dos i m at ™ . At the end of the enri chment time and after the r ep l a ce ment of the sample solution by the supporting electrolyte, the stripping voltammogram is recorded in a still sol ution . The analyte concentration is determined by the computer ( polarograph) with reference to a linked-up data bank. The analytical procedure becomes co m ple t el y automated when a flow­ through U .V. digestion apparatus for the sample pre-treatment is linked to the measuring equip­ me n t of F i gu r e 4.3 1 .

4.7 EVALUATION AND CALCU LATION Polarograms and voltammograms are eva l uated either graphically or by calculati o n . To obtain an accurate value of the height of a current wave or pe ak , the residual o r ba c kgro und current must be de ducted from the measured signal val ue . In Figure 4.32 possible w ays of graphically evaluating

1 20

Introduction to Voltammetric Analysis

a

Sampli ng

b

tube '-----

Waste Sample C h anger

Figure

4.31

Dos i no

Flow-th rough cel l

VA Sta nd

VA Trace Analyser

Photograph (a) a n d sch e m a ti c l ay o u t (b) o f equipment for fu l ly automated flow-th rough stripping vo lta mm etry with a sample changer and motorised burette (Courtesy of Metrohm) .

different types of curves and peaks a re compared. When graphically evaluating d.c. polarographic currents, the residual current is assumed to vary linearly with potential. However, the residual current-potential plot is actually non-linear since the double layer capacitance varies somewhat with potential ( § 1 .4.3 ) . T o a first approximation, i t can b e compensated instrumentally by adding t o the cell output current a counter current which increases linearly with potential (cha rge compensation ) . When both branches of a wave have different slopes, more empi rical evaluation procedures for deter­ mining the wave height are used ( Figu re 4.32b ) . In a series of measurements ( including the cali­ bration measurements) the same evaluation procedure must be used throughout. At higher sensitivities the plot of the residual current becomes curved in all cases, and the determination of the true wave height ( current ) is more difficult. The best approach is to record the residual cur­ rent of the supporting electrolyte alone (a blank determ i nation) and subtract the blank current from that of the test solution, either visually (by overlaying of transparent paper) or by calculation [ 50, 5 1 ] . I n the evaluation o f peak currents the residual current can be estimated visually. At low con­ centrations and with residual cu rrents having a pronounced curvature, the tangent method leads

Practical aspects

1 21

a

b I I c Figure 4.32

Eva luation of pol arograms and voltammograms: (a) eval uation of waves and peaks under ideal cond itions (constant residual cu rrent); (b) eva l uation of wave height u nder non-ideal cond i t ions; (c) eval uation of peaks with non- l i near residual currents (I) tangent method (II) eval uation with cu rved base current.

to calibration curves which do not pass through the origin and which also may be non-linear. The evaluation with a curved baseline is to be preferred in such cases (Figure 4.32c). The best values are obtained when a polynomial describing the approximate shape of the baseline is used. The coefficients of the polynomial are determined from experimental points which lie o n both sides of the current peak [ 5 2 , 53 ] . This evaluation becomes straightforward with computer controlled equipment in which the cu rrent-potential curves arc recorded digitally at a high data rate. The first step in the data processi ng consists of the automatic search for stray experimental data points. These are then eliminated and replaced by the mean value of both neighbouring data points. The line of best fit of the current-potential curve is then smoothed within a sliding window along the voltage axis. The smoothing factor which can be applied essen­ tially depen ds on the number of data points available. The pri nciple of the curve smoothing i s illustrated in Figure 4.33. After smoothing, the curve is automatically differentiated ( first derivative) and in this form used for peak recognition. This is achieved by seeking out successive maxima and minima in the differentiated curve ( F igure 4.34 ) . In the usual voltanunetric presentation ( increasing negative potentials to the left ) , a maximum followed by a minimum indicates a reduction peak, whereas the reverse order indicates an oxidation peak. The potentials of the maximum and m i nimum in the di/dE versus E plot are noted and the peak width is the difference between these values, while the peak potential is the midpoint between them. The qualitative identification of the analyte is achieved by comparing the peak potential ( ± a pre-set tolerance ) with data stored in the computer

1 22

Introduction to Voltammetric Ana lysis

•

•

•

•

•

•

•

•

•

E Figure 4.33

Curve smooth ing using a vo ltage window with nine data poi nts.

Peak width

E d ild E

Peaks of the derivative c u rve

E

Figure 4.34

Peak recognition and cha racterisation (Courtesy of Metrohm). Peak potentia l = peak width = I Emin - Emaxl·

(Emax + Em;n)/2;

memory. For a peak to be id e n t i fi e d it m u st be g re a te r than a pre-determined minimum peak height. After th e p ea k is recogn ise d a baseline is constructed and its value at the p e ak p o t ential is s u b tra ct e d from the maximum value of current to give the peak height. The result is pr es e n t ed as a peak or wave height or as the p e ak area all of which are p ro p o rti o n a l to the concentration of the analyte. The above procedure is summarised in Figure 4.35.

Practical aspects

dE

(front) I

dE

I

( rear) I

Foot of cu rve �:�1 : 1 (front) : Contact point (front)

1 23

I I I I I

Foot of cu rve (rear)

I I I I I

E

Hei g ht or a rea calcu lation

_k Figure 4.35

� eak hei g ht

11

Area

Wave hei g ht

Flow diagram of peak and wave eval uation procedu re (Courtesy of Metrohm).

When th e ba sel ine is curved th e polynomial wh ich approximately describes this baseline is used to mathematically establish it over the potential region across the base of the peak. This potential re gi on is determined from the potentials at which the change in the gradient (becomes more positive) of the current-potential cu rve occurs. The same procedure is used for the evalua­ tion of closely adjacent peaks. The peak- or wave-height ( h) or peak area ( A ) calculated by the above procedure can be conve rted either by a calibration curve ( c versus h or c versus A) or by th e method of standard

1 24

I ntroduction to Voltammetric Analysis

additiorls into the desired concentration units. When using a calibration curve its slope and lin­ earity must be checked at least daily. When experimental parameters are changed, in particular after changing the electrode capillary, re- newing a carbon paste electrode surface or after distinct changes in temperature, it must be re-determined. When using the method of standard additions, an aliquot, v, of a standard solution of known analyte concentration c is added to a volum e V of the sample solution in which the analyte con­ centration is ex. If h is the peak- or wave-height of the sample before the standard addition, and H the height after that, then the analyte concentration is given by

h cv

H( V + v) - Vh

The h i ghest accuracy can be obtained by using several (at least th ree) standard additions, pro­ vided the total concentration always lies within the linear range of the standard cu rve, p lotting h versus c, and extrapol ating the straight line plot back to the c axis. The distance from the origin to the intercept corresponds to c;. The addition of the standard solution can be carried out manually with a micropipette or automatically with a motorised burette such as Metrohm's Dosimat™ 685 or Dosino"" 700. In combination with further Dosinos for the addition of auxiliary solutions, for example for rinsing, and pumps for emptying the cell, polarography-voltammetry now can be extensively automated. References 1 2

4

3

5

6 7

8

9 10 11 12 13 14

15

16 17 18

19 20 21 22 23 24 25 26

27

28 29

l'. Tschopel, L. Ko lz, W. Schultz, M. Veb c r and G. Ti)lg, Fresenius Z Analytische Chem ic, 1 980, 30 1 , 1 .

G. Knapp, Tntern ational f. Environmental Analytiw l Chemis try, 1 98 5 , 22, 7 1 . Edn, VCH , Wcinheim, 1 990. H . -Y. C he n and R. Neeb, Fresenius Z. Analytische Chem ie, 1 984, 3 1 9, 240. P. A . Bruttel and J. Schafer, Sample Prepara t ion Tech n iq u es in Volta m metric Trace Analysis, Metrohm Monograph, Metroh m , Hcrisau, Switzerland, 1 99 2 . H. Kroder, K . H . Bauer and D. Saur, Vom Wasser, 1 994, 82, 269. M. Ko l b , l'. Rach , J. Schafe r and A. Wild, Fresenius f. A 1zalytica/ Chem is try, 1 992, 342, 3 4 1 . F. Wadhat and R. Neeb, Frese11i11s Z. Anr�lytische Chem ie, 1 989, 335, 748. ] . l'isch, J. S c h a fe r and D. F r a hn e , G/T Faclrzeitschrift fiir das Labora tori um , 1 99 3 , 37, 500. ] . K o th e a n d R. Bitsch, Fresmius ]. Analytical Chemist ry, 1 992, 343, 7 1 7. S. San der and G . H e n ze , GJ T Labor- Fachzeitschrift, 1 996, 40, 1 23 2 . P. Collet, L. Matter and B. Thomas, l.ebensmittelchem ie, 1 990, 44, 3. G . Knapp, Fresenius Z Analytische Chem ie, 1 984, 3 1 7, 2 1 3 . L . Dunemann, 5th Colloq ui um A tomspektrometrische Spurenanalytik, B . We l z , editor, Bodenseewerk Perkin-Elmer, Uberlingcn , 1 9 8 9 , p. 5 9 3 . P. Schramel and S . Hasse, Fresen i us /. Analytical Ch e m istry, 1 9 9 3 , 346, 7 9 4. W. Wasiak and A . Ci skewska, Analytica Clremica Acta, 1 996, 335, 20 1 . I . Eguiarte, R.M . Alonso and R.M. )imenez, A nalys t, 1 996, 1 2 1 , 1 83 5 . H . Hein a n d W. Ku n ze , Umweltanalytik m i t Spektroskopie u n d Chroma tograph ie, VCI f, 1 994, p. 1 00 . G.K. Rudnikov an d N . A Ulakhovic, Russian Chemiml Reviews, 1 980, 49, 74. P.T. Kissinger and W. R. Heineman, l.aboratory Tech n iques in Electroanalytical Ch em ist ry, Marcel Dekker, New York, 1 984. D.T. Sawyer and J . L . Hobcrts, F.xperimental Electrochem istry for Chem ists, J . Wiley, New Yo r k , 1 974. ]. Heyrovsky, Polarograph isches Praktikum, 2nd Edn, Springer-Verlag, 1 960. F. von Sturm a n d M . Res s e l , Fresenius Z Analytische Chem ie, 1 962, 1 86, 63.

K. Doerffel, Stat is t ik in der Analytischen Chem ic, 5th

U. Eisner, J.A. Turner and R.A. Osteryoung, Analytical Chemist ry, 1 976, 48, 1608. R. Necb, I. K i e h n a s t and A. N a rayanan, Fresenius Z. A n a lytische Chen1 ie, 1 97 2 , 262, 3 3 9 . J. Lindquist, /. Electroanalytical Chem istry, 1 974, 52, 37. M . I . Montenegro, M .A. Q u c i ros and J.L. Daschbach, editors, NATO, ASI Series, Series E, Applied Sciences, Vol. 1 97, Microelectrodes: Theory and Applicat ions, Kluver, Dordrech t, 1 99 1 . J . S c h i e we , A.M. Bond and G . Henze, Umweltdiagnostik mit Mi k rosys te men , G . H e n ze , M . Kohler a n d J.P. Lay, editors, Wiley- VCH, 1 999, p. 1 2 1 . J. Heinze, Angewandte Chemie, In tern a tional Hdition, 1 99 3 , 32, 1 268.

Practical aspects

30

R.M. Wightman a n d D.O. Wipf, Electroanalytical Chemistry, Vol. 1 5 , A.J. Bard,

0 . Niwa, Electro a n alys is, 1 995, 7, 606. York, 1 989. p. 267.

31 32

1 25

edito r, Marcel Dekker, New

M. Kohler, Umweltdiagn ostik mit Mikrosystemen, G. Henze, M . Kii h l e r and J . P. Lay, editors, Wiley- VCH, 1 999,

p. 99. 33

34

35 36

37

38 39

40 41 42

S. Fletcher, Chemis t ry in A ustra lia, 1 994, 6 1 , 80.

R.I.. D e u tsch e r and S. Fletcher, }. Electroanalytical Ch e m is try, 1 988, 239, 1 7.

S. Fletcher and M . D. Horne, Electrochemistry Co m m u n ications, 1 999, I , 502.

Kh. Br ain i n a and E.N eyman, Electroanalytical Stripping Methods, Wiley, 1 993.

Kh.Z. Brainina, N.A. Malakhova, A.V. Iva nova, B i ose n sors for Direct Monito r ing a( Envi ro n m e n ta l Pollutarzts in the Field, Kl uver Academic Publishers, Dordrecht, 1 989. K. Kalcher, Electroanalysis, 1 990, 2, 4 1 9. K. Kal cher, ).M. Kauffman n , J.Wang, I . Svancara, K . Vyteas, C. Neuhold and Z. Yang, Elect roamJ iysis, 1 ':1':15,

7, 5 . L. Gorton, Electroarzalysis, 1 995,

J . Wang, T. B aom i n a n d R.

45 46

47 48 49 51

50

52

53

1 994, 6, 3 1 7.

Kh.Z. Brainina, N. Stojko, C. Faller a n d G. Henze, Umweltdiagrwstik mit Mikrosystemen, G. Henze, M. Kohler editors, Wiley- VCH, 1 999, p. l 39. Kh.Z. Hrainina, H . Schafer, A. V. Ivanova and R. Khanina, Analytica Ch im ica Acta, 1 996, 330, 1 75. M. Kohler, Institut fiir physikalische Hochtechn ologie, Jena, p ersonal communication. N.Yu. Stojko, Kh.Z. Brainina, C. Faller and G . Henze, Analytica Chimica Acta, 1 998, 371, 1 45. C. Faller, N. Yu .Stojko, G. Henze and Kh.Z. Brainina, A n alyti ca Ch i mica Acta, 1 999, 396, 1 95. C. Faller, G . Henze, N .Yu. S tojko, S. Saraeva a n d K h . Z . Brainina, Fresen ius' f. Analytical Chemistry, 1 997, 358, 670. S. Sander, W. Wagner and G. Henze, A rzalytica Chemica Acta, 1 997, 349, 93. A.M. Bo nd and B.S. Grabaric, Analytica Chemica Acta, 1 978, 1 0 1 , 309. S.D. Brown and B.R. Kowalski, Analytica Clr im ica Acta, 1 979, 107, 1 3. D. F. Untereker, W.G. Sherwood, G . A . Martinchek, T.M . Reidhammer and S. H ru ckenste i n, Chemical Instrumen tation, 1 975, 6, 259. A . M . B o n d a n d B . S . Grabaric, A na ly t ical Chem istry, 1 979, 51, 337. and J.P. Lay,

44

Electroanalysis,

Kh.Z. Brainina, G. Henze, N. S tojko , N. Malakhova and C. Fal l e r, Fresenius /. Analytical Ch e m is t ry, 1 999, 364,

285.

43

7, 2 3 .

Setiadji,

5

Flow-throug h tech niq ues

W hen t h e am ount o r co m po si tion of a n analyte in a solut i o n o f a sa m ple is determin e d b y flowing the solution past a detector, the t e chn i q u e is referred t o as a flow-through technique. These tech­ niques are imp o rtan t in automated a nalyt ica l p rocedures a n d greatly facilitate the combination of two o r m o re analyti c al te chniq u es in series to give powerful tandem techn iques with improved s e l e ct i vi ty and/or sensitivity. Im por tan t fl o w th rough t ech niq u es which employ volta m metry are h igh performance liquid chro matography, HPLC, with voltammetric or amperometric detection, high performance capilla ry electrophoresis (HPCE), with vo lta mm etr ic or a m pero m etric detection, voltammetric flow injection analysis (FIA) and flow-through stripping voltammetry. In all cases the sa mp le flows in a stream of supporting electrolyt e solution ( mobile phase) at constant velocity th rough the flow channel of a flow-thro ugh cell which contains the working elec­ trode a reference electrode and in most cases a n a u x il i a ry electrode ( three-electrode cell) . The basi c requirements for a good fl o w - thro u g h cell are: -

,

(i) (ii) (iii) (iv) ( v) (vi)

well defined hydrodynamics, high mas s transfer rate to the working e l e c trode high signal-to-noise ratio, low dead volume, s imp le design and easy to c l ean , optim u m e l e ctr o de place m ent

,

.

The cell must be so constructed that as m u c h of the analyte as p o ssib le m ust rea ch the wor kin g el ectrode with the minimum of turbulence and parti c ipate in th e electrode reaction, particularly when wo rkin g with low concentrations. This req u ires that laminar flow conditions prevail in the fl owing support i ng e l e ct r o lyte solution so that the band of a nal yte carried along in the support i ng ele ctro l yt e solution undergoes minimum m i xin g with the adjacent solution segm ents as it passes through the cel l and reacts at the electrode. The continuous flow of the supp o rt in g elec tro lyte solution deans the ce ll and readies it for the follo wi ng measurement. The flow situation in a flow-through ce l l is illustrated in Fig ure 5 . 1 . The mass tra n sport in the flow c h annel is m a i nl y by convection whereas in t h e vicinity of th e working elect rod e ( in the sta­ ti o nary l aye r on its surface ) it is diffusion co n t r o lled Of the wide var ie ty of flow-through cell desi gns, the m ost co m m only used are th e wall-jet cell, a , the th i n layer cell, b, an d the tubular ce l l c, illustrated in Figure 5.2. The currents developed in these cells as a fun cti o n of va r ious p h ys i ca l parameters are given by the fo llowin g equ a t i ons [ 2 ] . .

,

( a ) Wall-j et cell

1 26

( 5. 1 )

Flow-through techn iques

1 27

Counter electrode

�

---� -----------

�

� � ----­

Working electrode

M ass transport through a flow-through cell [1 ] .

Figure 5 . 1

( b ) Thin layer cell

(5.2 )

(c) Tubular cell

( 5.3 )

where

i is the limiting current ( tlA), the number of electrons i nvolved in the electrode reaction, F Faraday's constant, D the diffusion coefficient of the analyte, v k the kinematic viscosity of the supporting electrolyte solution, a the diameter of the inlet jet of the wall jet electrode, A the electrode area, w the mean volume flow rate of the solution, c the concentration of the analyte, b the channel height of the thin layer cell, and r the inner radius of the tubular cell. n

In the thin layer cell the analyte solution flows parallel to the planar working electrode surface, in the wall-jet cell it strikes the centre of the planar working electrode a t right angles then flows radially across the surface, while in the tubular cell it flows through a cylindrical channel in which the tubular working electrode is arranged. Graphite, glassy carbon, carbon paste, noble metals and mercury are used as electrode materi­ als. Flow-through cells with carbon working electrodes are m a i n l y u sed for the anodic detection of organic compounds after they have been separated by liquid chromatography or capillary electrophoresis, whereas those with mercury working electrodes are used for trace metal analyses via flow-through stripping voltammetry.

I

I Figure 5 . 2

a

b

c

Voltammetric flow-th rough cells showing the w orking electrode only in each case.

1 28

Introduction to Volta mmetric Analysis

Detector

Chromatog ram

Amplifier

WE AE

RE E l u ate Figure 5.3

t

Outl ine of a m pe romet ri c detection. WE - working electrode; AE - auxi liary e l ectrod e; RE - refere n ce e l ectrode .

5.1 AMPEROMETRIC AND VOLTAMMETRIC FLOW-TH ROUG H DETECTION Flow-through cells in which the measuring principle is based on amperometry o r v o lta m m et ry [ 1 -8 ] serve as d e t ectors for liquid chromatography a n d capillary electro phoresis. T h ese two t yp e s of detector a l o n g with those based on the principles of conductometry, p o te n ti o m et ry and cou­ lometry are among the group of detectors known as electrochemical detectors. Amperometric detectors, which are the most commonly used [9 J , are of the three-electrode type. The potential of the working electrode is set relative to the r efe r en c e electrode, the iR dro p between the working and a uxiliary electrodes is co m p e ns ate d for by the potentiostat and t h e cur­ rent flowing thr o ugh the working electrode is the m e asu re d signal. The c u rr e n t , which is in the pA to f.IA ran g e , is a m pl i fi e d and recorded as a function of the time flow of the mobile ph as e This gives t h e concentration-time profi l e or ch ro m a to gra m of the analyte( s ) in t h e effluent. The p r i n ­ ci p le of amperometric detection is outlined in Figure 5 . 3 . .

first used a s a

des c r i bed b y K iss i n ge r in 1 973 [ 1 0 ] . T h e wall-jet principle h a d been d etec to r i n H P LC in 1 974 [ 1 1 ] . The development of amperometric cells

Historical n o te. T h e first thin layer cell was

1 967

back to the work of Ke m u l a [ 1 2 ] who used < 1 DME as a detecto r for col u m n chro m ato gra phy. Kemula, who

known since

and

was

p o l a ro gra p hy 'chromatopolarography', continuously flowing from the chromatographic column using the apparatus shown in Figure 5.4. The chromato p olarogra m was re c o rded at a constant potential. Fu rther developments resulted i n d i ffere nt geo met r ic arrangements and smaller cells which were used fo r voltammetric flow analysis and for flow-through stripping vol­ ta m m et r y as well as an ampero metric detector i n combination with c h r o m a tog r a ph i c columns [ 1 3 ] . With the in t ro ­ duction of glassy carbon and carbon paste el ec t ro des in a mperometric and voltammetric d ete c to rs for ch roma t o graphy and capillary electrophoresis, Kemula's detection principle was extended to the analysis of oxi d i s ­

goes

polarographed the el u a te

called the combination of column chromatography with

able substances [ I 0 ] .

Methyl­ para1hion

I [JJ A)

p- N itro· phenol

Parathion

0

Figure 5.4

2

3

4

5

t [min]

6

Kem ula's c h romatopolarograph and the current-time t race recorded for a m ixture of insecticides. EoMe -1 . 1 V vs Hg pool electrode. =

Flow-through techniques

Figure

5.5

1 29

Profile o f the diffusion layer i n a wal l-jet cell [1 4] .

Thin l ayer and wal l -jet cel l s with glassy carbon electrodes are

the main flow-through elec­ should be less than 5 f.!L in order to avoid any s e n s i tiv i ty loss or peak b roadening caused by an u nn ec e ssa r i ly l a rg e dead volume in the cell. With m o de rn m i cro - tec hnol ogy it is p o ssi b le to c o n str u ct cell s h avin g volumes in the n L range. Such miniature detectors are used in c onj u n ct i on with m i c ro - H P L C columns for the analysis of very small s amp le s . The a dvantage s of amperometric detecto rs over other types are their go o d hydrodynamic properties, th eir r a p id re sp o n se a nd t h e re l a t ive ly h igh se n sit iv ity of the m e asure d s i g nal. While the current flowi ng from the el ec tr o d e p roce s s i s p ro p o rt i o nal to the an alyte concentra­ tion in accordance with Faraday' law ( e q ua t i o n 1 . 5 ) , under hyd r o d yna m ic conditions only tha t p o r tion of the an alyte which reaches the electrode surface by diffusion from the flowing electro­ lyte can react. Consequently, increasing the electrode surface area will lead to an increase in sensi­ tivity i n a n a mper o m e t r i c detector. However, the increase in se n s itiv ity is limited as the noise l eve l also increases wit h i n c r eas i n g elec t ro d e s u rfac e a r e a . The s ign al - t o - no i s e ratio also d ep e n ds on t he electrode material and the detailed structure of the working el e ct rode s u rface. In am per o m etr ic fl o w-through cells, the analyte is convectively t r a nsporte d w ith t he flo w ing electrolyte and re ac h e s the working electrode surface by diffusion th rough th e stationar y di ffu sion l ayer on the surface; m igra ti o n is insignificant because of the presence of the supporting elec t r o ­ lyte. The thickness of the di ffusi o n la ye r , 8, an d hence the c u rrent , depends upon the cross-section of the cell and the electrolyte s olution flow rate ( e q u a tions 5 . 1 , 5.2 and 5 . 3 ) . The highe r the flow r a te , the th i nner the diffusion layer and, since the current is inve rsely p r opo r tion a l to 8, the higher the current. (Note that in all c ases , 8 is less than it is in still solutions. ) These relationships are not valid at very high flow rates wh en the t r ansp o rt velo c i ty of the analyte in the cell is much greater than the diffusion velocity th ro u gh the still layer. The profi l e of the diffusion l ayer d e pend s upon the tl ow con ditions as i s sh own for the case of the wal l - j et el ectrode i n Figure 5 .5 . Tn order to produce a suitable di ffu s i o n layer the flow rate of the so l ution perpendicular to th e wall - j e t electrode shou l d be si m i l ar to the paral lel fl ow rate of

trodes used as d e t e c t ors fol l ow i n g

HPLC

separation . The cell volume

the sol ution i n a th i n l ayer ce l l . A di sadvan tage of the p erpen dicular tl ow i s t h e tendency to create

a vortex wh i c h

can result i n broaden i n g and a l oss of symmetry i n the peaks. Thin layer cells with cell volumes < 1 00 f.!L h av e the a dva n t a ge o ver wa ll -j e t cells in th a t the de a d vo l u m e i s s m a l l eno u gh for peak b r o a d ening to be avoided . Thin layer cells can also be constructed with two w orki ng electrodes ar r a n g e d either in p a r a llel or in series in the flow channel ( F i gu r e 5 . 6 ) . With su ch a dual e l ec t r o d e ar r a ngeme n t d i ffere n t pote n t i al s can be app l i ed to each wo rk ing el ectrode so that a s ingl e analyte can be oxidised at o n e el ectrode and reduced at the other, o r if two an alytes are po o r ly s epara t e d c h r o m a t o gr a ph i cally but have

Introduction to Voltammetric Analysis

1 30

a

b Cou nte r electrode

Counter el ectrode

I I

Work ing e lectrode

Fig ure 5.6

...... 1 I .,. .,. I

1

---..

Working e lectrodes

Thin layer cel l with para l lel (a) and series (b) arrangement of the working electrodes.

dissimilar reduction ( o r o x i dation) p o ten t ials then one may give a signal at one of the elec­ trodes and both give a signal at the other electrode. If the same potential is ap pl ied to each elec­ tr o d e , then the increase in electrode reaction leads to an increased current signal-however the noise l evel also increases. Coulometric detectors are eith er tubular ce l ls or t h i n layer cells in which the working elec­ trode has a much l a rger surface area t h an is t he case for a m p e rom et ric detectors so that com plet e oxidation (or r edu cti o n ) of the band of a n a l yte as it passes the working electrode may be achieved. The electrode m at eria l in tubular cells is either metal or graphite arranged along the inside wall of the tube ( open tubular c ell ) or as a porous filling ( packed bed tubular cell) . The essential difference between amperometric and coulometric detection is that in the former it is the p eak curren t that is m e as ured and in the latter it is the area under the peak ( Q J id t) th at is measured. Because of the large electrode s u rfa ce area, lower signal-to-noise r atios are observed so that mea s uremen ts made wi t h coulometric detectors are rare ly more sensitive than those made with amperometric detectors. Moreover, the re gen erat i o n of pas s ivated electrodes (mechanically o r ele c tro chem ic a l ly) is m uc h more d i ffi c u l t in coulometric tubular cells than is th e cle an i ng of the p la n a r electrodes in a thin l aye r cell. These p roble ms are not so troublesome when the coulomet­ ric cell is not used as a dete c tor but as a fi lt e r for the el ectro lyt i c removal or conversion of interfer­ i n g substances prior to d e te c t i on , or for the electrochemical derivatisation of poorly detected analytes [ 1 ] . Th e main adva n tage t h at a mp e r o m et ric and c o ulom etric flow-through cells have over other HPLC d et e cto rs-o f which U. V. detectors are the main ones used-is their selectivity and high s en sit ivity. The main factors which effect the sensitivity of amperometric and coulometric detec­ 1 tors are: t h e cell geometry t h e surface (area and condition) and compo s ition of the working electrode t h e flow rate a n d c o m p os i t i o n of the eluate t he potential a p p l i e d to th e work i n g electrode.

=

•

•

•

•

!.

Flow-through techniques

1 31

The preferred electrode material for commercial amperom e tric and coulometric detectors is carbon in the form of glassy carbon or carbon paste. The electrochemical p roperties of carbon enable measurements to be made over a po te ntial range in excess of 2.0 V wi t h the limit in the cathodic direction being determined by the pH of the eluate. Glassy carbon is more resistant than carbon paste (a mixture of graphite powder and a higher hydrocarbon such as Nuj ol) especially t o wa rd s orga nic solvents. The stab i l ity of c a rbo n paste e lectrodes can be increased i f a polym er such as p olyethylene PVC, chloroprene gum or K el F is used instead of Nujol. The advantage of carbon pas te electrodes i s t hat they have a lower base current and a better signal-to-noise ratio than do glass y carbon electrodes. For a n u mb er of a n al ytes noble metal (Au, Pt) electrodes are preferred and mercu ry electrodes or amalgam at e d go l d electrodes ar e used for reductive determinations [ 1 3, 1 5-1 8 ] . In s pecial cases other metals are used as electrodes, for example cobalt and iron electrodes, modi fi ed with phthalocyanine, h ave been used for the determination of organic pe roxides or hyd r oxylamines [ 1 9, 20] . T he rep roducibility and sensitivity of measurements made with solid electrodes generally de p en ds on the preliminary t rea t m ent o r con d i ti o ning of t h e electrode surface. G las sy carbon is us ually polished with an alumina ( < 1 J-Im) suspension and rinsed with high purit y water while a carbon paste surface can be regenerated simply by mechanically removing the old surface layer. In ,

-

'

'

for the case where the chromatographic p ea k s a r e symm etrical (Gaussian bell shaped curve i n Figu re 5.7 ) the mass of a n a lyte that is j ust detectable can be calculated u s in g Fara day' law. The charge consumed by the a na lyte is propor­ tional to the area under the peak and is given by

and the

Q =

m ass

J i( t) d t

=

khB

with k "" 0.42

of analyte reacted at t h e e l ect r o d e , m, is thus

0.42��B

m

molar mass = 200 and two electrons arc requi red per molecule of el ect ro de reaction , then as li ttle as 1 pg of will pro du ce a peak with a height of 1 00 pA a n d a breadth of 23 s. In a co u l o m e t r i c detector the mass of ana­ lyte redu ced ( oxidise d ) should theoretically be equal to t ha t which comes off the colu mn. In a n a mperometric detec­ tor however, onl y from I to 5% of the analyte reacts al lhe electrode so t ha t 20- 1 00 pg of analy le is n eeded lo produce the above p ea k . Thus the d e termi n a t i o n limit of both detectors lies in the lo wer pg region 1 1 4 ] . The h ig h er noise levels in the coulometric detector tend to counter its h igher electrode reaction e ffic i en cy so that only ra rely is it more sensi tive than the a m p e ro m etri c detector. I t i s worth n ot i n g t h a t when amperometric de te c ti on is combined wi t h a m i c r o - b ore co l u mn the d etermination is more sensitive than mass sp ectrome t r y in many ca ses .

If the

a n alyte

I I

I I I

�+-- s -!

Figure 5 . 7

G aussian chromatographic peak: tR retention time. h pea k height, 8 peak breadth

(6a).

1 32

I ntroduction to Voltammetric Analysis

I njector

/

Pu lsation damper

F i tt i ngs Pump

Separation column Degaser

El uent A m pe romet ri c detector

Waste I ntegrator Figure 5.8

B lock diagram of eq u ipment for H PLC w i t h am perometric detection.

pro d u cts may be el ect roc h em i c al ly removed from the electrode su rface by cha n g i n g the pote n t ial app l i ed to t h e electrode between two suitable values ma ny times. When this process i s carried out di r e ct ly in the m easu ri n g cell between successive samples and becomes part of the electrode pote nt i a l program it is referred to as pulsed ampero­ metric detection ( PAD ) [ 2 1 , 22 j . The use of HPLC with a m pero m et ric detection is most efficient when both sys tems a re fully i n te gra t ed It is important th at the HPLC column is a reversed p has e type so that aqueous mixed so lven t el uents in wh i c h buffers or supporting e le c tro lyte salts are soluble can be used. The organic solvent part of the m i xtu re is usually a c eto n it r i le or methanol. The essential features of an HPLC-am perometric detecto r system are outlined in F i gu re 5.8. The first step in the p ro ce s s m ust be the rem oval of o xyg e n from the elution mixture in th e de gass er The pump must p ro v ide a cons t a nt rate o f flow of eluent, i n depen d ent of the co l um n back pressure and of the eluent viscos­ ity. Minor pulsations in the flow produce a n o i sy base line and a dec r eas e i n the s en si t ivty of the measu rements. The p ul sat i on s are rem oved by the pul s e da m pe r The pre-column which usually con ta ins the same materia l as the main column , should h o l d back troubl esome i m puriti es and i n s o d oing extend t h e life o f the m a in se p a ra t i on co l u m n The details of a three-electrode ampcrom ct ric detector cell, c on s tr u cte d from polychlorotri­ fl uo ro et hylene with a wall - j e t electrode arc shown in figure 5.9. T h e cell is thermostattcd in order

many cases accumulated electrode reactio n

.

.

.

.

,

Flow-through techn iques

Reference electrode

Figure 5 . 9

1 33

Auxil iary el ectrode

An amperometric detector with wal l -jet working electrodes and fitted with a water jacket for temperature control, cell vol ume < 1 !J L (cou rtesy of Metrohm) .

to minimise errors from temperature fluctuations. Working electrodes are e xchan g eab l e and made from glassy carbon or carbon paste discs. The cell is provided with standard fittings for direct attachment to any HPLC system . A cell in which a micro-electrode array replaces the s ingle disc electrode has also been de v e l o ped [ 23 ] . ln principle this electrode array enables several ana­ lytes to be determined simultaneously simply by applying a range of potentials to the v ari o us micro discs. The selectivity of amperometric detection de pends on the fact that only co m p o unds which contain groups that can be oxidised or reduced at the potential applied to the working electrode give useable signals. The inter- relation between selectivi ty, sensitivity and detection po t en t ial is illustrated for a m ixture of 3-nitrophenol and 3 - methyl-4- chlorophenol in Figure 5 . 1 0 [24] . The current is due to the oxidation of the phenol group in both compounds. Whereas at + 1 .0 V only 3-methyl-4-chlorophenol gi ve s a useful peak, at + 1 .2 V both analytes record good useable peaks in the chromatogram. Determinations using anodic detection are more selective and sensitive if t he least positive detection potential that g ives a usable signal is used ( minimise the presence of peaks due to other s pe c ie s that are oxidised at potentials more positive than the analyte) . For

3-Methyl -4-chlorphenol 1 .4 v 1 .3 v

1 .2 v

1 .1 v 1 .0 v 0.9 V 4

Figure 5 . 1 0

6

8

10

12

t

[min]

Selectivity and sensitivity of amperometric detection a s a function of detection potentia l .

1 34

I ntroduction to Voltammetric Analysis

E'

20

"ii

15

� :l:

Dl

.t: .II: Ill Gl 11.

10 5

0.85

E'

20

"ii

15

:l:

� Dl

.t: .II: Ill CD 11.

0.95 1 .05

1 .15

1 . 25

1 .35

E

[V]

5

6

7

10 5

b

8

0 0.85

Figure 5 . 1 1

1 . 45

0.95

1 .05

1 .15

1 .25

1 . 35

1 .45

E [V]

Hyd rodynamic voltam mog rams of polycyclic aromatic hydrocarbons [25, 26] : (1 ) benzo(a) pyrene, (2) benzo(k)fluoranthene, (3) benzo(g, h,i)perylene, (4) indeno(1 ,2,3-c,d)pyrene, (5) acenaphthene, (6) fluoranthene, (7) acenaphthalene, (8) benzo(b)fluoranthene. Eluent - metha nol : water 85 : 1 5). =

cathodic detection the least negativ e useful detection potential is used. The most suitable detec­ tion po tent ial is best determined experimentally from the cyclic vo l tamm o gram or the hydrody­ namic voltammogram of t h e analyte. A h yd ro dynami c voltammogram is a current-potential curve whic h shows the dependence o f the chromatographic p ea k h eigh t on the detection po t e nti al . The t ech n ique used to obtain the necessary informati on is voltammetric flow inje c t i on analysis in which an aliquot of th e analyte is i njecte d into the flowing eluent prior to th e detector and t he p e ak current recorded. This is repea ted many times, the det ec t or potential b e i n g c h ange d after each injection, until the pe ak cur­ rent-potential plot rea ches a plat ea u or a maximum as shown for some p o lycyclic a ro matic h yd r o carb o ns (PAHs) in Figu re 5 . 1 1 . Whereas indeno( l ,2,3,-c,d)pyrene, benzo(g,h,i)perylene, benzo{k)fluoranthene and benzo(a)­ pyrene give a maximum detection signal at potential s between + 1 .30 V and + 1 .35 V ( Figure 5. l l a ) , the current-potential curves of acenaphthalene, acenaphthene, fl uor an t he ne and benzo (b ) fluoranthene have not reached a maximum even at + 1 .45 V ( Figure 5. 1 1 b ) . A m easu re­ able sign al is first obtained at + 1 .35 V for acenaphthalene b e n zo ( b )fluoranthene and fluoranthene whereas the other PAHs are oxidised at less positive po t ential s . Based on these resul ts , it is possi­ ble to s e l e c t ive l y determine benzo(a)pyrene i n a mixture of PAHs by using a detection po t ential of + 1 .0 V. At more p o si tive potentials other peaks app e ar in t he chromatogram until finally, at a potential of + 1 .45 V, peaks for all th e PAHs are recorded as shown in the ch romatograms in Figure 5. 1 2. When using amp ero m etric detectors the composition of the el ue n t should remain as constant as po s s ibl e ( isocratic elution ) . If gradient elution is used in order to re d uce the analysis time, then base line drift is obse rved resu ltin g in a loss of s en si t ivity . This is partic u lar ly so with glassy c arb o n worki n g electrodes and is due to the slow adj u s tm e nt of the electrochemical equilibrium at the electrode surface ( long lead-in time) to the cha n gin g e l uent co m p os i t ion . Wo rk in g electrodes made of other materials such as the u l t ra - t r ac e gra phit e electrode, have m uch shorter lead-in times (see Figure 5 . 1 3 ) . The relatively rapid response of t he ultra-trace electrode arises from its

Flow-through techniques

1 35

2

3

6 5 8 7 4

c

b

a

10

0

Figure 5 . 1 2

30

20

t [min]

Determination of polycycl ic hydrocarbons by H PlC with amperometric detection : (a) detection potential + 1 .0 V - selective determination of 1 0 ng benzo(a)pyrene (6); (b) determination of 1 0 ng benzo(a)pyrene (6) and 1 0.5 ng benzo(g, h, i)perylene (7) at + 1 . 2 V; (c) determ ination of 8.6 ng acenaphthalene (1 ) , 1 1 .4 ng acenaphthene (2), 5 1 . 5 ng fluoranthene (3), 9.5 ng benzo(a)fluoranthene (4), 1 0.5 ng benzo(k)fl uoranthene (5), 10 ng benzo(a)pyrene (6), 1 0. 5 ng benzo(g, h,i)perylene (7) and 10 ng indeno(1 ,2,3-c,d) pyrene (8) at a detection potentia l of + 1 .45 V. Eluent - methanol :water 85 : 1 5 with 2 g l- 1 trichloracetic acid as supporting electrolyte. Working electrode - glassy carbon, reference electrode - Ag/AgCI in methanolic KCI. The detection and determination limits were i n the range 1 00-300 pg per i njection [25, 26].

b

a

0

Figure 5 . 1 3

10

20

30

40

50

60

70

80 t

[min]

Establishment of electrochemical equilibrium (lead-in time) for a glassy carbon (a) and an ultra­ trace graphite (b) electrode [26] . Supporting electrolyte - methanol :water 70:30 with 1 g l-1 UCI04 and 0 . 5 g l-1 H2SO� detection potential + 1 . 3 5 V (vs Ag/AgCI, 3 M KCI) .

1 36

I ntroduction to Voltammetric Analysis

4

3

8 5

I

25 nA b

2

2

0

Figure 5. 1 4

a

4

5

10

15

5

20

25

30

35

I I

85

I

90

) t [min]

Chromatogra ms of BaP and metabolites 1 (each 90 ng), amperometric detector at + 1 .35 V (vs Ag/AgCI, 3 M KCI) using an ultra-trace electrode with isocratic el ution (a) and gradient el ution (b) [27) : (1 ) benzo(a)pyrene-tra ns-9, 1 0-d ihydrodiol, (2) benzo(a)pyrene-trans-4,5-di hydrodiol, (3) benzo(a)pyrene-trans-7,8-di hydrodiol, (4) 9-hydroxybenzo(a)pyrene, (5) 7-h y droxyb e nz o(a) p y re n e , (6) 1 - h yd roxy benzo (a) py r e ne, (7) 3 - h yd roxyb e nzo (a )p yre ne , (8) benzo(a)pyrene. from the a i r.

1 T h ese result from t h e e n zymatic d e g r a d a t i o n of B a P i n a n i m a l s a n d h u m a n s aft e r u ptake in food or

micro-electrode like properties (§ 4 . 5 . 3 ) . When used as the wo rking electrode in a m pero m etri c d e t ect i on fo l lo w i n g g r ad ient elution, for example in the a n al y s i s o f b enzo ( a) p yrene (BaP) and its metabolites, the ra p i d adj ustment to steady s t a t e conditions is an i m po rta n t r equ i r e m e n t and le a ds to a m o re stable b as e - li n e ( Fi gu r e 5 . 1 4b ) . In comparison the chromatogram using t h e much slower isocratic elution is shown in Fi gu re 5. 1 4a [27] . The d e t ec ti o n limits for the an al ysis of benzo (a) pyrene and its m et a b o l i te s lie between 0.2 n g an d 0 . 4 n g and the determi n ation l i m its between 0. 3 ng and 0.6 n g when either the glassy carbon electrode or the ultra-trace electrode are used. The rapid adj u st m e nt of steady s tate conditions at micro -electrodes is not only i mp ortan t fo r a m pe ro m e t ric detection in HPLC with changing solvent co m p o sit i o n , bu t a ls o fo r fl ow­ th rou gh vo lt a mm e t ry [ 28 ] . While the current i s m e a sured at a constant voltage in amperometric detectors, in voltammet­ ric dete c tors it is measured while t he vo lt age is changing ra p i dly. Either linear scan, s ta irc as e , square-wave, pulse or a . c. vo lt a m m e try can be used for th e measurement. When compared to sta ­ tio n a r y m et h o d s , non - st a tio n a r y methods with superimposed voltage p ulses ofte n result in high e r sel ectivity and sensitivity an d fewer p roblems fro m the adsorption of i m p ur i t i es and electrode reaction pro d u ct s on the electrode surface [ 7 ] . Voltammetric detection is of particular importance for the determination of an a l y t es which are not, or a re on l y po o r l y, se p arate d on the ch romatographic column. If these an a l yt e s have significantly different redox properties, then by using very fast scan rates a three dimensional i-E-t chromatogram can be obtained from which the i n di v i d ual analytes can be detected a n d

Flow-through techniques

/

1

/

1 37

2

- 1 .2

1 50 Figure 5 . 1 5

Th ree-d imensional chromatogra m recorded at the D M E using a rapid scan square-wave voltam metric detector for the determ ination of 1 .2 x 1 o-s M N-nitrosoproline ( 1 ) and 1 .34 x 1 o-s M N-nitrosod iethanolamine (2) i n the presence of a n unknown i mpu rity (3). Mobi le phase: 1 % phosphate buffer, pH 3 . 5 . Square-wave voltage ampl itude, flfsq 2 5 mV. Squa re-wave frequency 1 00 Hz [29] . =

determined . Square-wave voltammetry is the most suitable techn ique to use for these fast scan measurements. In the rapid scan square- wave voltammetric detector typical operating parameters arc: voltage scan rate, about 250 mV s 1 ; square-wave amplitude, � Esq = 10 mV; frequen cy = 1 00 Hz [ 29, 3 0 ] . A typical three-dimensional chromatogram is shown in Figure 5 . 1 5. Amperometric or voltammetric detection in combination with a micro-bore column is often more sensitive than mass spectrometry and the equipment needed is much simpler and cheaper than a mass spectrometer. These factors are very important when large numbers of analyses are required such as in environmental studies and has led to the development of many methods for the determination of phenols [ 3 1 -33] , insecticides [ 34-40] and polycyclic aromatic hydrocarbons [ 2 5-27, 4 1 ] in environmental samples using voltammetric or amperometric detection. Pesticides and their metabolites are usually fi rst separated from the other components of aqueous samples­ and simultaneously enriched-by solid p hase extraction using a solid phase extraction packing with an appropriate polari ty. The analytes are then eluted from the solid phase column, sepa rated by HPLC and detected amperometrically [ 42, 43 ] . As an example of the high sensitivity that can be achieved when using this procedure, the chromatogram for the determination of the PAH load in a surface water sample is shown in figure 5. 16. The amperometric detection of PAHs, hydroxy­ PAHs and i nsecticides is about 5 to 1 0 times more sensitive than U . V . detection [ 1 7 ] , while for hydroxy-PAHs, amperometric detection with a detection limit between 3 ng L-1 and 8 ng is about 2 to 5 times more sensitive than the commonly used fluorescence detection. Amperometric detection following HPLC sepa ration is thus a very powerful combination for trace and ultra-trace analysis of electrochemically active compounds. The amperometri c detec­ tors are an important addition to the available range of detectors such as U.V. and fluorescence detectors. As the examples of the PAHs and BaP and its m etabolites illustrate, complex samples of

L-1

1 38

Introduction to Voltammetric Analysis

I so pA

F i g ure 5 . 1 6

0

20

10

30

t

[min]

Determination of t h e PAH l oad i n g in a surface water after solid phase e n ri c hm e nt [26] . HPLC c hrom a t ogra m recorded using amperometric detection . Separation col umn - BAS-C 1 8; eluent - CH30H: H20 (85 : 1 5) with 2 g l-1 CCI3COOH. (1 ) 7.8 ng l- 1 fluoranthene; (2) 5.4 ng l- 1 benzo(b)fluoranthene; (3) 1 .9 ng l- 1 benzo(k)fluoranthene; (4) 4.4 ng l-1 benzo ( a ) pyren e; (5) 9 . 7 n g l-1 b e nzo (g h i )p e ryl e n e; (6) 2 . 5 ng l- 1 i ndeno( 1 ,2,3-c, d)pyrene. ,

,

anth ropogenic pollutants and their metabolites in soils, waters and tissue [ 44] can be analysed using amperometric detectors after suitable sample pre-treatment. In addition to those compounds which are electrochemically active at carbon electrodes (see chapters 6 and 7 for detailed discussion) many electrochemically inactive compounds can be chemically converted to oxid isable or reducible derivatives. When using flow-through techniques, the derivatisation is best carried out in the flowing solution using the procedures of flow-injection analyses either before chromatographic separation (pre-column deriva tisation) or between the column and the detector (post-column derivatisa tio n) . When the derivatising agents themselves arc clectroactivc, it is usual to use pre-column dcrivatisation [ 45 ] . A mpero me t ric detection in combination with ion chromatography (IC) makes possible the selec­ tive determination of easily oxidisable anions in the presence of other ions. This is not possible with the more commonly used conductivity detectors. Chlorite, nitrite, thiocyanate, thiosulfate and iodide can be detected at a glassy carbon or carbon paste electrode at ca. + 1 . 1 to + 1 .2 V ( versus Ag/AgCl, 3 M KCl ) . Traces of thiosulfate, bromide, iodide, sulfide and cyanide can also be determined at 0 to +0.5 V using a silver electrode. Determination limits generally lie between 0.5 and 5 IJg L- 1 • The selectivity is affected by the composition of the working electrode, the detection potential and the pH of the eluent [ 46 ] . Practical examples are the determination of sulfide in the presence of cyanide in water samples [47] , of bromide in snow [ 4 8 ] and of iodide in serum [49 ] . High performa n ce capilla ry electrophoresis (HPCE) with amperometric detection is another very powerful technique for the determination of electrochemically active compounds f 50-53 ] . The separation is carried out in a quart7: capillary and depends on the differi ng mobilities of the ana­ l ytes in an electric field. In addition, the separation is affected by the electro -osmotic flow of the carrier electrolyte resulting from the surface charge on the inner wall of the capillary. Capillaries with an inner diam eter of 25- 1 00 flm and a length of between 50- 1 00 em are used and the required electric field is provided by a 20-30 kV high voltage d.c. power supply. Miniaturisation of the equipment components results in sample volumes of no more than 10 nL being inj ected into the capill ary. This means that the working electrodes of the amperometric detectors must be very small and careful attention must be paid to the interference arising from the high voltage field required for the electrophoresis.

Flow-th rough techniques

a

G ro u n d

1 39

WE R E AE

I ff !� J

___,

b

Figure 5. 1 7

WE R E AE G ro u n d

Amperometric detection followi ng capil lary electrophoresis showi ng (a) on-column, and (b) end-column detection. (Inner diameter of the capillary (a) > 50 11m and (b) < 50 11m with conical capillary ends.)

As shown in F igure 5. 1 7, the measurement is made using a three-electrode detector, the potent i a l being determined from the hydrodynam ic voltammogram of the analyte. The working electrode is made from a carbon or metal (Au, Pt, Cu or Ni) fibre and is placed either in an on­ column or an end-column positi o n. With optimal geometric adj ustment in the quartz cap i l l ary micro-electrodes with their characteristic transport properties give a signal which is nearly inde­ pendent of the carrier electrolyte flow rate. Micro-fibre electrodes placed directly in the separa­ tion column result in especially sensitive signals. The main problem with combining amperometric detection with capillary electrophoreseis is the influence of the high voltage field on the very small faradaic current produced at the working electrode. In order to isolate the electrode a d i a p hrag m of porous glass or Nafion ( Figure 5 . 1 7a ) is placed between the detector and the column. This diaphragm is not used when working with very narrow capillaries. Typical applications include the oxidative detection of catecholami nes [ 54 ) , indoles [ 5 5 ) an d heterocyclic aromatic amines [56) at a carbon fibre electrode, ol i g o - and polysaccharides at copper electrodes [ 57 ] , and the reductive detection of quinones at carbon fibre e l e c t ro d es [ 5 8 ) an d o f metal ions at gold amalgam electrodes [59] . ,

5 . 2 fLOW-THROUGH STRIPPING VOLTAMMETRY The automation of stripping voltammetry was developed initially using the batch method in w h i ch the sample solution and electrolyte are placed i n the cell batch-wise, but for routine analy­ ses the th rough-put was too low because of the time required for the degassing step and for rins­ ing the cell. F ar greater through-puts can be achieved if the batch cell is replaced with a flow­ through cell ( § 4 . 6 ) . Flow- through cells for stripping voltammetry generally use glassy carbon o r mercury film working e l ectro de s [ 5 ] . It would be an advantage if a hanging mercury drop could be used as the working electrode as this would lead to more reproducible measurements [ 1 4 ] . A three- electrode flow-through cell with an HMDE working electrode is shown in Figure 5. 1 8. The flow-through

1 40

I ntroduction t o Voltammetri c Analysis b

a

Worki ng elect rode ( H M D E ) Auxil iary e l ectrode

E lectrode pl ate

... =f· t

Flow d i rection

Figure

5. 1 8

-+l

3 . 0-5 . 0

0 . 5-2 . 0

mm

C hannel p late mm

Schema t i c diagra m of a flow-through cel l with a n H M D E working electrode: (a) genera l v i ew (b) cross-section through the H M DE and flow channel.

,

cell consists of two plastic blocks which are screwed together. In the upper block, the reference, working ( HMDE) and auxiliary electrodes are placed in that order in the direction of the flow. The semi-circular or semi-elliptical flow channel etched into the lower block is centred about the HMDE ( Figure 5 . 1 8b ) . The analyte i s accumulated on the working electrode from the flowing sample solution either by electrolysis ( A SV determinations) or adsorption (AdSV determinations ) . The sample solution is moved through the cell with as little pulsation as possible by means of a peristaltic pump. The accumulation process is controlled by the time elapsed or the volume of sample that has passed through the cell. In the first case the accumulation starts with drop formation and ends when the flow is stopped by switching a m ulti-way valve. With volume controlled accumulation, a definite volume of sample (0.5-5.0 mL) contai ned in a sample loop is added to the flowing electrolyte by means of a m ulti-way valve. A suitable arrangement for trace analyses by flow-through stripping voltammetry using a peristaltic pump, multi-way valve, flow-through cell and a microprocessor­ controlled polarograph to control the analysis procedure and to process the data is shown in Figure 5. 1 9. Prior to each measurement, the flow-through cell must be rinsed with the supporting electro­ lyte. The drop is formed and using either time or volume control, an ali quot (plug) of the sample ( in supporting electrolyte solution ) is introduced into the stream of deaerated supporting electro­ lyte and passed through the cell for the accumulation step . A short time after the sample plug has passed through the cell the flow is interrupted, the deaerated stripping electrolyte (which may or

Pe ristaltic pump

Sample Supporting

Waste

/ hl \ Waste

el ectrolyte

Sampling Unit Figure

5. 1 9

VA Stand

VA Trace Analyser

M icroprocessor control led e qui pme n t for flow-through stripping voltammetric analysis (cou rtesy of Metrohm).

Flow-through tech n iques i

[nA]

1 41

b

- 300

i

- 200

[nA]

- 350

Ni - 250

Co

- 1 00 - 1 50

- 50

- 0 .85

Figure 5.20

E

- 1 .05

- 1 .25

(V] (vs. Ag/AgCI, 3 M KCI)

0

10

20

30

40

50

Stri ppi n g voltam mograms (a) and ca l ibration curves (b) for the determ ination of cobalt and nickel by fl ow t h r ou gh stripping voltammetry. Enrichment from 0.03 M H N 03 and st r ippi ng into N HiNH4CI, pH 9.5; s a mp l e a liquots 0.5 m l; flow rate 1 .5 m L min· 1 • -

may not be the same as the supporting electrolyte) is introduced via the multi-way valve. When the stripping electrolyte has reached the cell the flow is stopped and the stripping current mea­ sured in the stationary stripping electrolyte solution . The recording and evaluation of the voltam­ mogram, for instance by means of a calibration c urve, is carried out by the pre-programmed polarograph. As an example, the calibration curves for the sim ultaneous fl ow-through Ad SV determination of cobalt and nickel as the dimethylglyoxime complexes arc shown in Figure 5.20. At the end of a run, the old mercury drop is washed away by increasi ng the flow rate. Instead of a pump to move the sample and electrolyte solutions through the cell, a motorised burette (Dosino™ 700 ) which is pulsation free and which can provide continuously variable flow rates may be used. When using a motorised burette, the enrichment time can be defined and controlled by the flow rate, the sample volume and by means of a repeated back and forth m ovement of the sample in the cell if necessary. The multi-mode electrode (Metrohm ) is suitable for use as the working electrode in the flow- through cell. Determinations in flow-through cells generally show an improvement in sensitivity, since in the flow channel substa ntially higher flow velocities and consequently grea ter separation rates can be obtained than with the usual cells. One advantage of flow-through stripping voltammetry is that the accumulation and determination steps can readily be ca rried out i n different supporting electrolytes (solution exchange) . By thi s means the sensitivity and selectivity of the determination can often be improved. An example of improved selectivity is the determination of thallium in the presence of lead or tin ( see Figure 3 . 5 ) . Thallium is accumulated from HCl solution. The stripping voltammogram in HCI shows the presence of another component, the peak of which interferes with that of thallium . It is therefore necessary to change the supporting electrolyte between the accumulation and determination steps. Using ordinary voltammetric cel ls, this is cum bersome­ the first cell and electrolyte must be replaced by a second cell containing the determination elec­ trolyte while maintaining an oxygen-free atmosphere about the el ectrode to avoid oxidation of the deposited metal amalgam. This problem does not occur when the solution exc hange is carried out in a tlow- through cell. Another benefit of solution exchange is the improvement obtained in the base current as illustrated in Figure 5 . 2 1 . The lower base- l ine and increased sensitivity result-

1 42

Introduction to Voltammetric Analysis

b

i [nA]

a

i [nA]

Zn Cu

Zn

Cu

1 00

1 00

50

50

0

0

- 1 .2

Figure 5 . 2 1

- 0.6

0.0

E [V]

- 1 .2

- 0.6

0.0

E [V]

ASV dete r m i n a tion o f zinc, cadmi um, lead and copper (each 1 0 119 L- 1 ) i n different supporting electrolytes: (a) accumulation and stripping from 0.03 M H N 03, (b) accum u lation from 0.03 M HN03 and record i n g th e st ri pping volta m m o g ram after exchanging the HN03 with an a cetate buffer solution (pH 4.6).

ing when the nitric acid used in the accum ulation step is replaced by an acetate buffer solution for the stripping step is quite evident. Flow-through adsorption stripping voltammetry is also helpful in elucidating reaction mechanisms. Whereas in the batch m ethods, all steps-the complex form ing process, the adsorp­ tive accumulation and the stripping process-occur in the vessel, in flow-through methods it is possible to consider the i ndividual steps s e parat ely . Thus it is possible, for example, to distinguish as to whether the analyte forms a complex in solution and the complex is then adsorbed, or the ligand is first absorbed and the analyte forms the complex on the electrode surface. Such studies have shown that in the case of the determination of uranium with chloranilic acid, the complex formation takes place essentially on the electrode surface [ 60 ) . References 1

H. Eamons and G. Jokuszies, Z. Chem ie, 1 988, 28, 1 97.

2

).M. Elbicki, D. M . Morgan and S.G. Weber, Analytical Chemistry, 1 984, 56, 978.

3

H. Gunasin g ham and B. Fleet, Electroanalytical Chemistry, Vol. 1 6, A.).

Bard, edi tor, Marcel Dekker, New York,

D.C. Johnson, S . G . Weber, A.M. Bond, R . M . Wightman, R. E. Shoup and l.S. Krull, Analytim Chimica Acta, 1 989, p. 89.

4

T. H. Ryan, Electrochemical Detecto rs, Plenum Press, New York, 1 9 84. R.J. Rucki, Talan ta, 1 980, 27, 147. K. Stulik and V. Pacakova, CR C Critical R e v iews in Analytical Chemistry, 1 98 2 , 1 4, 297. K. Brunt, Electrochem ical Detectors in Trace Analysis, Academ i c Press, New York, 1 98 1 , Vol. 1 . G. Henze, 'Electrochemical Detection ', in Ullmann's Encyclopedia of Industrial Chemistry, VCH 1 986, 1 80, 1 87.

6 7 8 9 5

P.T. Kissinger, C . Refshauge, R. Dreiling and R.N. Adams, A nalytical Let ters, 1 973, 6, 465. B. Fleet and C . J . Little, /. Ch romatographic Science, 1 974, 1 2 , 747. W. Kemula, Roczniki Ch emie, 1 953, 26, 28 1 . W.W. Kubiak, Hlectroanalysis, 1 989, 1 , 3 79. P. Surmann, Fresenius Z. A nalytische C:hem ie, 1 983, 3 1 6, 373. ) . Frank, Chimia, 1 98 1 , 35, 24. Verlagsgesel lschaft, 1 994.

10 11 12 13

14 15

Flow-through tech niques

16

17

18

20 19

21

22 23 24 25

26 27

28

29 30 31 32 33

34

35 36

38

37

39 40 41 42 43

44

45 46 47

48 49

50 51 52 53 54

55 56

57 58 59 60

1 43

P.T. K i s s in ge r, C.S. Brunett, K . Bratin a n d J.R. R i c e , National Bureau of Standards (US), Special Publication 5 1 9, 1 9 79, 7 0 5 . S. Ya o , A . Meyer and G. Henze, Fresenius' f. Analytical Che m is try, 1 99 1 , 339, 207. D.C. Johnson and W. R. LaCo u rse , Hectroanalysis, 1 992, 4, 367. X. Qi and R . P. Baldwin, Electroanalysis, 1 993, 5, 547. X. Qi and R.P. B al d w i n , Electroanalysis, 1 994, 6, 3 5 3 . W. R. LaCourse, D.C. Jo hns o n , M .A. Rcy a n d R.W. Sl i ngsb y, Analytical Chemis t ry, 1 99 1 , 63, 1 34. D.S. Austin-Harrison and D.C. J o h nso n , Electroanalysis, 1 91!9, 1, 1 89. S. F l e tch er, Ch em istry in Australia, 1 994, 6 1 , 80. D. Ramond, An a lysis , 1 990, 1 8, 1 3 3 . H . - P. Nirmaier, E. Fisch er, A . Meyer a n d G. Henze, !. Chromatography, A, 1 996, 730, 1 69. H . - P. Nirmaier, E. Fischer and G . Henze, GIT Lal,or-Fachzeitschrift, 1 998, 42, 1 2 . H . - P. Nirmaier, E. Fischer an d G. Henze, F.lec t roa n alys is, 1 998, 1 0 , 1 87. J. Schiewe, A.M. Bond and G. Henze, Um we ltd iagnost i k mit Mikrosystemen, G. Henze, M. Kohler an d J.P. Lay, editors, Wiley-VCH, 1 999, p. 1 2 1 . R . Samuelsson, J . O'Dea and J . Osteryoung, A n a ly tical Ch e m is try, 1 980, 52, 22 1 5 . S . A . Borman, A na ly t ica l Ch e m is t ry, 1 9 8 2 , 5 4, 69 8A . N . Cardellicchio, S . Cava l l i , V. Piangerelli, S . Giandomenico an d P. Ragone, Fresenius' f. Analytical Chemistry,

1 997, 358, 739.

M.T. Galceran and K . Jouregui, Analytica Chimica A c ta, 1 995, 304, 75. E.C.V. B u tl er and G . D a 1 Po n t , f. Chrom a tograp hy, 1 992, 609, 1 1 3 . M.W.F. Ni el e n , G . Koomen, R.W. Frei a n d U.A.T. l:lrinkman, f. Liquid Chromatography, 1 98 5 , 8, 3 1 5 . Q . G . von N eh r ing, J.W. Hightower a n d J.L. And erso n , Analytical Chemistry, 1 986, 58, 2777. W. S c hu s sle r, Ch ro m a t ographia, 1 990, 29, 24. K.D. McMurtrey, A.E. Holcomb, U.A. Ekwenchi and N.C. Fawcett, ]. Liquid Chromatography, 1 984, 7, 9 5 3 . A. Meyer and G. Henze, Fresenius Z. Analytische Ch e mie, 1 91!9, 332, 898. M.A. Alawi and H .A. Russel, Fresenius Z. Analytische Chemie, 1 9 1! 1 , 309, ll. W. Buchberger and K . Wi n s aue r, Mikroch imica Acta [ Wien ) , 1 980, 2, 2 5 7 . E. Fisch er, G . H e nz e a n d K . L . Platt, Fresenius' J. An a lyt ica l Ch e m is try, 1 99 1!, 360, 95. A. Di C orc i a and M . Marchetti, An aly ti cal Chemistry, 1 99 1 , 63, 580. B. Rossner and G . Schwedt, Fresenius Z. A n a lyt isch e Chemie, 1 9 8 3 , 3 1 5, 6 1 0. J. Hausen, A. Meyer and G. Henze, Frese n i us' f. An a lyti ca l C11emistry, 1 99 3 , 346, 76 1 . G . Ch i ava r i and C. B erga m in i, J. Ch roma t ograp hy, 1985, 3 1 8, 4 2 7 . T. Kolb, G. Bo ge n s chu tz and ). Schafer, LaboPraxis, 1 998, May, 66. P. Stei n man n and W. S ch o tyk , }. Ch roma tograpy, 1 995, A 706, 2 8 7 . S. S ee fe ld and U. Baltcnsperger, An a lytica Ch em ica A cta, 1 99 3 , 2 8 3 , 2 46 . B. M i c hal ke , P. Schramcl a n d S. Hasse, Fresertius' /. Analytical Chemist ry, 1 996, 3 5 4, 5 76. J.P. Lan de rs , Handbook of Cap i l lary Electrophoresis, CRC Press, 1 997. T. Kappes and P. C. Hauser, f. Ch ro m a tography, 1 999, A 834, 89. W. l:luchberger, Fresenius' /. Analytical Chemistry, 1 996, 354, 797. P.D. Cu rry, C.E. Engstrom - Silverman, Jr., and A. G. Ewing, Electroanalysis, 1 99 1 , 3 , 587. M . E . H adw ige r, S.R. To rch i a , S . P a rk, M.E. Biggin and C.E. Lunte, /. Chroma tography, 1 996, 8 68 1 , 24 1 . J.C. Olsson, P.E. Andersson, 1:1. Karlberg a n d A-.C. N ordst rom , f. Ch ro m a tograp hy, 1 996, A 7 5 5 , 2 8 9 . J.C Olsso n , A. Dyremark and 1:1. Karlberg, f. Chromatography, 1 99 7 , A 765, 3 2 9 . W. Zhou and R.P. B al dw i n , Electrophoresis, 1 996, 1 7, 3 1 9. M .A. Malone, P.L. Weber, M . R. Smyth and S.M. Lunte, Analytical Ch em is try, 1 994, 66, 3 7 8 2 . W. L u and R . M . Cassidy, An a ly tical Chemistry, 1 993, 6 5 , 1 649. S. Sander, Private communication.

6

Applications: i n o rga n i c species

Polarographic and voltammetric techniques can be applied to the analysis of many inorganic spe­ cies over a wide c on cent ra t ion range. The older literature contains many examples of th e analysis of substances in the I o-5 mol L 1 to I o-2 mol L 1 range using mainly direct current polarography ( DCP) with the dropping mercury electrode ( DME) [ I , 2 ] . No wadays, the main appli c atio n is to the dete r m i n ation of analytes at trace and ultra - t race concentrations ( down to 1 0-12 mol L-1 ) [ 3 ] . These low detection limits are achieved when the sensitivity o f the pulse and fast scan techniques are enhanced by the preliminary acc umulatio n of the analyte on the electrode in the stripping methods described in chapter 3 . For concentrations in the 1 0-7 t o 1 0-·l mol L-1 range t h e test solution i s usually analysed directly using differential pulse polarography (DPP ) and a DME or fast linear scan (or stai rcase) vo l tamme try with mercury ( SMDE, MTFE or HMDE) or solid e l ect ro de s . For the lower concen­ trations, stripping t e ch n i q u es following electrolytic or adsorptive accumulation at the electrode ( usually mercury) are used. Elect ro l yti c stripping of the ac c umu lat ed analyte is commonly achieved using a differential pulse, a fast linear or a staircase voltage scan and the concentration det erm in ed from the resulting current-potential plot. Alternatively, the accumulated analyte m ay be stripped from the electrode by m e an s of a constant current (chronopotentiometric stripping) or a chemical reaction ( potentiometric stripping) . In these latter two techni ques, the data is nor­ mally presented in the derivative form, d t/ d E ve r s us E (§ 3.3 and § 3 . 4 ) . T h e determination of simple and complexed inorganic i o n s is based on their reduction and oxidation reactions and any associated kinetic, catalytic or adso rptive processes that occur at the electrode surface. Noble metal or carbon electrodes can be used instead of a mercury electrode to voltammetrically determine noble metal ions and those compounds whose analysis depends on an oxidation process which occurs at the more positive potentials. Although not widely used as yet, chemically modi fi ed e l ectro de s are g ai n i n g in i mp o rta n c e as replacements for mercury elec­ trodes, particularly in field analyses. The use of micro-electrodes or m icro-electrode arrays is also inc re asi ng particularly where fast responses are re q uir e d such as in fl o w - th rou gh voltammetry. Polarographic and voltammetric analyses are usually carried out in aqueous media. When the solubility of the analyte in wa ter is very low, as is the case with many organic and organometallic compounds, and for the analysis of metal chelate extracts, m ixed aqueous-organic solvents or the p ure org anic solvent may be used. In these solvents, tetraalkylammonium salts are normally used as supporting electrolytes.

6. 1 POLAROGRAPH IC M ETHODS For an i norganic species to b e determined voltammetrically or polarographically the relevant electrode process must occur within the useful p o t entia l range of the supporting electrolyte­ electrode combination used for the analysis (Table 2. 1 and Figure 4.6 ) . This useful potential range is sometim es referred to as the electrochemical window of that supp orting electrolyte-electrode

Applications: inorganic species

Tl+

- 0.47 v Figure 6 . 1

2 mg L- 1

Cd 2 +

- 0. 7 1 v

1 mg L-1

- 0.98 v - 1 .23 v

1 mg L- 1

1 mg L- 1

1 45

- 1 .47 v

1 mg L- 1

Simultaneous determ ination of tha llium, cadmium, nickel, cobalt and i ron by DP-polarography. Electrode - S M D E . Supporting electrolyte - 0.03 M Hl04 + 0. 7 M NH3 + 0.04 M sulfosal i cylic acid. Potenti a l scanned from -0.2 V to -1 . 7 V (vs Ag/AgCI, 3 M KCI).

combination. I n ad d itio n to de t e rm i n i ng the useful potential range, t h e s u pp ort i n g e l ect ro lyt e may markedly influence the value of the half-wave p o t e n t i a l or p e a k potential of t he a nal yt e . T h e p o l a r o gr aphic h a l f- wav e p ot e n ti als for a selection of m et a l i o n s in va r i o us s u ppor t ing e l e ctr o l yt e s are listed in Table 6 . 1 . More e xt e n s i ve t a b u la t ion s a r e p r e s e n t e d elsewhere [ 1 , 4 , 5, 6 , 7 ] . The cited half-wave p o t e nti a l s are comparable with the peak p o te n t i al s of a. c. p ol a r o g r a ­ ph y o nl y for r ev e r s i b l e electrode processes. The r e duc t i o n of simple or c omp l exe d ions to m et a l s which are soluble i n m e r c u ry is u su a ll y reversible and gives rise to well formed cur­ re n t-p o te n ti a l curves t h a t a r e re a d i l y in t e rp re t e d . Exa m p le s of this a r e cadmium, copper, lead and th a l l i u m . Several metal ions which give well formed p ol a ro gra m s are not listed in T a b le 6. 1 . T h e s e include the alkali, alkaline earth, scandium, ytt riu m , lanthan u m and m o s t of the rare-earth met­ als. The y are rev e rs ibl y reduced in the p re s e n ce of tetraalkylammonium salts as s u p p o r t i n g elec­ troytes at about -2.0 vo lt . However, th e i r very similar half-wave p o te n t i a l s means that th ey are difficult or impossible to resolve when more t h a n one of them is pre s e n t in the sample and so they are not u su a l l y a n al ys e d polarographically [ 7 ] . Because the po te n t i a l at which a p o l a ro gr a p hic wave or current peak oc c u rs is c h a ra c t e r i s t i c of a given analyte in a p ar t i c u lar supporting electro l yt e , several analytes can be determined together in a sample pro v i de d that thei r current waves (or peaks) are sufficiently well separated-by a b ou t 1 20 mV for w a ves and 50 mV fo r peaks. This ab ility to si m u lt a neou s ly determine several c o ns t i t ­ uents present in a s a m pl e (metal ions and di ffe re nt chemical forms of c l e men ts ) is an advantage of pol ar o gra p hy and vo lt a m met ry wh e n c o m pa r ed with ot h er methods of trace m e tal analysis ( s uch as ato m i c ab so rp t i o n s p ec t r o sc op y ) . This is i l l u s t rat e d by t h e so- called polarograph ic spectrum of a sa m pl e co n ta i n i n g five metal ions in adm ixture shown in Figure 6.1. P o o r s ele c t i v ity arising from the overlap or s u peri m p o s ition o f respo n ses c a n u s ua l ly be overcome by using a different supporting electrolyte or by adding a complexing agent that

Table 6 . 1

....

Polarographic half-wave potentials o f selected metal ions i n various supporting electrolytes. 0.3 M Trieth2 M HAc +

I M NH3 +

Metal ion

2 M NH4Ac

I M NH4Cl

l M KCI

l M KCN

l M NaF

Al ( I I I )

n.r.

n.r.

- 1 .58 i

n.r.

As( I I I )

-0.94

-1.61 i

-0.60 i

n.r.

O. I M Na-

I M

0.1 M

I M

anolamine

O.S M Na-

0 . 1 M Na2-

citrate +

l M HCI

NaOH

KN03

KSCN

+ 0. 1 M KOH

Tartrate

EDTA

O. l M NaOH

n.r.

n.r.

n.r.

n.r.

n.r.

n.r.

n.r.

n.r.

n.r.

-0. 1 0 w

-0.37 w -0.6 1

-0.24

-0. 1 0 w

n.r.

-0. 1 5

n.r.

- 1 .26* w

0.25* w

� � a

c._ c

Po (5' ::::1

8'

< 0

3 3

�

Bi(III)

-0.20 w

n. r.

-0.05 w

i (c)

-0.43 w

-0.06 w

-0.67

- 1 . 1 6 it

n.r.

-0.86 w

-0.34 w ( c )

-0.57* w

-0.80* w

Cd ( I I )

-0.62 w

-0.76 w

-0. 6 1 w

-1.14

-0. 56 w

-0.60 w

-0. 8 1 t

-0.54 w

-0.6 1

-0.80

-0.67*

n.r.

-0. 7 1 * i

Co ( I I )

n.r.

- 1 .25

- 1 .37

-1.21 w

- 1 .40 w

n.r.

n.r.

- 1 .26 w - 1 .53

-1.10

n.r.

- 1 .56

n.r.

n.r.

Cr(III)

-1.19 i

- 1 .33 w t

- 1 .02

n.r.

n.r.

-0.99 w

n.r. t

-0.86 - 1 .00

- 1 .00 w

n.r.

- 1 .6 1 i

- 1 .23 w

n.r.

Cr(VI)

n.r.

-0.27 w - 1 .44 w -1 .67

-0.37 -0.87 w

-0.88 -1 .27 i

-0.33 -0.82

- 1 .02 i

-0.84 w

-0.3 1 - 1 .09

-0.40 -0.99 w

- 1 .26 w

-0.36* -0.89* - 1 .74*

- 1 .23* w

-0.76* w

Cu( I I )

-0. 24 w

-0. 1 9 w -0.45 w

-0. 1 7

n.r.

+0.03 w t -0. 1 9

-0.34 -0.42

n.r.

-0.63

-0.49

-0. 1 1 w

-0. 29 w

-0.44

Fe ( I I )

n.r.

- 1 .43 w

- 1 .56 i ( c ) n.r.

- 1 .42 w

i (c)

- 1 .54 i t

- 1 .30 - 1 .58 - l .SS i (c)

- 1 .0 1 w -1 .67

- 1 .54* i

-0. 1 2 * w

-0.86* w - 1 .60* w

Fe( I I I )

-0. 0 1 -0.23

n. d

-1.41 i

n.r.

n.r.

n.r.

n.r. t

- 1 .28 i

- 1 .55 w

-0.98 w - 1 .67

-0. 22* - 1 .56* w

-0. 1 4 * w

-0. 88* w - 1 .62* w

Mn(II)

n.r.

-1.58 w

- 1 .54 w

- 1 .29 w

- ! .SO w

n.r.

- 1 .67 i t

- 1 .45 i

- 1 .55

-0.43 - 1 .66 i

- 1 .55* w

n.r.

n.r.

�

;:; · l> ::::1 "' ..:;;: "' v; ·

Table 6 . 1

Polarographic half-wave potentials of selected metal ions i n various supporting electrolytes.

(continued) 0.3 M Trieth-

2 M HAc +

I M NH3 +

Metal ion

2 M NH4Ac

1 M NH4Cl

1 M KCI

I M KCN

I M NaF

Mo(VI)

-0.62 w -1.14 - 1 .29

n.r.

n.r.

n.r.

n.r.

- 1 .3 1

Ni(II)

- 1 .08

w

- 1 .06

w

- 1 .02

Pb( I I )

-0.46 w

-0.46

w

-0.40

w

w

n.r.

O. I M Na-

I M

0.1 M

I M

anolamine

O.S M Na-

O. I M Na2-

citrate +

I M HCI

NaOH

KN03

KSCN

+ 0. 1 M KOH

Tartrate

EDTA

0.1 M NaOH

-0.07 i

n.r.

n.r.

n.r.

n.r.

n.r.

-0.55* -0.78

n.r.

n.rJ

-0.99

w

-0.65

w

- 1 .35

n.r.

n.r.

n.r.

-o.nt

-0.35

w

-0.38

w

-0.90

-0.62*

- 1 .06*

- 1 .06

i

n.r.

-0.42

w

-0.40

w

w

-0.72*

w

Sb(III)

-0.38 -0.49 w

-0.80 w

-0.06 -0. 1 4 w

-0.87

-0.56 -0.76

-0. 1 1 w

-0.4 1 -1.17

-0. 1 8 w

-0.57 w -0.69

- 1 .30 w

n.r.

-0.67*

-0.36* -0.98* w

Sn(II)

-0. 1 2 w -0.62 w

-0. 7 1

-0.42 w

-0.86

-0.30 w -0.65 -0.77

-0.44 w

-0.82 -1.14 w

-0.34 w

-0.44 w - 1 .58

-0. 74 w -1.14

-0.57* -0.86 - 1 .09

-0. 1 6*

-0.86* w -l . l l * w

Sn(IV)

n.r.

n.r.

n.d

n.r.

n.r.

-0.45

n.d

n.r.

- 1 .56

n.r.

n.r.

- 1 . 19* i

n.r.

Tl( I )

-0.43 w

-0.45 w

-0.46 w

n.r.

-0.42

-0.45 w

-0.44

-0.42 w

-0.49 w

-0.44 w

-0.48* w

-0.47* w

-0.50* w

V(V)

i (c)

-1. 1 1 - 1 .29 i

- 1 .09 (c)

-1.18 w

- 1 .00 i

-1.01 i

-0.39

- 1 .02 i

-0.49 w

-0.34

-0.36 i

- 1 .26*

-0.79* - 1 . 14* w

W(VI)

-0.25 i

n.r.

n.r.

n.r.

n.r.

- 1 .00 i (c) n.r.

n.r.

n.r.

n.r.

n.r.

- 1 .29* i

n.r.

Zn ( I I )

- 1 .04

- 1 .3 1 w

-0.98 w

n.r.

-1.16 w

-0.98 i

-0.96

- 1 .00 w

- 1 .57

- 1 .30* w

n.r.

-1.41* w

Notes:

- 1 .57

i

n.r. - no polarographic wave observed in this solution; w - well developed wave; i - ill-defined wave; ( c ) - catalytic wave:

j, - precipitate forms.

Quoted from Metrohm Applications Bulletin No. 36. The half-wave potentials were measured at 25°C using the Metrohm Polarecord 506 + Polarographic Stand 663 and a Ag/AgCl, saturated KCI reference electrode ( drop time of the DME

=

0.4 s ) .

'Half-wave potentials measured against the SCE ( these values are thus ca. 0.04 V more negative than they would be if measured against the Ag/AgCl, sat. KCl electrod e ) .

The half-wave potential is characteristic of a particular metal-ion species in a given supporting electrolyte. This enables polarography to be used for the qualitative identification of metal -ion species in solution. However, the half-wave potentials are of more importance in judging the selectivity of a proposed polarographic analysis or in the development of a method for determining several metal-ion species simultaneously in a particular test solution.

)> "2.. ;::; · "' "0

o· ....

"' �

:; ·

..c 0

"' "' ;::; · "' "0 "' ,..,

iii ' "'

.... .co ....

1 48

I ntroduction to Voltam metric Analysis

2a

- 0 .7

- 0.5

- 0.7

- 0. 3

2b

1b

- 0.7

Figure 6 . 2

- 0. 5

- 0.3

E

- 0.7

/

In

- 0.5

-

I

- 0 .5

[V] (vs. Ag/AgCI,

1 3 .

In

3M

- 1 .0 - 0.7

Co

\

- 1 .3

KCI)

- 1 .0 - 0 .7

I nfl uence of com plexi ng agents on the sepa ration of DP-polarographic peaks: (1 ) Pb2 ... and n+ in 0.1 M KCI (1 a) and in 0 . 1 M KCI plus 0.1 M NaOH (1 b); (2) Cd 2 ... and l n 3 + i n 0.1 M HCI (2a) and in 0 . 1 M HCI plus 2 M K l (2b); (3) Co2 + and N i 2 + i n 0.1 M HCI (3a) and in 0 . 1 M HCI plus 0.2 M pyridine (3b). Analyte concentration in each case is 1 0-4 M .

markedly ch anges the redox behaviour of one of the analytes. T h e influence of complex forma­ tion on the separation of DP polarographic peaks is illustrated by the examples sh own in Figure 1 6.2. The determination of ( 1 ) Tl ... i n the presence of Pbl-1-, ( 2 ) of Cd 2 ' in the presence of In· ' or 2+ 2+ of (3) Ni in the presence of Co is not possible in aqueous KCl or HCl solution because of the proximity of the current peaks in each case. Howeve r , in each example the DP-polarographic peaks can be separated by add ition of an appropriate complexing agent. I n example 1, the addi ­ l+ tion of NaOH converts Pb to the hydroxyplumbate ion but does n ot affect Tl ' , in example 2, 2+ 1- is used to form a complex with Cd and in example 3 the a dd i t ion of pyridine forms com­ plexes with both ions. Because mercury reduces the platinum metal ions i n solutions of the common supporting electrolytes, it is necessary to have complexing agents present in order to obtain well formed polarograms. For example, rut henium in citrate solution [ 8 ] and palladium i n ammonia solu­ tion [ 9 ] can be determined in the mg L- 1 range by D P-polarography. I nert electrodes such as platinum, gold, glassy carbon or graphite are generally used for the voltammetric determina­ tion of these metals [ 7 ] . A summary of the polarographic behaviour of the platinum metals is given in reference [ 1 0 ] . M ultivalent ions are often reduced i n several steps, the mechanism and degree of reversibility of th e electrode reaction in most cases being dependent on the composition of the supporting

Appl ications: inorganic species a

b

1 49

- 1 .2 v

- 1 .15 V

-

Figure 6 . 3

0. 4

- 0.8

- 1 .2

E [V]

- 0 .4

- 0.8

- 1 .2

E [V]

DP-polarogram of chro m i u m (VI) in different su pporting electrolytes : (a) 0 . 5 mg l-1 Cr (V I ) in 0 . 1 M KN03; (b) 0.5 mg l- 1 Cr(VI) in 0 . 1 M Na2-EDTA. E l ectr o de - SMDE; poten t i a l scan from -Q. 1 V to -1 .4 V (vs Ag/AgCI, 3 M KCI).

electrolyt e solution. Two-step current-potential curves are ob tain e d with chromium (VI ) ,

copper(II ) , iron (III ) , molybdenum (VI) and vanadium ( V ) , among others, whe n suitable sup­ p orti n g e l e ctr o lyte sol utions are used. I n a m ult i - s te p process, the shape and height of the polaro­ graphic peak as so c i a te d with each step is de t e r m i n ed by t h e number of el e c trons involved and the reversibility of that step. For analyti c al purposes, the best s en s i t ivi ty is obtained when a support­ ing el ec t r o lyte can be fo u n d in which the multi-electron e xc ha n ge occurs in one step at a single po te n t i al, res u l t in g in a l a rge s i ngl e wave or p e a k i n the p o la ro gr a m . This is illu s tr a ted in the DP-polarographic reduction of Cr(VI ) shown in F i gu r e 6.3. In 0. 1 mol L' 1 KN0 3 s u pport i n g electrolyte (curve a), Cr(VI ) is r ed u c e d in two steps: fi rst to Cr (III) at -0 . 3 V and th en t o C r ( I T ) a t - 1 . 1 5 V. In 0. 1 mol L- 1 N a2-EDTA solution ( curve b ) , o n ly one pe ak is observed corresponding t o the four- electron reduction of Cr( VI) to Cr( I I ) . B eca u s e the three-electron reduction to Cr( I I I ) is irreversible, the first pe ak in curve a is sm a ller than the second peak for t h e catalytic o n e - ele ct ro n reduction of Cr( I II ) to Cr( I I ) . The l a r ges t peak, obtained for the fou r - ele c t ron reduction of Cr(VI) to Cr(II) in EDTA solution ( c u rve b ) , gives a determination limit of l0-6 mol L- 1 for the analys is of c h romi u m . For those sys t e m s which g iv e well behaved c at a lyt i c c u r r e n t s, qu i t e sensi tive polaro ­ g r a p hic a n d v o lt a m m e t r i c a n a l y se s a r e p o s s ible ( § 1 . 4 . 2 ) . E x ampl es are the determination o f Cr ( V l ) or M o ( V I ) in the presence of ni t r a t e ions or of Fe ( I I I ) i n t h e pr e s ence of broma te ions. For e xample , the determination limit for Cr(VI) u sing DPP c a n be l o w e r e d to ca. l0-8 mol L- 1 wh en N a 2 - EDTA contai n i n g n i t rate ions is used as the s u pp o rt i n g e l e c t r olyt e [ 1 1 ] . Th e Cr( I I ) fo rmed by the reducti o n of Cr(VI ) at the working e l e ctro d e is i mm e d i a t e ly oxidised back to the trivalent state by the n itrate ions and is available to be re du c e d aga i n in the e l e ctr o de reaction. Titanium, tungste n , u ranium a nd van adi um can also be d e t ermin e d via cat a lyti c currents in the presence o f nitrate or ch l o r a t e ions with limits of de te c t i o n in t he J.lg L- 1 ( ca . 1 0- R mol L ) ran ge [2, 7 ] .

1

1 50

Introduction to Voltammetric Analysis

Application 1

Determination of ch ro mium in

Sample: Pre-treatment:

Supporti ng el ectrolyte: C h romi u m standard : Analysis:

the presence of nitrate [ 1 2 ]

Surface, pota b l e or waste waters, biological samples. 1 Su rface and tap waters conta i n i ng no org a n ic matter. Cr(V I ) no pre-treatment. If C r(l l l) p resent, add 1 m l 0 .0 1 5 M H 2S04 and 1 0 ml 0. 1 % (N H4hS208 to 1 00 m L of sample. B o i l gently u nti l a l l S 2 0 t is destroyed . M a ke up to 1 00 m l with water. 2 Waste water with org a n i c content. Add 1 0 �L 30% H 202 to 1 00 ml sample, adjust pH > 4 a n d irradiate with U .V. for 1 -2 h at 90°C to ensure the decom­ position of o rganic matter and the oxidation of Cr to Cr(VI). At pH < 4, the oxi­ dati o n is i n complete. 3 Biological materia l . Digest with H 2 S04 and H202. 0.2 M NaOOCCH3 plus 0 . 0 5 M d iethylenetria minepentaacetic acid i n 2.5 M N a N 03• 2 . 8288 g K2Cr207 i n 1 L d isti l l ed water. Add 5 m l of su pporting electrolyte to 20 ml of sa mple and adjust pH to 6 . 2 with d i l . N a O H . Deaerate with n itrogen for ca . 5 m i n . Record the DP-pola rogra m from - 1 .0 V to - 1 .45 V. A peak appears at -1 . 3 V (versus Ag/AgCI, 3 M KCI) due to the cata lytic red uction of Cr(l l l) in the DTPA complex to Cr(l l). The determination limit is ca . 0 . 5 �g L- 1 when an H M DE is used with no accumulation. A platin u m work­ ing electrode may be used for this determination if preferred.

Elements that can exist in both positive and negative valance states can often be converted to the latter state polarographically. An example is the reduction of As( III) in hydrochloric acid solu­ tion. The polarographic curves indicate that the reduction of As( III) to As( O ) occurs at 0 45 V and that further reduction to As ( -III) occurs at 0 87 V. The peak (DP) or wave ( d . c . ) associated with either pr o ce ss may be used for the determination of As(III ) [ 1 3 , 1 4 ] . Voltammetric or polarographic methods are less applicable t o the analysis of anions than to the a n a lys is of cations. Bromate, iodate and periodate which can be reduced d i re ctly to the corre­ sponding halide ions [ 1 5 ] and sulfite ( reduced to SO/ , 520/- or Sp3 2-, d e pe n ding on the pH) [ 1 6 J are among the few anions that can be determined directly by means of polarographic reduc­ tion currents. A number of other anions that form compounds of low solubility or form stable complexes with Hg/+ ions formed by the oxidation of the mercury of the electrode can be determined indirectly. The anodic current produced as a result of the oxidation of the electrode mercury is proportional to the concentration of such anions in the sample solution. The determi­ nation of h al i de sulfide and cyanide ions a re typical exa m p les of this type of analysis [ 2 , 7 ] . -

-

.

.

,

Application 2

Determination of sulfide and sulfite [ 1 7]

Sample: Supporti ng e lectrolyte: Standards: Su lfide analysis:

Sulfite a nalysi s:

I nfusion solution, waste water, flue gas water, photographic solutions, etc. N a O H and acetate buffer made from 0.2 M NaOH plus 0.4 M C H3COOH. 2 1 g L-1 S - i n 0 . 0 1 M NaOH (02-free); 1 g sot L- 1 i n 02-free disti l led water. 2 The ana lysis is based on the production of Hg + and its convers ion to HgS. To 1 m l 2 M NaOH add 1 0 to 20 m l water, deaerate for 5 min and mix i n 1 to 1 0 ml of sa m p l e . Record the DP-polarogra m between -0 . 5 5 V and -0.90 V. The peak occurs at ca . -0 . 7 5 V (versus Ag/AgCI, 3 M KCI). The pea k current versus concentration plot is l i near between 2 1Jg L-1 and 2 mg L- 1 . The analysis, carried out i n acetate buffer, is based on the reduction of S03 2- to S204 2 -. D i l ute 1 0 ml buffer solution with 9 ml water, deaerate for 5 m i n a n d add 1 - 1 0 m l of the sample. Record t h e DP-polarogram between -0 .4 V a n d 2 M

Applications: inorganic species

1 51

-0 . 8 5 V. The cu r rent p eak occurs at ca. 0 6 1 V (v e r sus Ag/AgCI, 3 M KCI) and the l i near ra nge lies between 2 mg L - 1 a nd 3 00 mg L- 1 su l fite . 1 I n o r der to avo id loss of su lfide or s u l fi te deaeration of the solution m u s t not be conti n ued afte r the addition of the sample or sta ndards. 2 If su lfide and su lfite a re both to be deter m i n ed i n the same sa m ple, the polarogram of s u l f ide i n a l ka l i n e sol ution must be recorded fi rs t 3 If thiosu lfate is present a peak appears at -0. 2 8 V w h i ch can be used ana lyti ca l ly. -

N otes :

.

,

.

The more insoluble the mercury salt formed with the anion, the m ore negative is the half-wave or the peak potential. Ani ons whose mercury salts have very different solubility products can in some cases both be determined together. For example, in 0. 1 mol L 1 Na OH , }:; 12 = -0. 2 2 V for the 1 CN- wave and £1 1 2 -0.77 V (versus SCE) for the 52- wave [ 7 ] . These two ions can be determined together with a detection limit of 1 0- 7 to 1 0-6 mol L- 1 • This is the basis of the polarographic esti­ mation of cyanide and sulfide in sewage as given in DIN 38 405 part 13 of the Deutsch en Instituts fiir Normung ( German Standards Institute) . =

Application 3

[ 18] I n d ustria l wa ste water. Boric acid ( 1 2 . 4 g) and KOH ( 1 1 . 2 g) a r e d i ssolved i n wate r (800 ml). The pH is adj usted to 1 0 . 2 (if ne c ess a ry ) a n d the vol u m e made u p to 1 L. Dissolve 2 . 503 g KCN i n 1 L 0 . 0 1 M KO H ( 1 000 m g C N- L- 1 ) . Add 20 ml o f sa mple t o 20 m l o f supporting electrolyte a n d deaerate. Record the DP-polarogram from 0 V to - 1 .0 V. The peak p oten t ia l is ca . -0 .24 V (versus Ag/AgCI, 3 M KCI) and the li near ra nge is from 0. 0 1 to 1 0 mg L-1 cya n ide. 1 C ya nide can be determined i n the presence of a 1 000 fo l d excess of phos­ phate, nitrate and su l fa t e and i n a l a rge excess of chloride i o n . 2 K3[Fe(CN)6]. K4[ Fe(C N)6] a n d K 2 [N i ( C N ) 6] have no i nfluence on the a nalysis. 3 C y a nid e ca n also be determined i n the p rese nce of s u lfide (e.g. i n wa ste water disti l l ates). After disti l l i n g oft a nd col lecting the cya n i de as H C N a n i mproved detection l i m it of ca . 5 mg L- 1 CW ca n be ach ieved .

Polarographic determination of cyanide

Sample: S u p porti ng electrolyte : Cyan ide sta n d a rd :

A nalysis : N otes :

-

Phosphate and silicate can be determined polarographical ly after reaction with molybdate to give respectively 1 2-molybdophosphate or 1 2-molybdosilicate [ 1 9, 20 ] . The electroactive heteropoly­ acids are separated from the excess molybdate, each other and impurities by extraction with organic solvents. The polarographic determination m ay be carried out either directly on the organic extract after addition of an appropriate supporting electrolyte or by back extracting into an aqueous phase [ 2 1 ] . When an analyte is separated from the sample matrix by solvent extraction prior to the volta­ mmetric or polarographic measurement, the selectivity and sensitivity of the electrochemical analysis may be greatly improved. This com bined or tandem procedure is often referred to as extraction polarography (voltammetry) [ 2 2 ] . The analyte is extracted either as an ion-pair o r metal-ion chelate a n d by careful control of the conditions can b e separated from interfering com­ ponents in the sample. An additional advantage of extraction polarography is that a n umber of 'polarographically difficult' metal ions can be analysed as their metal chelates. Examples are (a) Al (III ), the polarograph ic wave of which, with an £1 11 of ca. - 1 .7 V in neutral or slightly acidic solutions, is distorted by concurrent hydrogen evolution, and (b) Ti ( IV) which is readily hydrol­ ysed and in acid supporting electrolytes gives poorly defined irreversible d.c. polarographic waves.

1 52

I ntroduction to Volta mmetric Analys i s

-

1 3 .

v

i [nA]

500

400

300

200

1 00

E

- 1 .0

I

I

- 1 .5

I Standard deviation

-6 -4 -2

0

[V] (vs. Ag/AgCI, 3 M KCI)

F i gure 6.4

2

4

6

8

Ti

10 12 [mg

L-1 ]

Polarographic determ ination of titanium in a ceramic sample after acid digestion and extraction into d ichloromethane. (a) Supporti ng electrolyte alone - 1 5 g L- 1 TBAHFP i n C H1CI1. (b)-(e) Digest with standard additions of {b) 0, {c) 4, (d) 8 and (e) 1 2 mg L_, Ti . Result from standard addition plot 5 . 7 mg L- 1 Ti.

Both of these ions can be determined in the organic phase after extraction as the oxinate complex with determination limits in the mid- J.Ig L- 1 range. Titanium ( I V ) can be determined in ceramic samples after digestion with Hf/HNO� and extraction into dichloromethane as the oxinate complex. Tetrabutylammonium hexaflu orophos­ phate (TBAHFP ) is added as supporting electrolyte. Good current peaks are observed in the DP­ polarogram a t - 1 . 3 V (versus A g/AgC l , 3 M KCl/DMF, 0. 1 M TBAHFP) as can be seen in Figure 6.4. Interference from fluoride ions in the d igest is overcome by the addition of Ae+ [ 2 3 ] . In a similar way, aluminium ( ll l ) can b e extracted from sample digests as the oxinate complex with chloroform. The chloroform is disti lled off and the residue dissolved in DMF, a teraalkylam­ monium salt added as supporting electrolyte and the polarogram recorded. An irreversible wave -2 . 2 2 V) and magnesium ( £1 12 = with £1 12 = - 1 .8 V (versus SCE ) is obtained . Calcium ( £1 12 -2.20 V) in the range 1 0-4 to 1 0- 2 M and water up to 2% do not interfere [ 24 ] . Aluminium can also b e indirectly measured by means o f the polarographically active com­ pounds it forms with some organic dyestuffs such as Solochrome Violet RS, Erichrome Violet BA, Superchrome Ga rnet Y or Pentachrom e Violet R. The A l ( ITI) is measured either by the decrease in the signal of the dyestuff or by the signal produced by the reduction of the coloured lake formed between Al( I I I ) and the dyestuff. Similar methods have been developed for the determination of Be ( l l ) , Th( IV) and Zr(IV) [ 7 ] . A n important advantage that polarography has over many other analytical techniques i s that with this technique it is possible to distinguish between various forms of the analyte in a sample depend ing on their oxidation state or chemical form. All that polarographic speciation r eq ui re s is that the current peaks of the different elemental s p ecies are sufficiently well separated. =

Applications: i norganic species

1 53

50 nA

0

Figure

6.5

- 0.2

- 0. 4

- 0.6

-

08 .

- 1 .0 E

[V]

Determi nation of Fe(l l) and Fe( l l l ) in a zinc galvanising bath containing 65 g l-1 Zn 2 +, 1 00 g l- 1 H2S04 and 1 00 g l-1 Na2504 after adding Na4P207, x H20 (0. 1 M) and rai s ing the pH to 1 0 . 3 . fpiFe(lllJ -Q .34 V, fp[Fe[HIIJ = -0.85 V (versus Ag/AgCI, 3 M KC I). Resul t s eva luated by standard additions - 608 mg l- 1 F e ( l l) and 441 mg L-1 Fe(l l l ) . =

It is often necessary to b e able t o distinguish between the various oxidation states and chemi­ cal forms of an element because of their differing toxicities. For example, Cr( VI ) is a strong cell toxin whereas Cr(III) plays an important role in the metabolism of carbohydrates and fats and is an essential trace elemental species. Cr(VI) can be determined polarographically in the presence of Cr( I l l ) [ 1 2 ] . Other examples include the polarographic determination of Fe ( I I ) and Fe(III) in a zinc galvanising bath ( F igure 6 . 5 ) , and the determi n ation of Sn( I I ) in the presence of Sn(VI ) and of Au ( I ) in the presence of Au( l l l) in galvani sing baths [ 2 5 ] .

6 . 2 STRI PPING VO LTAMM ETRY Anodic stripping voltammetry with a mercury working electrode ( HMDE, TFME or mercury modified ThfMGE) can be used for the trace level analysis of antimony, bismuth, cadmium, cop­ per, gallium, germanium, indium, lead, manganese, thallium, tin and zinc. In particular, ASV has been successfully applied to the determination of traces of the heavy metals cadmium, copper, lead and zinc in environ mental water samples [26-29 ] . In wa te r samples which are only slightly polluted such as rainwater, drinking water and sea water, the determination often ca n be carried out directly after sampling with no, or only minimal, pre-treatment such as adj ustment to th e desired pH value or the addition of a supporting electrolyte. More heavily polluted samples must first be digested. This is commonly carried out by irradiating the sample with U.V. light or by digesting the sample in a suitable acid oxidising mixture in a microwave oven ( § 4 . 2 ) . The effect of U.V. irradiation o n the determination of Zn and Cd in polluted surface waters is i l l ustrated in figure 6.6. Application 4

Determination of cadmium, copper, lead and zinc by ASV after U.V. digestion using an HMDE [30]

Sample:

1 Surface water po nd s n e a r H erisa u, Switze rla nd); DOC 4 . 6 m g L-1 (low in orga n ics). 2 I n d ustri a l w a s te water; DOC 3 7 2 mg L- 1 ( h i g h i n org a n i c materia l).

1 54

Introduction to Voltammetric Analysis

b

Zn

- 1 .1

E

- 0. 9

[V]

- 1.1

E [V]

- 0.9

I

b nr

Cd

- 0. 8

- 0 . 65

d i rect Figure 6.6

-

0.5

E [V]

- 0.8

- 0.65

- 0.5

E [V]

after U . V. photolysis

L-1 ) and Cd (4 !Jg L- 1 in a h ighly poll uted surface water (a) without U .V. digestion, (b) after U .V. d igestion . DP stripp ing voltammograms for the determination of Zn (1 0 j.Jg

Sample pre-treatment:

Supporti ng e lectrolyte: Standard solutions:

Fi lter the sample through a membrane filter (0 .45 j.Jm) a n d acidify with H C I (pH 2). P lace 1 0 m l of t h e fi ltered, acid ified sa m p l e with 20 j.J L 30% H 2 0 2 in the U .V. digestor. I rrad iate with U .V. at 90°( for 1 5 min for the slightly pol l uted sample 1 a n d for 1 80 m i n for the heavily pol l uted sample 2 (if necessa ry repeat the add ition of Hp 2 after 1 5 m i n and 30 m i n ) . Dissolve 5 5 . 9 g K C I and 20 . 5 g C H 3 COONa i n 5 0 0 m l o f pure water. P repare 1 g L-1 stock solutions of each metal ion. For zinc, cad m i u m and cop­ per, d issolve 1 g of the m etal in the m i n i m u m amount of HN03, and for l ead dis­ solve 1 . 598 g Pb(N03h in 1 00 ml p u re water. Ri nse i n to a 1 L vol u metric flask, add 1 ml cone. H N 03 and make up to the mark with pure water. Prepare work­ ing standards by d i l ution as requ i red. Sta ndards conta i ning > 1 0 mg L- 1 a re stable fo r 1 week a n d those conta i n i ng < 1 0 mg L- 1 should be made up dai ly.

Applications: inorganic species

Determ ination :

Notes:

1 55

Add 1 0 m l of the digested sample and 0 . 5 m l of the supporting e lectrolyte to the polarographic cel l and deaerate with n itrogen for 5 m i n . Accu m u l ate the metals on the H M D E for 60 s to 3 0 0 s at - 1 . 2 V (versus Ag/Ag C I ) . Record the stripping voltam mogra m (DP- mode) from - 1 . 2 V to + 0 . 1 V and determine the m etal concentrations by sta ndard add itions. The cu rrent peaks occur at ca . -0 . 9 8 V (Zn), -0. 58 V (Cd), -0 . 3 8 V (Pb) and -0 . 1 0 V (Cu). 1 As the orga n i c content of a sample increases, i nterference due to the organic matter becomes more a pparent in the AS-volta mmogra m . I n o rder to obtain opti m a l resu lts, U . V . irrad iation is an essential step i n the standard method D I N 3 8 406- E- 1 6 for t h e esti mation o f cad m i u m , coba lt, copper, lead, n i ckel, tha l ­ l i u m and zinc i n drinking, g round, surface and seepage waters . 2 For samples with h i g her concentrations (> 50 �g L-1 ) D P P is u sed i n stead of ASV. Using a S M DE, the DP-po l a rogram is recorded form +0. 1 V to - 1 . 3 V (no accum ulation). The experi mental conditions (sample pre-treatment, etc.) are the sa me as a bove. 3 This method can be used for the determ i nation of these metal ions in ch i l l i sauce powder [ 3 1 ] a n d whisky [ 3 2 ] .

Like ordi nary voltammetry and polarography the stripping techniques ( in combination with U.V. digestion) are also important in trace metal speciation analysis; that is the determination of the con centrations of the various forms in which a given metal may be present in a sample such as di f­ ferent oxidation states or in different chemical or physical forms. A number of excellent reviews have been published on this topic [ 3 3-38 ] . A simple speciation analysis designed to determine the distribution of a trace metal ion among its various chemical and physical forms may require three determinations of the metal content by ASV. First, i n the untreated sample at its natural pH, second, in the sample after diges­ tion by U.V. photolysis at its natural pH and third, after adjusting the pH to < 2 with ultra -pure HCl and digestion by U.V. photolysis. The metal concentration measured in the first determina­ tion is the fraction that is present as hydrated ions, labile organic and weak inorgan ic complexes with ligands such as chloride, sulfate, carbonate, etc. This fraction is readily influenced by the addition of organic material to the water system via waste water discharge. It is usually described as the biologically available fraction of the metal and as such is important in assessing the toxicity level of the water. The concentration measured in the second determination incl udes the amount of metal present as inert organic complexes in addition to that present as hydrated ions and labile complexes, while that measured in the third determination gives the total metal content of the sample. From these results the amounts of a trace metal present in the sample as ( i ) hydrated ions plus labile complexes, ( i i ) inert organic complexes to which belong organo-metallic compounds and metal-ion complexes of humic and fulvic acids as well as of NTA or EVTA, and ( iii) inert inorganic complexes plus colloidally bound metal may be estimated. A number of voltammetrically based schemes for the speciation of heavy metals have been developed [ 37, 1 6 , 1 7 ] . Comparison of results for labile and stable forms of an element in different samples can only be made if the same analytical procedure has been used in each case. The volta­ mmetrically determined labile fraction not only depends on the kinetics of d issociation of the var­ ious metal complexes but also on the time required to record the voltammogram (voltage scan rate). Different experimental procedures may lead to different results for the various metal species in a sample. There is a particular interest in determining the concentration of those metals in natural waters or waste waters for which there is a legally specified maximum concentration limit set out in government regulations for controlling the purity of drinking water and waste water discharge. In DIN 38406, Part 1 6 , for example, the concentrations of Cd, Co, Cu, Ni, Ph, Tl and Zn are

1 56

I ntrod uction to Voltammetric Ana l ysis

Sample

I nsol uble part

�

I

yes Filter

l

I I

j_

Analysis of the insol uble part Filter and residue

D igest

I

J J

j_

I Analysis of the soluble part

I

1

I I � I l

Filtrate I

Acidify

I

I nterference

J yes

Aliquots

I

I rradiate

J

I I I J J

Aliq uots

I Determ ination of Tl only

I

Adj ust p H to 4 . 6 with potassi u m chloridesodium acetate sol ution

I

j_ Determination of Zn, Cd, Pb, Cu Adj ust p H to 4.6 with potassi u m chloridesod i u m acetate sol ution

Complex Pb with E DTA solution

I

I Dete rm ination of N i , Co

I

�

Adj ust p H to 9.3 with ammonium chlorideammonia solution

I Add dimethylglyoxime as complexing agent

I

I Voltammetric determination Figure 6 . 7

F low d iagram of the procedure for the voltammetric and polarographic determi nation of heavy meta ls in drinking water, ground water, surface waters and rain water accord ing to D I N 38406 part 1 6 [39) .

Applications: i norganic species

1 57

monitored by mean s of polarograhic and voltammetric techniques. The procedure, outlined in Figure 6.7, is suitable for determining these metal ions at concentrations in the following ranges: Cd, Pb and Tl 0. 1 flg L- 1 to 50 flg L- 1 ; Co and Ni 0. 1 flg L- 1 to 1 0 flg L- 1 and Cu and Zn 1 flg L- 1 to 50 flg L- 1 • It is used for monitoring the heavy metal content in a variety of waters and identifies the amount of metal in solution as well as the amount which occurs in association with the suspended matter. The generally very small amounts of Co and Ni are determined as their dimethylglyoxime complexes by adsorptive stripping voltammetry (AdSV ) , while DPP or anodic stripping voltam­ metry (ASV) is used for the other metals, depending on their concentration. Sample pre­ treatment is determi ned by the state of the sample and except for rai n water usually i nvolves U. V. digestion with H202• Details in reference 39. Heavily contaminated waters such as sewage must be chemically digested usually by wet acid oxidation in a Digesdah l ( Figure 4 . 3 ) o r similar digestion apparatus. Sludge, soi l and sediment samples should be treated in a similar manner. To determine the heavy m etal content in sewage sludge or soil, the sample is dried, weighed and refluxed with aqua regia for several hours. Only the oxides of aluminium, titanium and chromium remain undissolved. After dilution of the digest the heavy metals are determined by DPP or ASV depend­ ing on their concentrations. Typical current-potential traces for the analysis of a sewage sludge are shown in Figure 6 . 8 . Zinc was determined by DPP and cadmium, lead and copper by ASV after accumulation at -0. 8 V at a SMDE. The concentrations were determined by standard addi­ tions [ 40] . As an alternative to a mercury drop electrode, a mercury thin film electrode formed on an Ultratrace graphite electrode with its micro-electrode like properties can be used to determine these metal ions by ASV. Application 5

ASV determination of cadmium and lead using an Ultratrace graphite electrode [41 ]

Sample: Sample pre-treatment:

Electrode preparati o n :

Solutions: Standard sol utions:

Determ ination:

Drinking, river, sea a n d waste water. Filter the sample thro u g h a membrane (0.45 �m) fi lte r and destroy a ny orga n i c matter b y U .V. irradiation as i n appl ication 4 or b y oxidative m icrowave d igestion. Polish the electrode surface with a s l u rry of 0.05 �m Al203 powder, rinse with p u re water and d ry on a soft paper tissue. When carryi ng out determinations on deaerated solutions the e lectrode o n ly needs to be polished once each day. If the sample solution is not deaerated, then the electrode must be re-polished after each determi nation. Once a g raph ite e lectrode has been coated with m er­ cury it is not suitable to be used for other applications . On the other hand, an el ectrode which, for exa m pl e, has been used fo r the determi nation of orga n i c com pou nds, is n o longer su itable as a base for a fi l m elect rode. 6 M H C I , 1 g L- 1 Hg(l l ) Prepare stock solutions conta i n i ng 1 g L-1 C d a n d 1 g L- 1 P b respectively. Sta n ­ d a rd solutions shou l d be prepared as req u i red b y d i l uting the stock solutions with 0. 1 M H C I . Place 0 . 1 - 1 9 m l o f sa mple (depe n d i n g on concentration) in a volta m m etric cel l with 0 . 5 m l Hg(ll) solution a n d 0 . 4 m l 6 M H C I a n d d i l ute t o 20 m l with pure water. Deaerate with nitrogen for 5 m i n . Accum u l ate the ana lytes for 5 m i n at - 1 . 1 0 V. After a rest period of 1 0 s record the stripping volta mmogram i n the DP mode (Figure 6.9). F rom the peaks at -0 .68 V (Cd) and -0 . 48 V (Pb) (versus Ag/Ag CI, 3 M KCI) determ ine Cd and Pb i n the sample by standard additions. Traces of Pt (cause catalytic h u d rogen evol ution) and Tl and In (si m i l a r stri pping peak potentia ls) i nterfere with this determ i nation.

... ¥:

Pb ( D P A S V) n

Cd

Zn

(DPP)

I

+ 30 J..1. 9

so nA

(DPASV) +

I

s nA

0 .4 )1g

3"

[ G.

+ 3.0 11 9

g

g

I

� 51 3 3

25 nA

�

n· )> :::J "' .:;: "' ;;; ·

Sample

Cu

(OPASV) Sample

I

- 0 . 9 - 1 .0 - 1 . 1

Figure

6.8

Determi nation of Zn

I

I

- 0. 7

I

I

- 0.6

T �

- 0.5 - 0 . 4 E [V] {vs. Ag/AgCI, 3 M KCI)

{by DPP), Cd, Pb and Cu {by ASV) i n sewage sludge [40] .

I

�- 1

- 0 . 3 - 0.2 - 0. 1

T 0

+

I

0.1

Applications: inorganic species

-

Pb

0. 48

1 59

v

Cd - 0 . 68

- 0.9

Figure 6 . 9 Note:

ASV

v

- 0.6

- 0.4

E

- 0. 1 5

[V]

determination of cadmium (2 . 1 IJg L- 1 ) and lead (0.50 IJg L-1 ) using an U ltratrace graph ite

electrode.

The total content of l e a d and

cad m i u m i n the s a m p le s h o u l d n o t exceed 1 0 IJg. H igher amou nts will res u l t in the electrode overload ing (exceed the sol ubil ity of these m et a l s i n mercury.

Because of the increased rate of transport of the analyte to the electrode surface of a micro-electrode or micro-electrode array, stirring of the solution has little effect on the accumulation of the analyte on the electrode surface. The increased rate also means that only short accumulation times are needed when micro-electrode systems are u sed for ASV. Furthermore, because the volume of mer­ cury on the surface of the micro-electrode is so small, during the stripping process the accumulated metal is very quickly and totally stripped off the electrode. Th is leads to very sharp and easily evalu­ ated current peaks in the voltammogram. An example of this is the ASV determination of cadmium, lead and copper using a thin mercury film formed in situ on a RAMn.• electrode shown in Figure 6. 1 0. The determination limits for these three metals using the mercury coated RAM™ electrode are in the 1 -2 J.Lg L-1 range after 90 s accumulation from an unstirred solution. As mentioned above several other elements can be determi ned by ASV using mercury elec­ trodes among them are two group V metals antimony and bismuth. Application 6

Determination of antimony and bismuth by anodic strippin g voltammetry [ 43 )

Genera l :

Solutions:

Anti mony and bismuth ca n b e determi ned tog e t h e r b y ASV i n 0 . 6 M H C I at the H M DE . C o pper i nte rferes with the antimony s ign a l but this can be ov e r c o m e by using 1 0 % H C I . H owev e r bismuth ca n not be dete r m i ned i n the latter soluti o n . 3 0 % H C I (Supra p u r qual ity)

1 60

Introduction to Voltammetric Analysis

Cu i [nA]

4

Cd

3

2

1

0 ,_ _,----.----.----.---.----,----.----,---, 0 6 - 0.7 - 0.8 - 0.9 0 - 0. 1 0 2 - 0 .3 - 0.4 - 0. 5 __

-

-

.

.

E [V] {vs. Ag/AgCI, 3 M KCI)

Fig ure 6 . 1 0

ASV determ ination of cad mium, lead and copper (each 1 , 2 , 5 and 1 0 11 g l-1 ) using a RAM ™ electrode with c a . 1 1 0 ind ividual electrodes. Supporting electro lyte acetate buffer (pH 4.6) with 0.6 mg l- 1 H g ( l l); face = -1 . 1 0 V; race = 1 20 s ( no sti rri ng) [42 ) . -

1 g L- 1 b ism uth a n d a n tim ony stock sol utions i n 1 M H C I . Pr e pa r e sta ndard solutions a s r e q u i r e d by d i l uti on with 0.3 M H C I . Determi nation i n s a m p les A d d 780 � L 3 0 % H C I t o 20 m l of s a m p l e s o l utio n co nt a i n i n g between 0 . 5 to 200 �g L -1 S b a n d Bi i n 0 . 3 M H C I a n d d e a e rate with n i t ro ge n . Accu m u late the wi th ou t copp e r : a n a lyte s at a n H M D E at 0 . 24 V for 1 20 s. Record the D P st ri p p i n g volta mmo­ gram from -0. 24 V to + 0 . 1 V and determ ine the S b c o nce n t rat i on from the peak at -0. 1 8 V a nd the b i s m uth concentration fro m the peak at -0 .04 V ( v e rs us Ag/AgCI, 3 M KC I) by standard additions. For concentrations in the ra nge 200 � g L- 1 to 5 m g L- 1 Sb and Bi are best determ i ned u s i n g a S M DE a nd re c o rdi n g the DP p o l arog r a m from +0.07 V to -0 . 2 4 V. Treat 20 m l of sa m p l e as above to dete rm i n e the b i s m uth conte nt. To deter­ Determination i n the m i n e the copper a n d a nt i m ony add a fu rther 1 0 ml 30% H C I to the sam p l e and presence of copper: deaerate aga i n . A c cum u la t e the a n a l ytes at -0. 5 V a n d record the D P s t rip p i n g volta m mogram from -0. 5 V to 0 1 V. The Sb peak is at ca . -0. 1 8 V a n d the C u peak a t c a . -0 . 3 0 V . E v a l u a te t h e concentrat i o n s b y standard a d d i t ions. For h i g he r co nce n trati o n s record the DP pol a rogram b e tw ee n 0 and -0 . 5 V. Note : I n 0 . 6 M H C I only S b( l l l ) is measured; Sb(V) m ust fi rst be red uced to S b ( l l l) w i t h hyd r a zi n e su lfate . I n 1 0 % H C I bot h Sb(l l l ) a n d Sb(V) can be dete r m i n e d so that it i s po ssi b l e to deter m i n e t h e a m o u nt of both oxidation states of Sb i n a s a m p l e (s p e c iat i on) Sta n d a rd s o l u ti o n s :

-

,

-

.

.

A n exampl e of the use of a gold electrode is the ASV determination of mer c u r y In the accumu­ lation step the mercury i s deposited as an amalgam on the gold electrode surface, and redis­ solved again in the anodic determination step. U sually a rotating disc e le c trode (Au-RDE) is .

Applications: inorganic species

1 61

used. Like all solid electrodes this must fi rst be polished with aluminium oxide powder and then electrochemically conditioned before use. The conditioning usually consists of cycling the electrode between an anodic and a cathodic voltage one or more times. This procedure for cleaning the electrode surface, repeated prior to each measurement, is necessa ry to obtain reproducible re s ul t s r 44) . Application

7

ASV determination of mercury on a gold electrode [ 45]

Sample: Sample pre-treatment:

Su pporting electrolyte: Determ i n at i o n :

Aquatic environmenta l sa mple, e.g sewage, tip water, seepage. Place a 1 0 m l sample i n the d i gestion vessel , adj ust pH to 2 with HCI, add 50 � L 3 0 % H202 and i rradiate with U .V. for 1 -2 hours at 80°C . For heavi ly pol­ luted samples, it is necessary to add a n additional 50 � L H202 afte r one hour i rradiation and conti nue i rradiation for a further 1 -2 h o u rs. Mix 0 . 3 3 6 g Na 2 - E DTA with 2 0 0 m l of 1 . 0 M H C I04 and 200 m l of 1 . 0 M H C I i n a 1 L vol u metric flask a n d make u p t o the m a ke with pure water. P lace 5 ml of the digested sample solution and 5 ml of s u p porti ng el ectro lyte in the polarogra p h ic ce l l fitted with a pre-conditioned Au- R O E . Accumulate the mercu ry at +350 mV for 60 s a n d record the D P-voltam mogram from + 3 5 0 mV to +800 mV. Using the cu rrent peak at + 5 8 0 mV dete rmine the concentration of merc u ry by sta ndard additions ( F i g u re 6. 1 1 ) Th is method can a l so be used fo r the determ i n ation of mercu ry i n orga n i c sam­ ples such as chi l i sa uce [46] and soya bean o i l [47] afte r microwave d igestion. Samples conta i n i n g more than 40 mg L- 1 Hg m ust be d i l uted or the sa mple vol u m e reduced. The supporting electolyte is stable for 1 week. The EDTA i s to complex interfering ions. .

Notes :

a

b i [nA] 2.0

.J

1 .5

1

1 .0

0.5 0.4

Figure 6 . 1 1

0.5

0.6 E [V]

o �----,---,---2 2.5 c [119 L-' ] 1 7.5 1 2.5 7.5 2 .5

A SV determination of mercu ry in waste water after U.V. irrad iation and enrichment at a Au-RDE. Evaluation of the mercury content was from the cu rrent at EP + 0.56 V by standard additions (a) or from the i versus c ca l ibration curve (b). =

1 62

Introduction to Voltammetric Analysis

Another application u s i ng a gold e l e c t rode is the determination of arsenic in a s a m p le In I So/o HCl at -1.2 V both o x i d a t i o n states {As( III) + As(V) } are r edu ce d a n d de p o sit ed on the go ld elec­ trode s u r fa c e The total As content is d et ermi ne d from the peak i n the anodic stripping voltam­ mogr am at +0. 1 5 V with a determination limit of 1 mg L_ , [ 48 ] . Recent studies have shown that As(V) can b e reduced at a mercury electrode i n the presence of 0-mannitol. This forms the basis of a method for de te r m i n in g the concentration of both As (III) and As(V) in g ro u n d water by CSV. It is a typical exa mpl e of valance st at e sp e c i ati o n [ 4 9 ] . .

.

Application 8 Determination of a rse n ic ( III ) and arsenic ( V ) by cathodic stripping voltammetry [50] Surface waters, se e p age w at e r . Sa m p l e : Dissolve 4. 1 7 g N a 2 HAs04• 7H20 i n p u re wate r a n d m a k e up to 1 L to give a Sto ck so l ut i o n s : stock so l u ti o n of 1 g L-1 As(V) a n d di sso lve 1 . 3 2 g As203 i n p u re water a n d make up to 1 L to g ive a 1 g L- 1 As( l l l ) stock s o l u t ion . Prep a re st a nd a rd s as r e q u i r e d by d i l u t i o n Ground water can be a n a lysed di rectly. Seepage is f i l t e r e d acid ified to pH 2 Sa m p l e p re t reat m e n t : wi t h sulfuric acid and, afte r addi n g 20 IJ L 30% H202 per 1 0 m l of sam p l e irra­ diated with U .V. fo r 1 5-30 min. T h i s t r e at m e n t converts all As to As(V). Determ inati o n : As( l l l): I n a s upp o rt i n g e l ect r ol yt e of 0.4 M H 2 S04 + 5 mg L- 1 C u 2 + accu mulate As for 4 m i n at -0. 5 5 V (versus Ag/AgC I , 3 M KCI) after d e a e ra t i o n with nitro­ g e n (5 min). R e c ord the DP stri p p i n g voltammog ra m from -0 . 5 5 V to -0 .80 V. As(V) : I n 0.4 M H2S04 + 0 . 2 5 M m a n n i t o l + 1 0 mg L- 1 C u2+ s u pporti n g electro­ l y t e acc u m u l ate As(V) + As(l l l) after d eae r at i o n with n itroge n for 5 m i n at -0. 5 5 V for 4 m i n a n d re c o r d the s t r i p p i n g volta mmogram. I n both cases t h e current peak occurs at -0 .695 V. Use sta ndard additions to d et er m i n e the concentrations. D eter m i n a t i o n l i m its: As( l l l ) 0 . 5 11g L-\ As(V) .

,

-

,

0 . 7 1Jg l- 1 •

Notes:

T h e st a b i l i ty of aqueous As( l l l ) solutions d ep e n ds on the As concentratio n . So l u ­ t i o n s with > 1 00 mg L- 1 As(l l l ) a re sta b l e over a long period . I n s o l u t i on s with < 1 mg L- 1 the As(l l l ) is co n v e rte d to As(V) with i n a few d ays. Therefore trace leve l s of As in g r o u nd and su rface waters a re a lways present as As(V) . The a b ove method is b es t su ited to the a na l ys i s of water with a l o w o rg a n i c content that d oe s n ot n eed any pre-treatment.

In addi tio n to a rsenic selenium and tellurium can also be determined by th e cathodic stripping of the inter-metallic compound formed with copper. The method is used for th e determination of selenium in m i n e ra l and b iologi c al sa m ples and can, as F i gure 6. 1 2 shows, be used fo r the deter­ mination of both selenium and tellurium to geth e r in a s am p l e [ 5 1 ] . Each el e m ent can be deter­ mined in a 1 0 4 -fold excess of th e other [ 5 2 ] . Selenium, an essen tial n utrient with a daily r equ ir e me n t for a dult s of 20- 1 00 f,!g, is a l s o qui t e toxic. This means that the dietary intake through fo o d and water must be carefully c o nt ro lled and to this end the u pp er limit for potable wat e rs in G e r m a ny has been set a t 8 flg L- t . CSV is used to monitor the drinking water levels of selenium. ,

Application 9 Determination of selenium by cathodic stripping voltammetry [ 53) Sa m p l e: Foodstuff, water. S a m p l e pre-treatment: The o rg a nic material i n biological samples and w a te r s with high o rga n ic content is removed by wet d i g e s t i o n with H2S04 and H202 in o rd er to avoid sel e n i u m loss . This step is o mi tte d for drinking waters.

Appl ications: inorganic species

3

- 0 . 4 V Se

Figure 6 . 1 2

4

Te

- 0.4 V

Se

Te

Determination of selenium i n the presence of tellurium by CSV. Supporting electrolyte: 0.5 M + H 2 S04 (pH 4.5) + 1 mg l- 1 Cu2+. face -Q.4 V (versus SCE), tac, 1 m i n . (1 ) 1 119 l-1 Se + 1 0 119 l- 1 Te; (2) 1 IJQ l-1 Te + 1 0 119 l- 1 Se; (3) 1 IJQ l-1 Se standard to 1 mg l-1 Te; (4) 1 119 l- 1 Te standard to 1 mg l-1 Se.

(N H4)zS04 + 4 mM EDTA

Selen i u m standard : Determ ination:

1 63

Dissolve 0 . 2 1 9 g Na2Se03 in water and m a ke u p to 1 00 m l i n a vol umetric flask. Adj ust the pH of the cooled digest to pH 2 . 2 with NaOH and d i lute to 50 ml with water. In the case of the u ntreated drinking water add 1 3 . 2 g ( N H4h504 to 50 ml and a dj u st the pH to pH 2 . 2 with H2S04. For each sa mple, place 20 ml i n t h e pola rographic cel l a d d 4 ml 0 . 1 M Na2-E DTA (to complex with interfering heavy metal ions) a n d 1 m l C u 2 • solution (0. 1 g L- 1 C u 2+}, deaerate with n itro­ gen for 5 min and accu m u late the Cu a n d Se at -0 . 2 V for 3 0-60 s with sti rri ng. Record the stri pping volta m mogram ( D P mode) from -0.45 V to 0 . 8 V and d eterm ine the concentration of sele n i u m from the pea k at ca . -0 . 6 5 V (versus Ag/AgC I , 3 M KCI) by standard additions. Alternatively, the determ ination can be carried out with greater sensitivity by adding a sod i u m ta rtrate b uffe r of pH 2 2.5 and 8 mg L- 1 Cu + to the sample i n the cel l . -

The development o f AdSV has greatly expanded the number o f m etals that can b e determined and markedly lowered the concentrations that can be measured by voltammetric techniques [ 26, 3 7 , 54-57 J. Usually the sample is first digested by U. V. irradiation or oxidative m icrowave digestion and the AdSV determination then carried out using the cathodic stripping of an adsorbed metal complex. A typical example is the determination of iron as its catechol complex in the lower f.lg L- 1 region [ 58 ] .

1 64

Introduction to Voltam metric Analysis

Application 1 0

Determination o f iron by adsorptive stripping voltammetry {59]

S a m ple : Sol utio ns:

Sample pre-treatment: Determ i n ation :

S u rface wat e r gr ou n d wa t e r d ri n ki n g wa t e r m i ne ra l water c o ol i n g water, etc. Stock. Disso lve 0. 1 00 g p u re Fe i n S u p ra p u r HCI a n d d i l ute to 1 00 ml w ith pure wate r. D il ute 1 m l of this sol ution to 1 00 m l with p u re water to g ive a 1 0 mg L- 1 stock so l ut i o n . Standard . D i l ute the stock solution as req u i red with pure water. Catecho l . Dissolve 2. 75 g pure catechol in 25 ml of deae rated pure water u n der a nitrogen atmosphere. This so l ution must be stored i n the dark and is st a ble for several days. Catechol can be p urified by s ubl i mation or recrysta l l isation from toluene. For s i ng l e d eterm i n ations si mp l y add a crystal of catechol to the sa m p le solution. PIPES buffe r sol u ti o n . D i ssolve 6 .047 g p i perazine- 1 , 4-eth a n es u lfo n i c acid i n 1 m l 30% NaOH and a few m l p u re water . Adju st t o pH 8 w i t h 2 5 % N H 3 solution and m a ke u p t o 2 0 m l . Destroy su rface active i nterferents b y U . V . i rradiati o n . Add 0 . 4 m l catechol solution to 20 m l s a mp le i n a v o lt a m m etric cel l, deaerate a n d add 0 . 4 ml P I P E S buffer. Wh e n nece s s a ry a dj u st the pH to 7 w i th 1 0% NH3 so l uti on . Accumulate t h e comp l ex on a n H M D E a t -0 .3 V for 30 s, record the D P p o l a rog r a m and d eterm i n e the i ro n content from the c u rrent p e ak at -0 .46 V by sta n d a rd additions. I n drinking wate r manga nese can be d e ter m i n e d i n th e p res e nce of i ron by ASV. An a l ka l i n e borate buffer a n d a Zn 2 + salt (to p revent i n ter m eta l l ic com­ p o u nd fo rmation) a re added to t he sample. (face = 0 0 7 5 V, fP - - 1 . 48 V). ,

,

,

,

-

-

Note:

,

-

-

.

A striking example of the very high sensitivity that can be obtained with adsorptive stripping i s the determination of platinum. In this method, formazone ( formed by condensing formaldehyde 2+ wi th hydrazine in situ) forms a complex with Pt( l l ) , { Pt (CH2= N -N H2 ) 2 } , which is adsorbed on the HMDE, reduces the hydrogen overvoltage at the mercury electrode and catalyses hyd ro ge n evolution . The height of the resultant current peak is proportional to the platinum con cen trati on. S urface active imp urities, which can be present even in drinking water at trace levels, interfere with the analysis. The determination limit is quoted to be 1 0 pg L- 1

[ 60] . This method

is

well

suited for determining traces of platinum in sea water and for monitoring changes in enviro n ­ mental Pt levels arising from automobile exhaust gases. Another application i s t o monitori ng Pt traces in body fluids following chemotherapy with the cytotoxically active cisplatin (cis-diamm i ­ nodichloroplatinum ( I l ) ) .

Application 1 1

Determination of platinum by adsorptive stripping voltammetry [ 6 1 ]

Sa m p l es : S a m p le p re-t reat m e n t :

Wa te r bi o l o g i ca l m a t e ri a l Aq ueo u s s a m pl es . Aci dify 1 0 m l of s a m pl e to pH 2 w ith HCI, ad d 50 � L 30% H202 a nd i rradiate with U .V. for 1 hour. U r i n e samples. Pl ace 2 m l sa m p l e, 50 IJ L H2S04 and 300 � I H 2 0 2 i n a d igestion vessel a nd i rradiate for 3 hours. Du ring th i s per i od at three d ifferent times add 1 00 �L H202• For p la nts body fl u ids material, etc. Completely deco m pose the sa mp le with H N 03 i n the p r esen c e of ad d ed c h l oride in a h igh pressure asher. Disso l ve the decom posed m a te r i a l in 1 0 ml p u re water. ,

.

,

,

Applications: inorganic species i

[nA]

1 65

ca. - 0. 9 V

- 1 50

- 1 00

- so

E

- 0 .80

Figure 6 . 1 3

Standard solution:

Notes :

- 1 .0

3 M KCI)

Determination of platinum i n a urine sa mple by adsorptive stripping voltammetry. U .V. i rradiation for 3 h; face == -0 . 70 V , tacc == 60 s. Eval uated by three-fold standard addition . Result: 2 . 5 ng L- 1 Pt.

Su pporti ng electrolyte :

Determ inatio n :

- 0.90

[V] (vs. Ag/AgCI,

Place 40 m l pure water, 2 5 � L 3 0 % forma l i n and 1 m l 96% H 2S04 i n a 5 0 ml vol umetric flask. On coo l i ng add 1 . 5 m l 0. 1 M hydrazine and make up t o the mark with pure water. This solution m ust be fresh ly made up each day. A com mercial stock sol ution was d i l uted with 1 M H 2S04 to g ive a 1 �g L-1 standard . Place 1 0 m l o f U . V. irradiated water sample or 1 0- 1 0 0 � L o f the solution from the h i g h pressure asher plus 1 0 ml p u re water in the voltam metric cel l with 1 . 5 ml of supporti ng electrolyte and deaerate. Altern atively add the d igested uri n e sample to 1 . 5 ml of support i n g electrolyte a n d deaerate. Accu mulate the plati n u m com plex on a n H M D E for 1 min at -0 . 7 V (versus Ag/Ag CI, 3 M KCI). Record the D P cathodic stri pping volta mmogram; the peak at ca . -0 . 9 V i s due to the plati n u m complex. The en richment ti me should be decreased below 6 0 s for Pt concentrations > 50 n g L _ , _ Pt(IV) is redu ced to Pt(l l) by the supporti ng electrolyte. Too h i g h a con centration of H N 03 rema i n i n g i n biological samples after h i g h pressure ash i ng interferes with the d eterm i n ation by oxidising the Pt( l l ) complex.

In addition to using the c a t a lyt i c reduction of Cr as its diethylenetriaminepentaacetic acid ( DTPA) complex in the presence of nitrate ions as the basis of t he DPP determination o f Cr(V I ) ,

1 66

I ntroduction to Voltammetric Analysis

this catalytic process has been applied to the adsorptive stripping voltammetric determination of Cr at ultra-trace levels [ 62 ] . Application 12

Determination of ch romium as its DTPA complex by adso rptive stripping voltammetry [ 63)

Sample: Sample pre-treatment: Supporti ng electrolyte: Standard sol ution: Electrode: Determ inati on:

Water samples and biological materia l . See Appl ication 1 . Dissolve 1 . 64 g NaOOCC H3, 1 . 96 g DTPA and 2 1 . 3 g NaN03 i n 1 00 m l of p u re water. Dissolve 0.2829 g K2Cr207 in 1 L p u re water. This stock solution ( 1 00 m g L- 1 Cr) i s twice d i l uted to give a working standard solution of 20 �g L- 1 Cr. H M DE Place 1 0 m l sample and 5 m l supporti ng electrolyte in a vol u m etric cel l, adjust the pH to 6 . 2 with NaO H ( 3 0 % ) and deaerate for 5 m i n with n itroge n . Accumu­ late the C r( I I I ) - DTPA com plex (formed from C r(VI) by red uction) at -0 . 1 V with stirring for 60- 1 2 0 s. Record the DP-voltam mogram from - 1 . 0 to - 1 . 5 V. Use standard additions to determi n e the Cr concentration from the cu rrent peak at - - 1 . 2 8 V. The determi nation l i m it is 20 ng L- 1 . All the Cr in the sample should be i n the C r(VI) state. With C r(l ll) present, the peak height is u nstable as the complex formed d i rectly from Cr(l ll) g radually changes to an elect rochem ica l ly inactive state. M agnesium at concentrati ons > 1 00 mg L- 1 interferes. Such samples (e.g. sea water) m u st be d i l uted with p u re wate r. After U .V. i rradiation at p H 6-7 all the C r i s present as Cr(VI). The U .V. i rradia­ tion of acid solutions conta i n i n g chloride can resu lt i n the formation of volatile C r0ll2 and lead to erroneous resu lts . -

Notes:

Alternatively, chromium(VI) may be determined by the product of its reaction with 1 ,5-diphenylcar­ bazide (DPCI)-the Cr(III) -diphenylcarbazone(DPCO) complex-which is adsorbed onto an Ultra trace graphite electrode and then cathodically stripped from the electrode [ 64, 65] . Since Cr(III) itself does not react with DPCI, the method can be used for the speciation of Cr in waters with low organic content. After first determining the Cr(VI) content, the sample is treated with (NH4)2SpR to convert Cr(IIT) to Cr(VI) and, after destroying the excess oxidant, the total Cr is determined. Only rarely can ultra-trace concentrations of metals in natural samples such as sea water be determined by AdSV without sample digestion because surfactants present in the original sample normally interfere with the adsorption of the metal complex onto the electrode surface. One example, shown in Figure 6 . 1 4, where no pre-treatment of the sample is required, is the direct determination of uranium in river water by AdSV using chloranilic acid (2,5 -dichloro-3,6-dihy­ droxy- 1 ,4-benzoquinone) as the complexing agent [ 66, 67] . In the potential region of analytical interest, the uranium-chloranilic acid complex is more stron gly adsorbed on the electrode than are the surfactants that commonly occur in river waters and so the latter do not i nterfere with the determination in this case. Application 13

Determination of uranium by adsorptive stripping voltammet ry

Sample: Stock solution :

[ 67 )

Gro u n d water, river a nd other s u rface waters. Dissolve 0 . 1 7 8 g ( C H l 00)2 U 02 . 2 H 20 i n pure water, a d d I ml 6 5 % H N 03 and d i l ute to 1 L. Make standard sol utions as req u i red by d i l uting this 1 00 mg L- 1 U sol ution.

Appl ications: inorganic species

1 67

E - 0. 075 V P i

d

[nA]

- 50

i

c

[nA]

- 40 - 40

b

- 30

- 30

l

573 ng L-1 U

- 20

a

- 20

- 10

- 10

E

- 0 .05 - 0 . 1

Figure 6. 1 4

Determinati o n :

0

- 0. 1 5

[V] (vs. Ag/AgCI,

3 M

1 .0

KCI)

2.0

3.0

4.0

JlQ L-1 U 5.0

Direct determination of uranium i n water from the Mosel river by adsorptive stripping voltammetry. Conditions: 2.5 x 1 0-'� M chloran i l ic acid, pH 2 .4, face = +0 . 1 V, tacc = 90 s . (a) sample, (b)-(d) after sta ndard addition of 1 .2 , 2 .4 and 3.6 1Jg l-1 U . The u ranium concentration may also be determ ined from the ca libration curve shown.

Chlora n i l i c acid - Dissolve 0 . 5 5 g ch lo ra n i l i c acid in p u re water and d i l ute to 1 l. T h i s solution s h o u l d be prepared weekly. Pi pette 1 0 m l o f sa m p l e a n d 1 m l c h l o r a n i l i c acid sol ution i nto a volta m metric ce l l adj u st the pH to 2 . 5 with HCI or NaOH, a nd deaerate with n itr o g e n . Accu­ m ulate the U-chlora n i l i c a c i d co m p l ex o n an HMDE at + 0 . 1 V (ve rsus Ag/AgCI, 3 M K C I ) f r o m 0 to 2 m i n (dep e n d i ng o n concentration). Record the DP str i p p i n g vol t a m mogra m between face and -0. 3 V. Eva l uate the concentration by s tan dard additions or from a ca l i bration curve. The sample of rive r water (Figure 6. 1 4) was found to conta i n 573 ng L- 1 U . The d e t e ct io n l i m i t after 6 min a ccu m u l ation wa s found to be 24 ng L- 1 . The dete rmi nation of u ra n i u m with ch l o r a n i l c acid is a m ong the m ost sensitive a n d sel ective m eth o d s H i g her c o n c e n t rat i o n s of u ra n i u m may be d e te r mi ne d using chlora n i l ic acid eithe r by d i l uting the s a mpl e or u s i n g other methods of analysis n a m e l y : DP-polarography with a SMDE fo r 0 . 1 - 1 . 5 mg L- 1 U, DP-volta mmetry with an H M D E a n d no e n r ich m en t for 1 0-500 �g L-1 U, and DP-a dsorptive stripping v o lta m m e t ry with an H M DE and various accumulation times for 24 ng L- 1 - 40 �g L-1 (F i g u re 6 . 1 5). ,

­

­

Comments:

.

1 68

I ntroduction to Voltammetric Analysis

AdSV

�

�

1 . 1 0-8

1 . 1 0-6

Uranium (g L-1 ]

Concentration ranges for the determ ination of u ranium in the presence of chloranilic acid by DP­ polarography, DP-voltammetry and D P-adsorptive stripping voltammetry.

Figure 6 . 1 5

Other metals which form complexes with chloranilic acid (Table 6.2) do not interfere with the uranium determination either because their chloranilic acid complexes have different accumula­ tion potentials or they have different stripping peak potential s. In most cases the current peaks measured in the voltammetric determination of metal ions as their complexes are due to the reduction of the central metal ion. There arc a few cases where it is the ligand of the metal complex that is reduced instead. The metal ions involved a re difficult to reduce in water as th ey usually have very negative reduction potentials. The determination of alu ­ minium as its 1 - 2 -dihydroxyanthraquinone-3-sulfonic acid ( alizarin-S or DASA) complex is typ­ ical of this type of determi nation. Metal ions which can be determ ined b y adsorption stripping voltammetry of their chloranilic

Table 6 . 2

acid

compl exes.

Supporting electrolyte

Analyte

U�anium

(CAA)

.

--i.5 x 1 0-·l M CAA, pll--z.s-·

Antimony( I I I ) Antimony( V ) Molybdenum

Tin(JV)

Vanadium

V- · 0+.1

5 x 1 0- M CAA, p H 3

+0. 1

1

-0.5

5

5 x 1 0 - 3 M CAA, pH

J

x

2.5

1 .5

1 0-3 M CAA, pH 2 . 5

x

x

1 0--'� M CAA, pH 4 . 2

10 5 M CAA, pH 4 . 7

67--

Reference

--

v

_. - -8-_ 0. 0 V

--

v

-0.2 v

-0.2 v

-0.2 V

-0.42 v

-0. 1 3 v

-0.65 v

-0.35 v ( 1 ) -0.52 v (2)

-0.45 v

--

68

68 69 70

71

Application 1 4

Determination of aluminium as its alizarin-S complex by AdSV (72]

Sample:

S o l utions:

G round water, sea water, d rinking water. DASA - 0 . 288 g i n 1 L p u re w at e r (1 mmol L-1 ) . B E S buffer - 0 . 1 M N, N-bis(2-hydroxyethyl)-2-ami noeth a n esu lfonic a c i d + 0.04 M N a O H ; p H == 7 . 1 Zn masking solution - C a E DTA O . Q 2 mol L-1 Standard Put 50 ml pure water, 1 ml DASA solution, 1 ml B E S s o l ution and 1 00 � L A I st o c k s o l ution (1 g L- 1 AI) i n a 1 00 ml vol u metric flask and make u p to the mark. -

Applications: inorganic species

Determi nation :

P lace 1 0 m l sample i n the vo lta m m etric cel l with 1 0 m l p u re water, 1 00 � L Ca­ E DTA, 200 �L BES and 2 00 � L DASA and deaerate with n itrogen . Accumulate the AI complex on the H M D E at -0.9 V with stirring for 3 0-60 s a n d then record the DP-stri pping vo l ta m mogra m from -0 . 9 V to 1 3 V. The pea k pote nti a l is at - 1 . 1 V. Use standard add itions to ca lcu late the concentration of AI . The p H of the sample is critical a n d m u st be pH 7 . 1 ± 0 . 1 after add ition of the b uffe r. Aluminium concentrations i n the range 1 - 1 00 �g L . , can be determ i n ed by the a bove method . For higher concentrations DPP should be used . The amounts of B E S and DASA m ust be i ncreased when determ i n i n g AI in the mg L-1 reg i o n . T h e determi nation l i m it i n d r i n k i n g water i s - 1 � g L- 1 • The l i near iP versus c ra nge extends u p to 40 �g L -1 for 60 s accu m u l ation time. -

Notes:

1 69

.

Potentiometric stripping analysis ( PSA) and chronopotentiometric stripping analysis (CPS ) are alternative techniques that may b e used for the analysis o f metals such as Bi, Cd, Cu, Pb, Sn, Tl and Zn at the trace level in water samples [ 73 ] . With micro-processor controlled equipment, limits of detection are in the sub- 11g L- 1 range. The main advantage of PSA and CPS is that organic material does not interfere with the analyses to anywhere near the same extent as it does in strip­ ping voltammetry. Also, with PSA atmospheric oxygen dissolved in an aqueous sample does not h ave to be removed, thus markedly reducing the ti me required for the determination (§ 3 .4 ) . Application 1 5 Determination o f cadmium and lead i n h igh organic content fluids by differential PSA [74 -76] Waste water, drinks (wi ne, apple j u ice, etc.), urine, bl ood . Sample: E l ectrode : I n situ prepa red t h i n mercury fi l m on a n U ltratrace g raph ite e l ectrode . The el ec­ trode su rface is prepared by tri m m i ng off the surface l ayer and pol i s h i ng with a 0 . 0 5 � m Al203 s l u rry. Insufficiently p repared e lectrodes give resu lts with poor reprod u c i b i l ity. Solutions: P repare working sta ndard sol utions of 1 m g L- 1 Pb and of 1 m g L- 1 Cd from the respective 1 g L- 1 stock solutions. Cetyltri methylammon i u m b rom ide (CTAB) 0 . 60 g L- 1 i n p u re wate r. Triton X- 1 00 0. 5 g L- 1 i n etha n o l . Sample p reparation : B lood m ust be sta b i l i sed by the addition of 2 m g m l -1 N a 2 - E DTA (i n soluti o n ) . B lood a n d u ri n e should be d i l uted 1 : 1 or 1 : 1 0 with pure water a n d triton X- 1 00 added. S u rface waters and drinks - do not d i l ute, add CTA B . T o 1 0 m l of sa m ple (or 1 0 m l o f d i l uted sa m p l e) i n t h e ce l l a d d 1 m l 30% H C I , Determi nation : 8 0 � L H g(N03h sol ution ( 1 g L-1 H g ) and, depend ing o n t h e orga n i c content, 1 00 �L of CTAB or triton X - 1 00 sol ution . The meta ls were accumulated from sti rred sol utions-cadm i u m at - 1 . 2 V a n d lead at -0 . 9 V (versus Ag/Ag CI, 3 M KCI). Depe n d i n g on the concentration the accu m u lation time varies between 1 and 5 m i n . The concentrations were evaluated by standard addi­ tions . (Fig ure 6 . 1 6). After 5 m i n accu m u lation the d eterm i n ation l i m i t is 0 . 4 �g L-1 and the l inea r range is up to 1 00 �g L-1 •

Modified carbon electrodes are being developed to replace mercury electrodes for stripping volta­ mmetry (§ 4.5.4) . One application is the determination of tin and lead in the presence of each other using a carbon electrode manual ly modified with Hg2Cl2 and pyrrolidinedithiocarbamate. Tin (IV) and Pb ( I I ) were accumulated as metals at - 1 .2 V (versus Agl AgCl, 3 M KCl) from a

1 70

Introduction to Voltammetric Analysis

dt/dE

[s v -1 1

- 1 .0

Figure 6 . 1 6

i j

dt/dE

[s v -1 1

- 0.6

- 0.2 E

[V]

-

j I

1 .2

- 0 .8

- 0.4 E

[V]

Determination of lead and cadmium in urine (NSB standard) by differential potentiometric stripping analysis. Resu lts after two standard additions - lead 7.52 1Jg L- 1 , cadmium 8.28 1Jg L-1 .

1

sup porting electrolyte c o ntainin g 4.6 g L- 1 NH4Cl, 9.6 g C ( NH4) 2C204, 9.4 g L- 1 HCl and 10 mg L-1 methylene blue [ 77 ] . The metals were determi ned by DP-anodic stripping vo l tamme try. As Figure 4.26 shows the c urrent signals are well s ep arated and readily evaluated. The linear iP versus c range for the Sn (IV) a ft er 1 20 s accumulation at - 1 . 1 V is from I to 1 00 f.tg C ' and the detection limit is 0.9 flg L- 1 • The method is n o t affected si gn ificantly by the presence of surfactants and can be used for the direct determination of Sn ( TV) in fruit juices which have been canned in tin- p lated containers. Cu(II) and Tl( I I I ) were found to interfere with the determination. Other appl ications of carbon based modified electrodes have been to the determination of silver [ 7 8 ] a n d techne ti um [ 79 ] . A method of m odifying a MTFE t o r emove interference from or ganic mol­ ecules is to coat the electrode with cellulose acetate [ 80] or a co-polymer film [ 8 1 ] . These coatings act as a semi-permeable membrane, a llowing the smal l metal ions to pass through to the electrode surface wh ile preventing the larger o r ganic molecules from adsorbing on the surface.

6.3 AMPEROMETRIC M ETHODS

­

The proportionality between curren t and concentration is exploited analytically in am perometric gas sensors, in amper o metric bio-sensors, and in amperometric flow-through detectors used in flow injection analysis and l i qu id chromatography. In addition, ampcrometry is used to detect the end p o i nt in the Karl Fischer determination of trace levels of water in org a n ic solvents ( § 2 . 2 ) . The sensors that have been developed for t h e dete r minati o n o f gases i n aqueo us media i n gen­ eral consist of a two- or three-electrode a r r an gement i n com bination with a vo l ta ge source and a potentiostat. The p o te n t i al applied to the worki ng electrode must be set in the l im it i ng current region for the o xi d a t ion or reduction of the gaseous analyte and must be held constant during the course of the measurement. The measured current should be proportional to the concentration of the gas in the flg L- t to mg L- I ran ge Since the sensitivity of the measurement is control led by the rate of the overall rea ctio n i n cluding the transport of the an alyte to the working electrode, the movem en t of the analyte to the e lectrode must be controlled. Thi s is achieved with the help of a gas-permeable membrane m ade from teflon, polyethylene, polyvinylchloride, or cellophane, etc. The m e m b r an e is placed at a fixed distance from the work ing electrode and s e p arat e s it from the test solution . This enables a definite, repro du c ible resistance to the diffusion of the analyte from .

Applications: inorganic species

1 71

the test solution to the working electrode to be established. Wh en u si ng an amperometric gas sensor it is necessary to allow time for the diffusion gradient to be e sta bl i s h e d and the measured current to reach a ste ady value. Because this may take a considerable time, in p r actice the cu rre n t is measured at the so-called reference time. This i s the time required for the current to reach 95o/o of its m ax imum value and depe n ds upon the diffusion coefficient of the g as and the thickness of th e membrane . It amounts to ab o u t 2 5 s for a 2 5 j.lm thick m embrane . Amperometric detecto rs whose op erati on is b a s e d on the fo llowing redox reactions have been developed. Oxi d a t ion reac ti ons

CO +

+ 2H20

so!- + 4H + + 2e-

NO + 2 H 2 0

NO� + 4 H ' + 3 e-

N02 + H20

NO� + 2H

S02

2

Reduction

H -----7 2H+ + 2e­ z + C02 + 2 H + 2eHP

+

+

e-

rea ct i on s 03 + 02 +

2H+

+ 2 e- --� 0 2 + H p

4 H + + 4e-

N02 +

H+

+

e

-

2H20

HN02

The con s t r uc t ion and p erforma n ce of a m perometri c gas se nso r s are va ri ab le . More detail s are available in the rev iew articles [ 8 2 , 83) and the re fe re n c es therein. Probably the most im p o rtant gas sensor is the oxygen prob e used to determine the oxygen content in surface, marine and waste waters and b io l ogi cal fluids s u c h as bl o o d [ 84 ) . This electrode is ba s e d on the Clarke cell [ 85 ] , the o p erat i n g pri n ciple s and construction of which is illustrated in Figure 6. 1 7. The cell consists of a Pt cathode and a AgCI co a ted Ag a node immersed in the internal solution of KCI, all of whi ch is se parated from th e test sol u t ion hy the oxygen-permeable membrane. A p o tenti al of 0.6 to 0.9 V is ap p l ied to the el ectr ode s . The oxygen di ffus es th ro u gh the m embrane and inte rnal solution to the cathode where it i s reduced. This con s u m e s some a n alyt e so that in order to m a i n t ai n the oxygen diffus i o n g r a d i e nt th ro u gh the membrane and inte r n al s olu t i on d uri ng the course of t h e measurement, it is necessary to maintain fresh test solution at the membran e surface either by stirring th e solution or by s treaming it a c ros s the membrane. The Clarke cell can be miniaturised [ 86) and in this form is useful for the determ ination of the p art ial pressure of oxygen in arterial blood and other b io lo gic a l systems [ 8 7 ) . In a d d i t i o n to the oxygen micro-sensor, an am p ero m et ri c micro- sensor has been developed for the determ ination of hyd r o gen s ul fi de i n aq u at i c environmental s a mple s [ 8 8 ) . Th e construc­ tion of these sensors and an en l arge m e nt of the sensing tip arc s h o wn in Figure 6. 1 8 . The inner e l ec t rolyt e contains K, [ Fe(CN )6] which oxidises the H2S diffusing i nt o the sensor. W h en a p oten ­ tial of + 85 to + 1 50 mV is appli e d to th e Pt anode the K4 [ Fe(CN)6] re s u l tin g from t h e reaction with H2S is t he n oxidised at t h e anode with the r esul t in g c urrent being proportional to the H2S concentra t ion . The reaction scheme is pr es ented in F i gu re 6 . 1 9. Amperometric de t ectors p l ay an important role in t h o s e systems where the analyte is in a fl owin g solution such as in flow inj ection analysi s and the eluate fr o m l iqu i d c hromatogra phic co l u mn s ( cha p te r 5 ) . In a dd i t i o n to determining the metal ion d i rectly in the flowing soluti o n , the analysis of trace levels of h eav y metals can be carried out by first conve r t i n g the metal ion to an uncharged complex which can then be sep arated from interfering substances by HPLC. Alterna-

1 72

I ntroduction to Voltam metric Analysis

Gas-permeable membrane

Sample

b Electrolyte solution

Anode

Electrode casing

(insulator)

M e mbrane

Figure 6. 1 7

Schematic representation (a) and typical construction (b) of the Clark cell used for the amperometric determi nation of oxygen. Anode reaction: 4Ag + 4CI- -t 4AgCI + 4eCathode reaction: 02 + 2 H20 + 4e- -+ 40W

tively, the metal-ion complex can be e nriched by solid phase extraction and then separated on a reversed phase column. Methods for the detection of the dithiocarbamate complexes of copper, n ickel, cobalt, chromium(Vl ) , chromium ( III), cadm ium, lead and mercury at a glassy carbon electrode have been developed [ 89, 90 ] . Other complexing agents that have been used include 4- {2 -pyridylazo) resorcinol for copper, cobalt, nickel and iron [ 9 1 ] and N,N-diethyl-N' -4hydroxybenzoylthiourea for the determi nation of palladium [92 ] . I f conditions are chosen s o that the complex formation is rapid, then using th e principles of flow injection analysis, on-line complexation can he carried out by injecting the complexing agent into the flowing eluent. The detection can then be either amperometric, voltammetri c, or after accumulation stripping voltammetric. These flow-through applications are becoming more widely used as the in creasing demand for the environmental monitoring of toxic heavy metals can only be met by increasingly automating the analytical methods. A selection of methods for the determination of inorganic species in water samples by voltam­ metric-polarographic techniques is given in Appendix 1 . The examples listed cover a range of measurement techniques, suitable for the different concentration ranges encountered in practice. Emph asis is placed on adsorptive stripping methods because of their importance in determining metal ion concentrations in water samples at the trace and ultra-trace levels required in many environmental studies.

Applications: inorganic species (PI,

Counter electrode

l _r"�

\

r l ��'I I IJ

cathode)

I

I! 1\

\"'\

1\

\11 I

i

(silicone)

Membrane

Figure 6. 1 8

--

G uard electrode (anode, Pt)

G lass

casin g

(pasteur pipette)

"''f/i.!f...::

IyJ----... .... .... il ,,/:'

� �1t: I

1 73

Working electrode (a node, PI)

1---------1

f-]

'"'' ��

wj

�

\i

Membrane

Iif /,

/

Amperometric m icro-sensor [88] .

Methods based on voltammetry or chronopotentiometry are increasingly being appl ied to the determination of metals in a range of medical and biological samples. These analyses are required in studies to determine the physiol ogical function of the metal or, in the case of heavy metals such as cadmium and lead, the toxic effect of the metal on the organism being studied. Methods have been developed for determining metal ions in blood, serum, urine, animal and plant tissue and in pharmaceutical preparations. For higher concentrations DPP may be used, b ut for trace level analyses stripping methods are preferred . A selection of voltammetric and chronopotentiomctric methods of determining metal ions in biological and medical samples is listed in Appendix 2 . Voltam metry a n d chronopotentiometry have been applied t o the determination of metal ions i n samples from a wide range of other sources-agriculture (plant, soils, fertilisers) , foodstuffs, minerals, electroplating baths, alcoholic beverages, metallurgical products, gases, fuels, and high purity chemicals. Exam ples of these applications are given in Appendix 3. The methods developed for trace metal analyses in water samples (Appendix 1 ) can be used without modification for the purity control of salts of the alkali and alkaline earth metals since the salt matrix in these cases does not significantly interfere in the determination [ 93 ] .

2

Silicone membrane

Figure

6. 1 9

[Fe(CN)6] 4 -

2 [Fe(CN)6] 3 -

Reaction scheme of the hydrogen su lfide sensor.

Working electrode

I ntroduction to Volta mmetric Analysis

1 74

REFERENCES I . M . Kolthoff a n d

J,J. L i n ga n e , Polarography, 2nd E d n , Interscience, New York, 1952.

Meites, Polarographic 'Jixlmiques, 2 n d Edn, Interscience, New York, 1 96 1 .

2

L.

3

l'.A.M. Fa ri as , A.K. Ohara, I. Takase and S . L Ferreira, Talanta, 1 993,

4

5

6

L. Meites, Ha ndbook of A n a ly t ical Ch e m is t ry, McGraw- H i l l , New York, 1 963.

40, 1 1 67.

D.T. Sawyer a n d J , L. Rob e r t s, Experi m e n ta l Electrochemistry for Chem ists, Wiley, New Yo rk , 1 974.

T.A. Krjukowas, S . I . Sinjakowa a n d T. M . Arefjewa, Polarograph ische Analyse,

Grundstoffindustrie, Leipzig.

D e u t sc h e r Ve rla g fiir

Pierre, 0. Vi tto r i and M. Porthault, A n alys is, 1 978,

7

G. Henze and R. Neeb, Elektrochem ische Analytik, Springer Verlag, Berlin, 1 9 86.

8

6, 334. Ap p l i cat i o n Note No. V 54. F. E. Beamish and J.C. va n I .oo n , Recent Advances in the A n alyti ca l Ch em is try of the Noble Metals, Pe rga m o n , Oxfo rd , 1 972. J. Z a reb s k i , Chemia Analityczna, 1 977, 22, 1 037. Metrohm Ap p l i c a t i on Bulletin No. 1 1 6 1 2 e ; Metrohm A p p l ica t i o n N ote N o. V 2. G . Henze, A.P. Joshi a n d R . Neeb, Fres e n i us Z. Analytische Chem ie, 1 980, 300, 267. D. T. Meyers and J. Os te r yo u n g, Analytical Chemist ry, 1 973, 45, 267. Y.M . 'lemerk, M . E . Ahmed and M.M. Kam a l , Fresen ius 7.. A naly tische Chemie, 1 980, 30 1 , 4 1 4. B. Takano, M.A. McKibben a n d H . T . . Barnes, Analytical Chemis try, 1 984, 56, 1 594. Metrohm Application Bu llet i n No. 1 99 I 1 d. Metrohm Application Bulletin No. 1 l0 I 1 d . F. Po t t k am p , F. Umland and H . Reimann, Fresenius Z A rt a ly t isch e Chemie, 1 972, 260, 1 8 5 and 261 , 102. A . G . Fogg a n d A . A . Osakwe, Analytical Lette rs, 1 976, 9, 23. J . P. Alimarin, E.N. Dorokhova, L.P. Kazan skii and G . V. Prokhorova, /. Analytical Ch e m istry ( USSR), 1 979, 35,

10

9

11 12 13 14

15 16 17

18 19

20

21

22

J.

Metrohm

1 300.

G.K. B udnikov and N.A. Ulakhovich, Russian Chem ical Reviews, 1 980, 49, 74.

K a i s e r , Fresenius' f . Analytical Ch e m is t ry, 1 994, 349,

23

U. G r e ul a ch , G . I lenze a n d G .

24

G. H e n z e , Unpublished results.

25

j.S. B urm i cz, The Applica tion of VA Tech n i q u es to the Galvanic/Plating Jndustr}> M e t r o h m , Herisau.

27

26

28 30

29

31 32

33

M . G . Pa n cl i and A. Voulgaropoulos, Elect roa n a lys is, 1 993, 5 , 355.

C.M.G.

v an den Berg, Analytica Chi mica Acta,

1 99 1 , 250, 265.

M . L . Te rc i e r a n d J. Buftle, Elec tro ana lysis, 1 993, 5, 1 8 7. M . Es te r ba n and E. Casassas, Tre nds in Analytical Chemistry, 1 994, 1 3, 1 1 0 . M. Kolb, P. R a ch , J, Sc h a fe r and A. Wi l d , Frese11ius ' /. Analytical Chemistry, 1 992, 342, 34 1 . M e tro h m Application Note No. V 35. M e trohm Appl ication Note N o . V 39. G . Henze, Me tal Speciation i n the Environment, S . Giicer and F. Adams, Editors, NAl'O AS! S e ri es G 23, 1 990, H. W. N ii rnb e rg , P. Valen ta, L. M a r l , B. R a s p o r and L. Sipos, Fresen i us

p. 3 9 1 .

34

T. M . Florence and G . E . Batley, C. R. C. Crit ical Rev iews in Analyti ca l

35

T.M . Florence, An a lyst, 1 986,

36

H . W. N iirnberg, Fresen i us Z. A n a ly tisch e Clzemie, 1 983, 3 1 6, 557.

37 38 39 40

41

42 43

44 45 46

47

48

50

49 51

52 53

688.

J . Wan g , P. Tu z h i a n d T.

1 1 1 , 489.

Z Analytische Ch e m i f, 1 976, 282, 357.

Che m is t ry, 1 980, 9, 2 1 9 .

Mart i n ez, Analytica Ch imica Ac ta , 1 987, 20 1 , 43.

No. 2 3 1 / l d. Chevallerie-Haaf a nd G . Henze, Fresenius Z. Analytische Chemie, 1 987, 328, 565. Met r o h m Ap p l i cation Bulletin No. 24 1 / l d . I T . - P. Nimai cr, M . La u e r and G. Henze, Electroa nalysis, 1 998, 5, 326. Me t r o h m Application B u l l e t i n No. 7412d. Me t r oh m Application B u l l et i n No. 9614d. M. Kolb, P. Rach , j . Sch afer a n d A . Wild, Fresenius' /. Analyt ical Chemistry, 1 99 2 , 344, 283. Mctrohm Appl i c a t i on N o t e No. V 36. Metrohm Appl i c a t i o n Note No. V 4 2 . M e t r o h m Application Bulletin No. 226/ l d . U . Greulach a nd G . H e n ze , Analytica Chi m ica Acta, 1 995, 306, 2 1 7. G. Henze, W. Wag n e r and S. San der, Frese 11 i u s ' f. Analytical Ch e m is t ry, 1 997, 358, 7 1 4. G . Henze, P. Monks, G . Tolg, F. Uml a nd , a nd E . WeBling, Fresenius Z. Analytische Ch em i e, 1 979, 295, I . G . Henze, M ikro ch e m ica Acta [ Wie n ] , 1 98 1 , 2, 343. C. Arlt a n d R. N a um an , Fresen ius 7.. Analytische Chem ie, 1 976, 282, 463.

Mctrohm Application Bulletin

A. Meyer, U. de Ia

Applications: i norganic species

54

1 75

R. Kalvoda Imtrumelltation in A n a ly tica l Chemistry, ) . Zyka, Editor, Ellis H o r woo d , New York, 1 994, Vol. 2 ,

W.F. S myth, In o rgan ic A ds o rp ti ve S tripp i ng A n alys is, Mc tro hm M o n o g ra p h, Herisau, 1 99 1 . Ch . 3 .

55

56 57

58 59

C.M.G. van den Berg, Ana lys t, 1 989, l l 4,

1 527.

Z.Q. H u a ng and C.M.G. van den Berg, f. Electroanalytical Ch e m is try, 1 984, 1 77, 2 6 9 . C.M .G. va n d e n B er g, Analytical Proceedings, 1 988, 25, 265. Mctrohm Application Bulletin N o . 1 2 3 / 2 e .

60

C.M.G. va n den Berg an d G.S. jacinto, An a ly tica Chimica Acta, 1 988, 2 1 1 , 1 29.

61

Metrohm A p p l i ca ti o n Bulletin No. 220/ 2d.

62 63 64 65

66

67

68 69

70 71 72 73

74 75

76 77 78 79 80

81 82 83 84

85

87 88

86

R9

90

91 92 93

) . G o l im o ws k i , P. Va l e n ta and H .W. N ii r nb erg , Fresenius /:. Analytische Chemie, 1 98 5 , 322, 3 1 5. N .A. Malakhova, A.V. Chernysheva and Kh.Z. Brainina, Electroanalysis, 1 99 1 , 3, 803. N.A. Malakhova, A.V. Chernysheva and Kh.Z. Bra i n i n a , Electroanalysis, 1 99 1 , 3, 69 1 . S. San de r, W. Wagn e r and G. Henze, Analytica Ch im ica Acta , 1 995, 305, 1 54 . S. Sander and G. Henze, Fresenius' f. Analytical Ch em is t ry, 1 994, 349, 654. W. Wagn e r, S. S an d e r and G. Henze, Fres e nius' f. Analy tica l Chemistry, 1 996, 354, 1 1 . M. Ka ra kap la n , S . G ii<;er and G. Henze, Fresenius' f. Ana ly tica l Chemistry, 1 992, 342, 1 8 6 . F. Heppler, S. Sander and G. I lenze, A nalytica Clzimica Acta, 1 996, 3 1 9 , 1 9 . S. Sander and G . Henze, Fresenius' /. A n a ly tical Chemistry, 1 996, 356, 2 5 9 . C .M . G . van den Berg, K. Murphy and ).P. Riley, A nalyt ica Ch i mica A c ta, 1 986, 1 88, 1 77. J.M. Es tela, C. Tomas, A. Cladera and V. Cerda, Critical Reviews in Analy tica l Ch emistry, 1 99 5 , 25, 9 1 . D. J ag n e r, M. )osefson and S . We sterl u n d , Analytica Chimica Acta, 1 98 1 , 1 28, ! 5 5 . D. J agn e r, M. Joscfson, S. Westerlund and K. Aren, Analytical Chem ist ry, 1 98 1 , 53, 1 406. Metrohm Ton A n alysis No. D4-079- 1 094e. C. Fal l e r, G. H en ze, N . Yu. Stojk o , S.Saraeva a n d Kh.Z. Hrainina, Fresenius' J. Analytica.l Chem is t ry, 1 99 7 , 358 , 6 7 0. S. D o n g and J. Wang, Analytica Ch im ica A cta , 1 9ll8, 2 1 2 , 34 1 . ) . M . To r res Llosa, H . Rut K. Schorb and H . ) . Ache, A n a ly tica Ch imica Acta, 1 98 8 , 2 1 1 , 3 1 7 . ). Wa n g and LD. Hutchins-Kumar, Analy t ical Chem istry, 1 9 8 6 , 58, 402. B. Hoyer and "[M. F l o re n c e , Analytical Che mistry, 1 987, 59, 2 8 3 9 . Z. Cao, W.J. Buttner and ) . R. Stetter, Hlectroanalysis, 1 9 9 2 , 4, 2 5 3 . S.C. Chang, ).R. Stetter and C.S. Cha, "Jala nta, 1 993, 40, 46 1 . M . L. Hitchman, Measuremen t of Dissolved Oxygen, Wiley, New York, 1 978, Ch . 3. L.C. Clarke, J.R. Wolf, D. Granger and Z. Taylor, f. Appl ied Physi ology, 1 953, 6, l ll9. N . P. Revcbech, Limnology and Oceanog raphy, 1 989, 34, 474. M.A. Lessle r and G.P. Br ie rl ey, Methods of Biochemical A n alys is, D.Glieck, Editor, Interscience, 1 969, Vol . 1 7 . P. Jeroschewski, C. S te u ckar t and M . Kiihl, A n aly t ical Ch em is t ry, 1 996, 68, 435 1 . A.M. Bond and G.G. Wa l la c e, Analytical Ch em is t ry, 1 982, 54 , 1 706. A . M . B o n d and G.G. Wal l ace , A n aly tica l Chen1istry, 1 98 4 , 56, 2085. D.A. Roston, A n a lytica l Che m is t ry, 1 9114 , 56, 2 4 1 . M. Lauer, Disserta tion, Technische Un iversitat, Cl austh ai- Zcll crbad, 2000. R. Naumann, W. Schmidt and G. Hiihl, Fresenius' f. A n a ly tical Ch e m is try, 1 99 2 , 343 , 746.

M . Boussemarl, C.M.G. v an den Berg an d M. G h addaf, Analy tica Chimica Acta, 1 992, 262, 1 03 .

7

Appl i cations: organ i c species

7 . 1 POLAROGRAPHIC AND VOLTAMMETRIC M ETHODS The polarographic, voltammetric or chronopotentiometric determination of organic compounds usually depends on the reduction or oxidati on of functi onal groups [ 1 -7 ] . In addition there a re some applications where the method of determination is based on the reaction of the organic compound with the electrode material or on the change in the double layer capacitance associated with the organic species being a dsorbed on the electrode surface. Although aqueous solutions are to be preferred, the low solubility of m any organic and organometallic compounds requires the use of water-organic mixed solvent systems or pure organic solvents for the voltam metric deter­ mination. Methanol, ethanol or acetonitrile are the most commonly used organic solvents. Adsorption processes play an important role in the polarographic or voltammetric determi­ nation of organic compounds. On the positive side, the adsorption of organic and organometallic analytes is the basic process whereby ultra-trace levels of the analytes are accum ulated on the elec­ trode surface prior to their determination by anodic or cathodic stripping vo ltammetry. On the other hand, if surface active compounds are present in the sample they may be strongly adsorbed, displace the analyte from the surface and i nhibit the electrode reaction used for its determination. Organic functional groups which are electroactive and the oxidation or reduction of which is the basis of analytical determinations are listed in Table 7. 1 together with the most important classes of compounds containing these groups. The half-wave or peak potential at which a partic­ ular functional group is oxidised or reduced occurs over a range of potentials because it depends not only on the functional group of interest but also on the presence of other functional groups in the molecule, and on the stereochemistry of the molecule. Correlations have been established between the half-wave or the peak potentials and structural characteristics of molecules within the vari ous classes of organic m olecules [ 8 , 9 ] . However it is in the quantitative rather than the quali­ tative analysis of organic compo unds that voltam metry and polarography are mainly employed. The ranges for the half-wave and peak potentials of selected groups of substances are pre­ sented in Figure 7 . 1 . These potential ranges give an indication as to which compounds may be determi ned voltammetrically and whether the electrode process i nvolved wi l l be a reduction or an oxidation. Compounds with reducible functional groups predomi nate. Polyaromatic hydrocarbons, aromatic hydroxy compounds and amines, amides and various nitrogen heterocyclic com­ pounds may be determined anodically. In most cases, the electro -oxidation or -reduction of an organic compound involves the rem oval or addition of hydrogen to the molecule and so the h alf-wave and peak potentials are pH dependent. For this reason, determinations based on redox processes arc carried out in buffered solutions. The composition and concentration of the buffer systems used generally have little or no influence on the position of the half-wave or peak potentials. If its concentration is sufficiently high , the buffer also perfo rms the function of the supporting electrolyte. 1 76

Applications: organic species

1 77

Electroactive functional groups for the voltammctric d eterminati o n o f orga n i c compounds.

Table 7 . 1

Reducible groups

-------

Compounds

u n satu rated aliphatic hyd roca rbons with co nj u gat ed double or triple bonds, a ll e n e s ar yl substituted ethylenes, pol yaromatics

------- - ·

> C = C< -C

=

C-

,

7C - X

ha l o ge n substituted aliphatic and aromatic

hydrocarbons with the

exception of fluoro compounds

aliphatic and aromatic aldehydes, a ro m at i c ketones, qui nones

> C = O -O-O-

aliphatic an d aromatic peroxides a n d hyd ro pc rox i d es n i t r ate e s te rs

- O - N02 - N02

aliphatic and a romatic nitro compounds

- NO

al i p ha ti c and aromatic nitroso compounds

- NH - OH

hydroxyIa mines

-N= N-

azo com p oun ds

- NH - NH -

hyd raz o co mp ou n d s

> C = N-

benzodiazepines, pyri di n e s , qu inolines, acridincs, pyri m idi n es, triazines, oximes, sernicarbazones

- NCO

isocyan atcs

-S-S-

disulfidcs

>C=S

th iobenzophcnoncs

- S0 -

diary! and alkylaryl sulfoxides

- 50 2 -

sulfones

- S0 2 NH -

sulfonamides

7 C - Me

Oxidisable groups

organometallic compounds

- OH

phenols

- NH 2

aromatic arnines

- CO - N <

ami des

- SH

thiols

- N - CS2-

dithiocarbam ates

Groups reacting with mercury ions

- N H - CS - NH -

thioureas, thiocarba mates

- CS - NH -

thioamides

-

NH

-

CO

-

- NH - NH2

NH

-

derivatives

of barbituric

acid and of uracil

hydrazine derivatives

When the analyte is present in a complex m atrix contai n ing compounds which may interfere with the voltammetric or polarogrtaphic an a l ysis it will be necessary to s e p a r a t e the analyte from the matrix by HPLC, solid phase extraction or solvent extraction, or extract the interferents from the sample. When the analyte has been separated from th e matrix by solvent extra ct i on the sol­ vent is e i ther ev apora t ed and the residue dissolved in the supporting electrolyte, or the analyte back-extracted into an aqueous solution in order to minimise any i n t e rferen ce arisin g fro m the organic sol vent in the subsequent voltammetric measurement. An a dva n tage of using an extrac­ tion step is that in many cases the analyte is concentrated i n this step and so t h e sensitivity of the determination is markedly increased. ,

1 78

I ntroduction to Voltam metric Analysis

unsaturated aliphatic and polyaromatic hydrocarbons aromatic and polyaromatic hyd rocarbons aliphatic and aromatic halogen compounds ali phatic and aromatic aldehydes ketones acids peroxides qui nones phenols n itro-compounds nitroso- and azo-com pounds aromatic ami nes am ides N-heterocyclics sulfur compounds thiols and phenothiazines

+ 3.0 F i g u re 7 . 1

+ 2.0

1----1

+ 1 .0

0

- 1 .0

- 2 .0

- 3 .0

E [V] (vs. SCE)

Potential ranges for the polarographic h a l f-wa ve and volta m m etric pea k potentials of organic compounds.

Sa tura ted hydrocarbons and t h os e with isolated do ub l e and t r ip l e bonds are not polarographi­ cally reducible. Unsa tura ted hydrocarbons with conj u g ate d double or triple b o n d s or c u m u lat ive double bonds, for example allene, arc reduced at very negative potentials (-2.0 V or more ) . The reduction potential becomes more p ositive on increasing the number of double bonds and aro­ matic rings in the molecule [ I 0 ] . The reduction of hydrocarbons usually proc ee d s by one of the followin g mechanisms: ( 7. l a) R ... + H +

___.

RH"

(7. l b )

Appl ications: organic species

1 79

t he n eith e r

( 7. l c) ( 7. I d)

or (7 le) .

with one of th e electron exc hanges b ei ng the fas t e st step . D i m e ri sat i o n of the radical ( equation 7. I e) is more likely in a p ro ti c media or in a queo u s media at low proton concentrations ( h i gh p H ) o r i n those cases where the radical is relatively stable. Because polyaromatic h yd r oc a rb o ns are dif­ ficult to red u c e (E 1 12 - -2 V t o -2 . 7 V i n 75% dioxan using TEAl as supporting electrolyte) t h ey a r e n o r m a lly d ete r mine d a n o dically using a p l a t inu m electrode. Double bonds conj ugated with a ca rbonyl g r o u p or in a heterocyclic ring (such as pyr idi n e or q u i no l i n e ) are polarographically active. The 3-keto steroids cortisone and testosterone

0

0 HO

Testosterone ( I I)

Cortisone ( I )

can be d ete r m in ed b y the reduction of t hei r C4-C5 do uble bonds in 50% methanolic solution with TEACI04 as s u p p orti n g elec t rolyte Peaks are o b ta i n ed in the differential pulse p olarograms at -1 .77 V (I) a n d at 1 84 V ( I I ) (versus SCE) f 1 1 ] . Li kewise the determination of coumarin is based on the reduction of the double bond conj ugated wi t h the c a rb o n yl group f 12 1 . I n this c as e the p ro du ct is n ot dihydrocoumarin but th e d im e r ( equation 7 . l e ) . .

-

.

roo Coumarin

Except for the fluorides, halogen substituted hydrocarbons are polarographically reduced according to the scheme: ( 7 . 2a )

( 7 . 2 b)

then or

R" +

R"

___.

R2

( 7.2c)

Wh i c h p a t hway the e l ec t ro d e reduction o f a particular organic halide follows is d ete r m i n e d by the relative rate s of the up t a ke of the second electron ( 7.2b) and the dimerisation ( 7.2c) [ 1 3 ] .

1 80

I ntroduction to Voltammetric Analysis

For a n y given R g r o u p either aromatic or a l iph at i c the half-wave or peak potentials bec o m e more posit ive i n th e o rde r R-1 > R-B r > R-C l . For examp le , in dimethylformamide w i t h 0.05 M TEABr as s up p o rt i n g ele c t rol yte the half-wave potentials for iodobenzene, bromobenzene and c h lorob e n zen e are - 1 .64 V, -2 .24 V and -2. 5 5 V (versu s SCE ) , r esp ect ive ly In m o st cases, multi­ halogen com p o u n d s will gi ve mul ti-wave polarograms with the fi rs t wave m ovi n g to more posi­ t ive potential s as the number o f h a l o ge n substituents in c reas e s For e xa m p l e the reduction of CC14 in 75% dioxan-water c o n t ai n i n g 0.05 M TEABr g ive s two waves. The second ( more nega­ tive) wave is identical wi th that obtained for th e reduction of CHC13 un de r the same conditions. The reduction potentials also d ep e n d upon the steric arrangement of t he halogen at oms in iso­ meric c o m p o u nd s such as the hexac h l orocyclohexanes whe re the y- isomer, which is the most e ffect iv e i nsecticide, is reduced at the most pos i ti v e p o te n ti al [ 1 4 ] . The pol arographic properties of t h e haloge n s ub s tit uted carb oxyl ic acids also depend on the number and nature of the substituents. Monochloroacetic acid is polarographical ly inactive; di c h l o­ ro a ce t i c a c i d give s a pH-dependent wave in b as i c solutions, while trichloroacetic acid gi v es a s in gle wave at pH < 3. 1 a n d two waves between pH 7.7 and pH 1 1 .3. The thyro i d hormone t hyroxine (3,5,3',5' tetraiodot hyrosine p he nol ether) i n 40% ethanol co n tai n i ng Na2C03 (0.5 M) and TEAT ( 1 %) give s three waves with E1 1 2 values of - 1 .20 V, - 1 .42 V and - 1 .70 V ( ve rs us SCE) [ 1 5 ] . The first step i n the p o l a ro grap h i c reduction o f ali p h a t ic aldehydes i n alkal ine s ol u ti o n i s a c hem i c a l step, namely their dehyd r a t i on to the reducible unhydrated fo r m It depends on the rate of this step as to whether or not the polarographic wave is a kinetic wave. Either an alcohol (7.3b) or a glycol ( 7.3c) is produced according to the equations: ,

,

.

.

,

-

.

D eh ydrat io n R - HC( O H ) 2 � R - HC

=

{7.3a)

O + H20

Re d u c t i on

( 7 . 3b)

or

2 R - HC

=

0 + 2H+ + 2e-

___.

2R - He " - OH

___.

R - HC(O H ) - HC(OH) - R

(7.3c)

Sat u rat e d al d e hyd e s g i v e a s in gl e wave the height and potential of which is pH d e pe n den t Aliph at ic d ialdeh yde s such as glyox al a nd glu t a r a l de hyde, and monosaccharides ( a l do s e s) can be determined pola rographically. Monosaccharides g i ve kinetic currents b e c a us e the c l ect r o i n ac t ivc cyclo hem i - acetal for m must first be c o n v e rt e d into the electroactive a ld ehyd e form and this step is r at e determining. T ra c e s of m a n n ose i n m a nni to l (used in d r u g production ) , traces of acetalde­ hyd e in ethanol and t r a c e s o f fo r m al d eh yde (> I mg kg- 1 ) in s ta r c he s can be re ad i ly determined by differential p ulse p o la ro graphy [ 1 6 ] . .

,

Application 1 6

Polarographic determination o f formaldehyde [ 1 7 ]

Sample: Supporti ng e l e ct ro l yt e: Form a l d e h yd e sta ndard :

Wa ste water. Ga l va n i c b a th s pla st ics textiles, a i r, fi lm materi a l , f i s h . Dissolve N a O H (8 g) a n d Na2E DTA. 2 H 20 ( 7 . 44 g) i n water and make u p to 1 L. Vol u metrica l ly a na lyse concentrated (ca . 3 7 % ) for ma l d e h yd e sol uti o n and d i l ute to g ive a s t a n d a rd sol ution c o n t a i n i n g 1 g L- 1 formaldehyde . The solution has l i m i t ed sta b i l ity and m u st be made up d ai l y 1 Wast e waters a n d galvanic baths can be d i rectly a n a l ysed . 2 P l a st i c s and texti les. Red uce to sma l l fragments a n d extract by shaking with 0 . 0 5 M N aO H . Fi lter the extract. ,

,

.

Pre-treatment:

Applications: organic species

1 81

Acetone - 1 . 35 v

I50nA - 1 . 00

- 1 . 25 E

Figure 7 . 2

- 1 . 50

[V] (vs. Ag/AgC I , 3 M KCI)

DP-pol arograp h ic determ ination o f forma ldehyde, acetaldehyde and acetone i n a s a m p l e of methanol .

3 Air s a m ple s Forma ldehyde is a bsorbed by pass i n g the air t h ro u gh 0 . 0 5 M NaOH. 4 S o l i d s a m p l e s . Blend 1 -5 g of sa mple i n 20 m l water, add 1 m l 3 0 % H2S04 and ste a m disti l . C o l l ect t h e forma ldehyde i n 0.05 M NaO H . Deaerate 1 0 m l o f s u p po rt i n g electrolyte with n i t ro g e n a n d a d d 1 0 m l of sample sol utio n . After further dea erati n g , record the pu lse polarogram betwee n - 1 .4 V a n d - 1 . 8 V. The pea k potentia l of formalde hyde occurs at -1 . 6 5 V (vers u s Ag/Ag C I , 3 M KC I ) . Determ i n e t he concentration by standard addition, de a e ra t i n g with n i t ro g e n after each addition of the sta n d a rd solution . Because of the toxicity of formaldehyde and its suspected ca rci nogen i c proper­ 3 t i es, a maxi m u m workpl ace conce ntrat i o n of 0.6 m g m- has been set . DPP can measure forma l dehyde i n the sub-�g L-1 range and so is well su ited for deter­ m i n i n g the levels of forma l de hyde enco untered in envi ron menta l sa m p l e s such as r oo m a i r, b u i l d ing materi a l s, t ob a c co smoke etc. The formaldehyde, aceta lde hyde and a ceto ne content of m e t h a n o l ca n be determ i ned di rectly using a citrate- phosph ate buffer (p H 6. 5) conta i n i ng hydra­ zi ne s u l p hate . The DP-pola rogram, reco rded between -0 . 8 5 V and - 1 .60 V (versus Ag/AgC I , 3 M K C I ) i s shown in Fi g u r e 7 . 2 . The c u rren t peaks a re at - 1 . 0 5 V ( H 2CO), -1 . 2 V ( C H 3 C H O) and - 1 . 3 5 V (CH3hCO. The determi nation l i m its a re i n the mg L-1 ra n g e . .

Analysis:

Notes:

In contrast to al ip h ati c aldehydes, wh ich can only be red uc ed in n e u t ral or alkaline media, aro­ matic aldehydes are polarographically active over t h e whole pH range. The pH of the supporting electrolyte d et e rmi n es whether one- or two-wave p o l a r o g ra m s are obtained. The half-wave potentials for a s e l e c t i o n of aldehydes, sugars and ketones are listed in Table 7.2.

1 82

I ntroduction to Voltammetric Analysis

Table 7 . 2

Polarographic half-wave potentials of selected aldehydes, ketones and sugars [ 1 4] .

-------

Compound

Supporting electrolyte

Formaldehyde

Buffer solution,

Aldehydes

Ellz( V)

-

pH 10.7

pH 8.0

pH 1 2.7 Acetaldehyde

0. 1

M

.

1 4 6 ( vs SCE ) - 1 .59 -1.7 1 - 1 . 89

LiOH

Propionaldehyde

0. 1 M LiOH

Acrolein

Buffer solution, pH 8.7- 1 1 .0

(1) (2)

- 1 .04 - 1 .44

Crotonaldehyde

0.2 M TMAOH in 50% ethanol

(1) (2)

- 1 .37

- 1 .92

Benzaldehyde

0. 1 M LiOH

Phthaldialdehyde

Acetate buffer, pH 5 w i t h 1 .5% ethanol

Nitrobenzaldehyde

-1.51

0.092 M H3P04 + 0. 1 96

M TMAOH

in

ethanol-dioxan

Hexahydrobenzaldehyde

- 1 .80

(1)

-0.72

(2)

- 1 .09

(l) (2 ) (3)

-0 .8 ( vs Ag/AgCl )

-1 . 1

- 1 .7

- 1 .9

0.25 1\rl LiOI I

Ketones

NH,-(N H .1 )2S04 buffer

Acetone

2.5 M

Cyclohexanone

0.05 M TBAOH in 50% isopropanol

Acetophenone Benzophenone

1 M NHrNH4Cl buffer with 1% ethan ol

Buffer (pH

7) plus 0 1 M KCl in 25% ethanol .

- 1 .62 ( vs Ag/AgC]) - 1 .65

-!.15 - 1 .35

Anthrone

Buffer solution, pi I 7 i n 40o/o dioxan

Benzylacetone

0.1

a- lonone

0 . 1 M LiCl in 50% ethanol

- 1 .60

Flavone

0.05 M TBAOH/CH J COOH in isopropanol

-1 .35

Alloxan

Phosphate buffer, pH 7

Bem:oin

BR buffer - pH

Sugars

Phosphate buffer - pH 7

Allose

M N H 4Cl

-1.41

-1 .06

- 1 .02

l .3

pH 7.0 pH 1 1 .6

-0.90 ( vs SCE) - 1 .39 - 1 .53

- 1 . 74 (vs SCE)

Arabinose

Phosphate buller - pH 7

- 1 .54

Fru ctose

0 . 1 M LiCI

- 1 . 76

Galactose

Phosphate buffer - pH 7

Glucose

Phosphate buffer - pH

7

-1 .55 -

1 . 54

Lyxose

Phosphate buffer - pH 7

- 1 .50

Maltose

0 . 3 M KCL I KOH

- 1 .60

Mannose

Phosphate buffer - pH 7

-1.51

Ri bose

Ph osphate buffer - pH 7

Sorbose Xylose

0 . 1 M I .i Cl

Phosphate bu ffer - pH 7

-

1 . 77

-1 .76

- 1 .50

Applications: organic species

1 83

Fumaric acid - 1 --. 1 .5 mg L

Maleic acid

- 1 .0

- 1 .2

1 . 5 mg L-1 __.

-

1 .4

- 1 .8

- 1 .6

E [V] (vs. SCE) Figure 7 . 3

DP-polarogram of 1 . 5 mg L- 1 mal ei c acid and 1 . 5 mg L-1 fumaric acid in a NaH2 P04-NH4C I/NH3 buffer (pH 8.2) [14) . Peak potentials (versus SCE) : ma leic acid -1 .35 V; fumaric acid -1 .60 V.

Keto nes, like ald ehyd es , are also pol arographically active. Satu rated aliphatic keton es are more diffi c ult to reduce than the unsaturated ketones. The reduction of the unsaturated ketones pro­ duces either a d ike ton e or a s at urat e d ketone which m ay then be reduced further at a more nega­ tive po tential . Th e d iketon e s yie ld enediols in a two - el ect ron reaction. The reduction of aromatic ketones is gen e rally complicated. The n umber a n d p ote n t i al of th e waves are strongly influenced by the composition and pH of the supporting e l ect rolyt e . In acid solu tion the first wave ( E 1 2 > 1 - 1 .0 V ) is d ue to a on e - ele c t ro n reduction to the free radical which then dimerises to give the cor­ respo nding pinacol. At more n egative potentials a second wave occurs which arises from the reduction t o the corresponding carbinol. At h igh e r pH the waves merge into each other. The car­ bonyl fu nc t ion in steroids is reduced to a carbinol and gives a po l a ro g r aphi c wave in alkaline solu­ tion at ca. -2.0 V [ 1 8 ] . M a ny ph arm aceuti ca l p r o duc ts contain electroactive arom atic ketone gro ups, the reduction of which forms the basis o f their voltammetric determination [ 1 9 ] . The carboxyl pa rt of the carboxylic acid gro u p is, in general, n o t polarograph ically active. Polarographic wa ve s observed b etwee n - 1 .5 V and -2 .0 V ( v e rsu s SCE) are usually due to the reduction of the di ssoc i a ted p roto n. Ca r b o xylic acids with reducible functional gro u p s in the mol e c u le give ris e to two -wave p ol a rogr ams ( see Table 7.3) . In t he case of unsaturated carboxylic a ci ds , the double bond is reduced to give the corre sp o n di n g saturated acid. For exampl e , maleic acid ( cis-form) and fumaric ac i d ( trans-form are redu c ed to succinic acid, a t potentials which are pH and structure dependent. At pH 1 , the DP -polarographic peaks of both acids occur at EP -0.6 V ve rsus SCE, however in alkaline s o l ut i o n at pH 8.2 ( s u p port in g elect rolyte: 0.2 M NaH2P04 + 1 M N H4Cl + NH1) the signals are well separated with EP 1 3 5 V for maleic acid and EP - 1 .60 V for fumaric acid ( Fi g u re 7.3) [ 1 4 ] . T hi s behaviour forms the basis of the p o l ar o ­ grap h i c determination of fumaric acid in foods a n d c o n di m e nt s , in plasti cs as well as in fats and oi ls which may contain m ale i c acid as a preservative. =

=

=

HO - CH, -

OH

&

�

HO

-

.

o

OH

As co rbi c a c id ( vit a m in C) c a n be determined thr o u gh the oxidation of the enediol. In a Britton-Robinson buffer of pH 2.9, th e D P - polarographic p e ak poten ti a l fo r t h e oxidation o cc ur s

1 84

I ntroduction to Voltam metric Analysis

Table 7.3 -----

Polarographic half-wave potentials of sel ected organic acids [ 1 4 ] .

-----

-----

-----

-----

Compound

Supporting electrolyte

Acetic acid

0.2 M LiCl

Salicylic acid

0.05 M TEAl -----

----- -

Acetylsalicylic acid

0. 1 M Li Cl

Tartaric acid

0.05 M TMAHr

Oxalic acid

-----

----- ·

-----

----- · · -----

-----

-2.06

----- -

- 1 . 72

Ph o sphate buffer - pH 7 in 1 0% ethanol

( 1)

Phosphate buffer - pH 7 in 1 0% ethanol

(2)

- 1 .50

Ba(CH1C00 ) 2

--

-----

Acetate buffer - pH 4.64

0.05 M TEAl

-----

-----

----- ·

- 1 .65

0.1 M KCl

2 - Nitrobenzoic acid

Sulfanilic acid

-----

- 1 .89

Biphthalate b uffer - pH 4 with 0. 1 M

Qui naldinic acid

- --- .

- 1 . 70

Phthalic acid

3 - N itrobem:oic acid

---

E112(V) (vs Ag/AgCI)

-----

(1) (2) (1) (2)

-----

-----

-O.HO -1.10

-0.70

-1.10

-0.90 -1 . 1 0 - 1 . 5 !! (vs NCE)

at +0. 1 4 V (versus SCE) using either a d ro pp i n g mercury el ectrode or a carbon paste electrode as working electrode. As co rb ic acid can be determined in fruit and vegetables and in vitamin prepa­ rations with a determination limit in the mg L- 1 range [ 20, 2 1 ] .

Application 1 7

Polarographic determination of ascorbic acid (vitamin C) [22]

Sample: Pre-treatment:

Solutions: Standard : Determ i n ation :

N otes :

Vita m i n prepa rations, drin ks, fruit j u ice, foodstuff, etc. 1 L i q u i ds, fruit and vegetable j u ices. Fi lter if n ecessary. 2 Solid foodstuff (fru it, vegeta bles, etc) . Cut i nto small pieces, homogen ise with oxalic acid sol ution and filter. 3 Other vita m i n conta i n i n g foods or preparation soluble i n oxa lic acid. If neces­ sary add trich l o roacetic acid to p reci pitate prote i n and filter. Acetate buffer p H 4.64. Oxa l ic acid 1 g L- 1 • Dissolve 5 0 m g ascorbic acid i n oxa lic acid and make u p to 1 00 m l with the oxal i c acid soluti o n . Prepare a fresh sta ndard each day. Place 1 5-20 ml of acetate buffer i n the polarographic cel l , deaerate with nitro­ gen a n d add 1 ml of sample. After further deaerati ng record the a nodic DP­ polarogra m using a D M E o r DP-volta m mogram using a Pt working electrode from -0 . 2 V to + 0 . 2 V. The ascorbic acid peak occurs at ca . + 0 . 0 8 V. Determine the concentration using sta ndard addition s. By using various polarographic methods and su pporting electrolytes it is possi­ ble to determine s i m ulta n eously, a n u mber of electrochemically active vitamins i n a m u lti-vita m i n p reparatio n . An example is s hown in Figure 7.4. Using differ­ ential pulse polarography and an acetate b uffe r of p H 4.4, vita m i n s B2 and C ca n be determi ned together (Fig u re 7 .4a ) . Record i n g the a . c . polarogram of the same solution (Figure 7 . 4b) e n a bles the concentrations of vita m i ns K3 and 82 and fol i c acid (fP -0 . 58 5 V) to be determ ined. Addition of NaOH to the same sol ution and a ga i n recordi n g the DP-po l a rogram a l l ows n icoti nam ide ( fP - 1 . 7 6 5 V) to be determi ned (Figure 7 .4c). Amount of vita m i n s per ta blet d eterm i ned polarographica l ly - vita m i n B2 3 mg; vita m i n C 50 mg; folic acid 0 . 5 mg; vita m i n K3 0 . 5 mg and n icoti­ n a m i de 2 0 mg. ==

==

Resu lts:

==

=

==

==

=

Appl ications: organic species

-

Vitamin C

a

+0.065 v

�

1 . 32 v

�

Vitamin 8 2

- 0 .285 v

�

- 0.2

Vitamin �Vit K,

1

+

0.2

0

- 0.6

E

82

�

- 0 .045 v

�

- 1 .4

[V) (vs. SCE)

b

- 0 . 285 v

+0. 1 0 v

- 1 .0

Folic acid

- 0 . 585 v

�

+

0.2

0

- 0 .2

- 0.6

- 1 .0

Ni cotinamide

Vitami n C - 0.28 v

- 0 .2

Fig ure 7.4

- 1 . 765 v

c

�

- 0.6

- 1 .0

- 1 .4

E [V] (vs. SC E )

�

- 1 .4

- 1 .8

E [V] (vs. SC E )

Polarographic analysis of a m ulti-vita m i n preparation [ 1 4] .

1 85

1 86

I ntroduction to Voltam metric Analysis

Organic acids such as nitrilotriacetic acid (NTA) and other p o lya min oac ids such as ethylenedi­ a m i netetraacetic acid ( EDTA) wh ich form s ta b l e chelates with heavy metal io n s may be determined b y indirect polarographic methods. Because of t he i r ability to c o m p lex with metal ions th es e co mpo un d s are u s ed as water softening age n ts . Along with p h o sph at e they are often p r ese nt in detergents a n d cleansers and so may be present in wa s te waters. Their determination is b a s e d on th e formation of stable h eavy metal complexes which have polarographic p ro p er t ies dif­ ferent to those of the a n a lyte itself. Bismuth salts are commonly used a n d are added in a small excess to the s a mp l e [ 23 ] . For the a n a lysi s , use is made of the cur r e n t s i gn a l s that arc caused either by the excess metal ions or by t h e n ewly formed m etal ion complex. In ac c ord a nc e with DIN 384 1 3, Pa r t 5, both NTA a n d EDTA arc d ete rm i n e d after the addition of bismuth (I II) nitrate to the sam p le acidified to pH 2 wi th nitric acid as detailed below. Application 18 Polarographic determination of nitrilotriacetic acid (NTA) and ethylenediaminetetraacetic acid ( EDTA) [DIN 38 4 1 3, part 5] S a m ple : Waste water, surface water. 1 Add 1 m l 69% H N 03 to 1 L of sa mple, adjust to pH 2 a n d f i l te r th ro ug h a 0.45 �m Pre-treatment: m e m bra ne filter. The sample may now be stored for up to one week at 4°C. 2 To rem ov e su rfacta nts and other i n t e rfe r i n g o rg a n ic i m p u riti e s , add 10 g KN03 to 1 00 m l of acid ified s am p l e , pass i t throug h a n ad so rpt i o n col u m n fil led with XAD 2, pre -co nd i t io n e d with methanol and d i scarde the fi rst 20 ml. Pass the re ma i n i n g 80 ml t h ro u g h a c a t i o n e x c ha ng e col u m n with t he res i n in the Na form . Di sca rd the fi rst 20 m l. The rem a i n ing 60 ml is rea dy for t h e polaro­ gr a p h i c dete r m i n a ti o n . 1 B i - NTA ( "" 1 g H3 NTA L-1 ) . (a) Di ssolve 4. 5 g B i ( O H h N 0 3 i n 3 0 ml 69% H N 03 Stock sol utions: and d i l ute to 400 ml w i th d i sti l led water. (b) D i s sol v e 1 g N T A in 20 m l 2 M NaOH a n d d i l ute to 400 m l with d isti l l ed water. M i x a w it h b and make up t o 1 L. Th e solution should have pH 0. 7 a n d is stable for 4 weeks . 2 Bi-EDTA ("" 1 g H 4 E D TA L-1 ). (a) Dissolve 3 . 1 g Bi(OHhN03 in 30 ml 69% H N 03 and d i l ute to 400 ml with d isti l l ed water. (b) D issolve 1 . 274 g N a 2 E DTA. 2 H 20 i n 20 m l 2 M N a O H a n d d i l ute to 400 m l with p u re water. Mix a wi t h b a n d m a ke up to 1 L. The so l uti o n is sta ble for 4 weeks. 1 B i ( 2 g L-1 ). D i ssolve 2.8 g Bi(OH)2N03 i n 2 5 m l 69% H N 03 a n d d i l ute to 1 L Sta n d a rd s : w i t h p u re wate r. 2 Bi- NTA ( "" 1 00 mg H 3 N TA L- 1 ). Mix 10 m l B i - N T A stock solution with 50 ml pure water a n d 1 5 ml 2 M H N 03 i n a 1 00 m l vol u metric f l ask and make u p to the mark. The sol ution is stable for a bo ut 1 week . 3 B i - E DTA ( "" 1 00 mg H 4 E DTA L-1). M i x 1 0 m l B i - E DTA sto ck solution with 50 m l p u re water and 1 5 ml 2 M H N03 i n a 1 00 ml volumetric flask and m a ke u p to the mark. The solution is sta ble for about 1 week . Determi nation: P lace 20 m l of pre-treated s a m ple i n the pola ro g ra p h i c c e l l , a dj ust the pH to 2, add 800 mg asco rb i c acid, deaerate with n itrogen fo r 1 0 m i n a n d record the DP- p o la r ogram between +0 . 1 V a n d -0.6 V. Th i s g ives the base cu rre nt w h i ch should h ave n o current pea ks, o t h e rw i se the s am p l e pre-treatment is i n ade­ q u ate and s h o ul d be c h e c ke d . Add 5 0 � L B i standard solution, deaerate a n d record the D P-po l a rogra m . N ote t h a t th e h e i g ht of the p u re Bi p ea k m u st be at l east twice that of either of the peaks due to the B i - c omple xe s (see F ig u r e 7. 5). If the B i peak is less than this, the B i concentration i n the sample solution m ust be increased and t h e D P - p o l arog ra m re-recorded . =

Appl i cations: orga n i c species

1 87

Bi3+

i [nA]

(E

700

p

= -

0 . 05

V)

4

600

500

B i - E DTA

Bi - NTA (E p

400

=

-

0 .25

( Ep

V)

=

-

0.48

V)

4

4

300

200

1 00

+0.05 0

Figure 7 . 5

- 0.4

- 0.2

E

- 0.6

[V] (vs . Ag/AgC I , 3 M KC I)

Polarographic determination o f NTA and E DTA: (1 ) residual current; (2) after additi o n of Bi 3 + sta ndard; (3) after first addition of standard B i -NTAIE DTA solution; (4) after second addition of sta ndard Bi-NTA/EDTA solution.

H 3 N T A a n d H 4 E DTA were eva l uated b y sta n d a rd additions ( 2 ) of the respective sta n d a rd B i - NTA and B i - E DTA s o l u t i o n s as i l l ustrated in F i g ure 7 . 6 .

Eva l uati o n :

T h e concentration o f

I n o rder t o c h eck t h a t the s a m p l e p re-tre a t m e n t d oes not i ntrod u ce erro rs i nto

Note s :

the a n a lysis, it is necessary to determ i n e the recovery rate of the p roced u re . T h i s is d o n e by s u bject i n g 1 00 m l a l i q u ots o f e a c h of the

N TA

and

E DTA

sta n d a rd

solutions to the s a m e p roced u re as t h e s a m p l e ( a dsorpt i o n col u m n , cati o n

exc h a n ge, etc . ) . The recove ry rate, R, m ust be > 90% a n d i s co n s i dered i n the eva l u at i o n of the resu l ts. R i s given by:

R (% ) = A X 1 00/8

w h e re A i s the m e a s u red concentrati o n and 8 i s the added concentrati o n of

H3NTA o r H 4EDTA .

The va l ue of R i s used to co rrect the m e a s u red concentrati o n of each a n a lyte i n the s a m p l e a s fo l l ows: Cfound X 1 00/ R cm rrected =

Peroxides ( R - 0 - 0 - R) and hydroperoxides ( R - 0 - O H ) arc polarographically reduced in a two-electron step as follows: R - O - O - R' + 2H ' + 2e(R

=

H, alkyl or

•

R - OH + R ' - OH

a cy

l ( R - CO -) ) .

Hal f-wave potentials for selected peroxides a n d h yd ro pero xi d es are listed i n Table 7.4. I n a homologous s e r i e s of acyl and al kyl hydroperoxides and of dialkyl peroxides the half-wave

1 88

I ntroduction to Voltammetric Analysis

i

[nA] 400

300 1 . Addition

7

6

5

8

9

10

Concentration NTA or EDTA

F i gure 7 . 6

Table 7.4

Concentration of N TA or E DTA in the sample in mg L- 1

added to the solution in

mg L-1

Evaluati on o f polarographic data by t h e method o f standard additions.

Pol aro g ra p h i c half-wave potentials of peroxides and hydropcroxidcs [ 1 4 ] .

E112(V vs SCE)

Co mpound

Supporting electrolyte

Methylhydroperoxide

0. 1 M LiCl

-0.75

Ethylhydroperoxide

0. 1 M LiCl

-0. 2 5

n-Hexy lh ydro p eroxi d e

0.3 M

n - Octylhydroperoxide

0.3 M LiCl

(I:I )

-0.02

t - Butylhydroperoxide

0.3 M LiCl in benzene-methanol ( 1 : 1 )

- 1 .00

n - A mylhydroperoxidc

LiCI in benzene-methanol ( 1 : 1 )

0 . 3 M LiCl i n benzene-m ethanol ( 1 : 1 )

in benzene-methanol

-0.20 -0. 1 2

-0. 7 1

Dimethylp eroxide

0 . 1 M LiCl

Diethylperoxide

0.1 M LiCl 0.3 M LiCl in benzene-methanol ( 1 : 1 )

0.00

D i l a u roylperoxidc

0 . 3 M LiCl in b e n z en e-m eth a n ol ( 1 : 1 )

-0.09

D ib en zoyl p e roxi de -- · - -

-0.25

p o t e nti a l s become more positive with increasi ng molecular weight. Because of the range of h al f­ wave pot e nt i a l s encountered it is possible to polarogroaphically determine the components i n a mixture of peroxides. for i nsta n ce , in a mixture of hydrogen peroxide, methyl hydroperoxide, t-butylhydroperoxide, acetylhydroperoxidc and diethylperoxide, each component can be deter­ mined polarographically in one run [ 24 ] . D e pendin g o n the pH, 1 , 4 - benzoq uinone i s r e duce d a t the m ercury electrode i n aqueous solu­ tions between +0. 1 and -0.8 V ( ve r sus SCE ) to give a single two-electron wave. The reduction

Applications : organic species

0.0

Figure

7.7

- 0.2

1 89

- 0.4

E [V] (vs. SCE)

Pola rog ra ms of 5 x 1 0-4 M vita m i n K1 : (a) d .c. pola rogram; (b) a . c . pola rogram; and (c) DP pola rog ram recorded at pH 7 using a D M E [2 5 ) .

actually proceeds via two one-electron steps through the intermediate sem iquinone radical (which is unstable in aqueous media) as follows: o

=C)=

o

+ e +

H

Quinone

+

._

�

0

=0--

0H + e + H

+

_...

HO

-o-

OH

Hydro q u inone

( S e m i q u i no n e )

In aprotic media such as acetonitrile o r dimethylformam i de, in which the semiquinone radi­ cal intermediate is stabilised, 1 ,4-benzoquinonc gives two one- electron waves. The half-wave potential for the reduction of qui n ones is influenced by the nature, position and number of sub­ stituents as can be seen from the half-wave potentials listed in Table 7.5. Two important quinones which have been studied polarographically are vitamin K 1 ( T ) (2-methyl-3-phytyl- 1 ,4-naphtho­ quinone) and vitamin K3 (II) (2-methyl- 1 ,4-naphthoquinone) . The d.c.- , a.c.- and pulse-polaro­ grams of vitamin K3 are shown i n Figure 7.7. Using pulse polarography, the determination limit for vitamin K1 was found to be 0 . 1 1 mg L -I an d that for vitamin K3 to be 0 . 1 7 mg L- I [ 25 ] . 0

0

H 0

(I)

(II)

1 90

Table

Introduction to Voltammetric Analysis

7.5

Polarographic half-wave potentials of selected quinones [ 26 ) .

--- ---

Compound

--- ---

---

---

---

---

---

--- - - --- · --- --- · - --- - - --

Supporting electrolyte

---

---

E112(V vs

O . l M TEAP in acetonitrile

-0. 3 1

1 ,4- Benzoquinone

0. 1

-0.5 1

M TEAP in acetonitrile

2,6-Dimethyl - 1 ,4-benzoq uinonc

-0.66

2,6- Di- isopropyl- ! ,4 -benzoquinone

-0 .70

2,6-Di -t-butyl- 1 ,4-benzoquinone

-0.74

2,6-Diphenyl- l ,4-benzoquinone

-0.34

2,3,5,6- Tetramethyl- 1 ,4-benzoquinone

-0.84

2 - Chloro- l ,4-benzoquinone

-0 .34

2 , 5 - Dichloro- 1 ,4-benzoquinone

-0. 1 8

2,3- Dichloro - 5 ,6- dicyano- l ,4-benzoquinone

I ,4-Naphthoquinone

-0. 5 1 Phosphate buffer - p H 3

+0.08

pH S

-0.07

pH 7

-0. 1 7

9 , 1 0 - Ant hr a qu i n o n e

0.1 M

9, I 0- Phenanthrcnequinone

0.2 M Me1NOH

9 , 1 0 - Naphthacenequinone

---

---

---

----

----

SCE)

--- - - --- - --

1 , 2 -Benzoquinone

Me4NOH in 40o/o dioxane

0. 1 M LiCl in 80o/o ethanol

----

---

---

---

-0.60

-0.40 0 6 3 -1 .28

-

.

---

,

---

In acid solution, aliphatic nitro compounds are reduced at relatively positive potentials in a single step to the corresponding hydroxylamine.

The further reduction to the amine

occurs at quite negative potentials and can only be observed polarographically in alkaline media. In acid solution the polarographic wave corresponding to this process is obscured b y the hydro­ gen evolution current . Aromatic nitro-compounds likewise are reduced through the hydroxylamine to the amine in two steps. In most cases both waves can be observed in acid solution. Some nitrophenols and nitroanilines are reduced directly to the amine in a single step; while in alkaline solution the reduction proceeds only to the hydroxylamine. Tetranitromethane gives only a single polaro­ graphic wave in both acid and basic solution, while 1 ,2,3-tri-nitrobenzene gives two waves and picric acid is reduced in three steps. Substituents in the benzene ring influence the reduction potential of the nitro group( s ) ; the m agnitude of the effect depending on the electrophilic charac­ ter and position of the substituent group. In all cases the half-wave potentials become more posi­ tive as the acidity of the solution is increased. Polarographic data for selected nitro-compounds are given in Table 7.6. The reduction potential range of both aromatic ( E 12 from ca. -0. 3 to --0.8 V) and aliphatic 1 (E112 > -1 . 3 V) nitro compounds, and the num ber of electrons i nvolved in the electrode reaction ( n == 4 or 6 compared with n = 2 for most other organic compounds) means that many important pharmace uticals, agricultural products and explosives can readily be determined by the polaro­ graphic reduction of their nitro groups with high sensitivity, for example < 10 8 mol L- 1 using DPP [ 19, 27, 28 ] . The presence of the reducible n itro group in the carcinogenic 4-nitroquinoline-

Ta ble 7.6

Polarographic half-wave potentials for selected nitro-compounds.

4) E11 2 [ 1 (V vs SCE)

Compound

Supporting electrolyte

pH

Nitromethane

BR buffer in 3 o/o methanol

1 .8

-0.79

4.6

-0.89

8.0

-0.94

1 1 .6

-0.94

BR buffer in 30o/o methanol

Tetranitromethane

1 .0 M NaOH/2.0 M NH4CI/ 1 0o/o methanol ( 1 : 5 )

Nitroethane

0.3M

1 - Nitro p ro p ane

BR buffer in 30o/o methanol

LiCI - methanol/benzene ( 1 : 1 )

1 2.0

4.6

-0.8 1

1 1 .6

-0. 8 9

4.6

-0.79

- 1 . 20

0.3 M LiCl - methanol/benzene ( 1 : 1 )

- 1 .20

-1 . 3 5

2,2- Dinitropro pane

-0 .90

1 -Nitrobutane

- 1 .26

2 -Nitrobutane

- 1 .35

I ,3- Dinitropropane

- 1 .20

Buffer solution

with d ioxa n

1 .0

-0.28, -0.88

7.2

-0.66

1 2.0 1 ,2 - D initrobenzene

) E112 [ 29

(V vs SCE)

-0.4 1

2- Nitrop ro p ane

Nitrobenzene

pH

)> "Q_ '0

-0.32, -0.58

2.5

-0. 1 2, -0.32, - 1 .26

I , 3 - D initrobenzenc

5.7

-(J.35 , -0. 5 1

2.5

-0 . 1 7 . -0.29

1 ,4-Dinitrohen zcne

5.7

-0 .32, -0.59

2.5

-0. 1 2 , -0 . 3 3

4. 1

-0. 1 9, -0 .3 1 , -0.45

1 ,3 , 5 -Trinitrobenzene

NaO H/ N H 4 Cl ! ! Oo/o meth a nol

2,4,6-Trinitro p h enol

Buffer solution

2,4,6-Trinitrotoluene

Phthalate bu ffer

1 2.0 1 1 .7

-

0 . 35 , -0.44, -0 . 6 1

-0.40, -0.60, - 1 .0

!':. c;·

;:; ·

-0. 8 7

5.7

KHPhthalate-NaOH buffer in 8o/o ethanol

::::J � 0

.a "' ::::J ;:;· "' '0 "' n "'

;;; ·

.. � ..

1 92

Introduction to Voltammetric Analysis

N-oxide enables it to be determined in the presence of 4-hydroxyami noquinoline-N-oxide and 4aminoquinoline- N-oxide using differential pulse polarography [ 30 ] . Nitrazepam, parathion, nitrofurantoin and the nitroimidazoles in blood, plasma or urine samples can also be determined via the reduction of the respective nitro groups. At pH 0-5

( 7.4a)

and at pH 9- 1 2 Alkyl

'\ � - N0 + 2e- + 2H+

"'-.

-H2C

N - H + .l N 2 0 + .l H , O 2 2 -

/

(7.4b)

-H 2C

C-rzitroso compounds are reduced to either the amine or the hydroxylamine. In weakly acidic to basic solution nitrosobenzene is reduced to phenyl hydroxylamine while 4-nitrosophenol is reduced to 4 -aminophenol. The mechanism for the reduction of N-nit roso compounds (nitro­ samines) is summarised in equations 7.4a and 7.4b. The determination of nitrosamines by differ­ ential pulse polarography is quite sensitive, for example, N-nitroso -N-methylan iline in blood, serum or albumin, is read ily determined in the jlg kg- 1 range [ 3 1 ] . The polarographic and voltam­ metric behaviour of the nitrosamines, quinones, steroid hormones and imidazoles is important in connection with cancer research [ 32 ] . A number o f nitra tes, that is compounds containing the -O-N02 group, used i n medicine can be analysed polarographically via the reduction of the nitrate group. For example, the determina­ tion of glycerol trinitrate in pharmaceutical products is carried out by dissolving the product directly in the supporting electrolyte (0. 1 M NH3 + 0. 1 M N H 4Cl in 20o/o ethanol) and recording the DP-polarogram [ 1 , 3 3 ] Similarly, isosorbide din itrate { 1 : 4, 3 : 6,-dianhydrosorbitol-2,5dinitrate-( I ) } and isosorbide-5-mononitrate ( I I ) can be determined in medical preparations and the endo and exo forms of the mononitrate distinguished polarographically ( E1 12(endoJ -0.45 V and E112(exoJ -0.38 V versus SCE) [ 1 9, 34] . .

o2N -b:J

=

=

(en do)

(I)

(exo)

(II)

In the absence of conjugated systems, aliphatic N-oxides are reduced at relatively negative potentials. Similarly the reducti on potentials of th e pyridine and quinoline N-oxides are in the

Applications: organic s pe c i es

1 93

Cl

0

- 0. 9 E

Step A:

+ 2e -

= N-

+ 2H

•

0

'

Step B:

Step C:

[ V] (vs. Ag/AgCI)

/

Cl

+ 2e

c=N-

J:X.

N=

O

H"

-

+ 2H C

C-N

, �

+

+

•

' /

CH - NH -

,. NHCH3

:

CH2

'H

Cl

�

Figure 7 . 8

D.c. polarogram of 0 .35 mM chlord iazepoxide i n a c eta te buffer (pH

4.2) [ 1 4, 35].

region of - 2 . 3 V t o -1.8 V ve rsus SCE. The presence of other polarographically active groups in the molecule facilitates the r edu c ti o n of the -N�O group. In the c a se of chlordiazepoxide (see Figure 7.8) the reduction of the -N�O group and the two azomethine ( >C==N-) groups occur at different potentials resulting in three wel l separated waves with that associated with the reduction of the -N �0 group occurring at the least negative potential ( E112 - 0 5 7 V versus Ag/AgCl) [ 3 5 ] . Chlordiazepoxide i s one o f a series o f 1 , 4-benzodiazepines which are extensively used as psy­ chotropic d r ugs , particularly as anti-depressives and tranquillisers. The parent compound, 1 ,4benzodiazepine contains two electroactive azomethine groups in the diazepine ring. These com ­ pounds can be dete rmi n ed in the jlg L- 1 range by differential pulse polarography [ 3 5 , 3 6 ] in which the peaks arising from the reduction of the azomethine groups or, if present, other electroactive -

.

1 94

I ntroduction to Voltammetric Analysis

substituents may be used for the analysis [ 1 4 ] . Peak potentials associated with the reduction of the electroactive functions in several of these drugs are listed in Table 7.7.

o:-> 5

4

1 , 4- Benzodiazepine

The current peak arising from the following reduction which is common to all of these com­ pounds is the one normally used for their determination.

Cl

n

Application 1 9

CH 3 I

0

N-C

C=N

'

C H2

I

+

-

2e

+

2H

+

-----1•�

CI

6

0 It

f"""y N - C> I

C H3

It

� CH - N

H,

'

6"

Polarographic determination of diazepam i n body fluids and pharmaceutical products [37]

Sample: Pre-treatment:

Ta blets, seru m , u ri ne 1 Ta blets. Grind 1 0 weig hed tablets i n a mortar. Pl ace a known weight of the powde r in a beaker, sti r with 3 5 m l methanol for 20 m i n , rinse the m ixtu re i nto a 50 ml vol u metric flask with methanol : water ( 1 : 1 ) a n d make up to the mark. Store in a r efr i g erato r to a l l ow the insol u b l e material to settle pr i or to the anal­ ysis of the d rug . 2 Blood, serum, urine. Add 1 5 m l blood (hep a r i n added to prevent coagulation) to 40 ml n-penta ne in a separating fu nnel, shake 2 m in and sepa rate the phases. Extract the blood phase with two further 20 ml a l i quo t s of n-pentane. Combine the n - pe n ta n e e xtrac ts an d ce ntr ifuge for 1 0 min at 7 500 rpm . Sepa­ rate from any solid and disti l the n-pentane at 70°C. Dissolve the residue in 500 �L methanol and rinse into the polarographic cel l with 1 4 . 5 m l of supporti n g electrolyte . Use the same scheme to p re-treat the seru m a n d urine sa m p les B ritton-Robi nson buffer of pH 2 . 8 i n 20% methanol. Dry 0. 5 g d iazepam at 70°( for 24 h . Dissolve 1 00 mg of this i n 50 m l metha­ nol and m a ke u p to the mark i n a 1 00 ml vol u metric flask with pure water. Store in a cool dark place . Th is 1 g L -I stock sol ution is stable for a b ou t 1 week. Worki n g standards of 50, 1 00 and 200 m g L- 1 a re p r ep a re d by d i l u ti ng the stock solution with 50% methanol. They m ust be stored i n a cool d a rk place a n d a re stable for 2-3 d ays . Place 1 ml tablet extract with 1 9 m l s u ppo rt i n g electrolyte i nto the polaro­ gr aphi c cel l (or use the 1 5 ml of solution from the pre-treatment of a blood, serum o r urine sa mple) and deaerate with n itrogen for 5 m i n . Record the DP­ pol a ro g ra m from -0 . 5 V to -0. 9 5 V. The d iazepam peak a ppears at c a . -0. 7 3 V .

.

.

S u p po rtin g e lectrolyte: Diazepam standard :

D ete r m i na tion :

Table 7 . 7

DP polarograph i c peak potentials of selected 1 ,4 -benzodiazepincs measured at p H 4 a n d p H 1 2 i n Britton-Robi nson buffers ( �, V v s SEC) [ 36) .

Supporting electrolyte

Functional groups: Compound

-N02

=N-tOH+ -0. 3 7

Chlordiazepoxide

>C=NH+-

Medazepam

.

1 12

-0. 1 5

-0.60

-1 .22

-0. 1 6

-0.73

-0 .6 1

- 1 .50

Cl o n a ze p a m Flunitraze pam

Flurazepam

- 1 .24

-1 . 1 7 -

>C=N-

-0.73

-0.78

P r aze p am

-N02

-0.99

-0. 1 6

Lorazepam

-N=C<

-0.6 1

Nitrazep am

Diazepam

Pyridyl

-0.73 -0.40

Bromaze p am

Oxazepam

pH 1 2 buffer

pH 4 buffer

-0.76

-0.74

-0.74 -0 .73

- 1 . 23

- 1 .20

- 1 .45 -1.15

- 1 .22

-0 . 7 2

-1.10

-0.82

- 1 .2 3

{

g·(5" ::::l !'! 0

.a

"' ::::l ;::;"'

� n �-

...

�

1 96

Introduction to Voltammetric Analysis

(versus Ag/Ag C I , 3 M KCI) . Determ ine the concentration of d i az e p am using the m e t h od of standard additions {two additions}. 1 The total amount of diazepam in the cel l solution ( i nc l u d i n g that from the standard addition aliquots) should not exceed 200 j.Jg, i.e. c > 10 mg L _ , _ in order for the conce ntration to remain in the l i near iP vs c ra nge. 2 The reproducibility of the measurement is not influenced by the slight asym­ metry of the curve. 3 As w e l l as d i a z e pa m , 1 , 4-benzodiazep i n e and other members of the 1 ,4-benzodiazepine group (e. g . nitrazepam} may be determined by stripping voltam metry.

No t e s :

In addition to the diazepi nes, other pharmaceutical products and pesticides which contain the reducible azom ethine group can be determined polarographically [ 3 8 ] . In oximes, the a zomethinc group is reduced in acid solution as follows:

>

>

>

C

=N-

N - OH + H + � > C = N - OH ; OH; + 2e- + 2H+ > CH - NH - OH;

C

=

CH - NH - OH� + 2e- + H '

� >

CH - N H 2 + H 2 0

A single pola rographic wave is obtained for th e reduction to the amine. Information on the biotransformation of drugs and the excretion of the drugs and their metabolites is important pharmacologically. Numerous methods for the polarographic and volta­ mmetric determination of pharmaceuticals and their metabolites have been developed for the assessment of such relationships. The determ ination is often preced ed by separa tion from the sample ma trix by liquid-liquid extraction with diethyl ether or ethyl acetate. As an example, the flow diagram for the polarographic analysis of flurazepam and its metabolites in body fluids [ 3 9 ] i s presented i n Figure 7 . 9 . Other pharmacollogically active 1 ,4-diazepines can b e determined polarographically via the reduction of their azomethine groups. In addition to flurazepam, the related drugs, bromazepam , chlorazepam, chlordiazepoxide, diazepam and lorazepam may be determined in th e flg L- 1 range using AdSV or DPP [ 1 4 ] . The po larographic behaviour of the azo compounds depends o n the p H and o n the nature and position of the substituents in the benzene rings. Azobenzene is reduced to hydrazobenzene in a si ngle step:

I n acidic solution, 4-aminoazohenzene, and 2 - and 4-hydroxyazobenzene a re reduced in a four-electron process to the respective amines, for example 4-hydroxyazobenzene gives aniline and 4-aminophenol. Nitro groups or hyd roxy groups in the 3 -position stabilise the hydrazo prod­ uct. In general, the presence of substituents shifts the h alf-wave potential to more negative values. Azo dyestuffs, used in a va riety of matrices, are commonly determined by pola rographic and vol­ tammetric methods [ I , 1 9, 40 ] . Nitrogen heterocyclic compounds, such as pyridine, are reduced in a two-electron step: A number of alkaloids which are derivatives of pyridine can be determined on the basis of this reaction. Nicotine ( 3 - ( l - methyl-2-pyrrolidinyl) pyridine) is one such alkaloid which can be easily determined in the Jlg L - 1 range. +

2 H

+

+

2e

Applications: orga nic sp ec i es Extract s a m pl e with 2

8

Fl urazepam, O H , COOH, N H , x

NH-3-0H

m l ethyl acetate

Organic phase

Aq ueo u s phase (COOH)

(fl u razepam , OH, N H , N H -3-0H)

2

residue

3, e xt ra ct with 1 2 ml d i eth y l et h er

Aqueous

phase

in 10 mL B R buffer, e xt ract 1 0 ml d i ethyl ether

Evaporate to d ryness, take u p

Adjust p H to x

1 97

with

2

x

ic ph a s e

Organ ic phase

Aqueous phase

O rgan

(COOH)

(fl urazepam)

(OH, NH, N H-3-0H)

2 ml (pH 4),

with 2

formate buffer

9, e xtract 1 4 ml diethyl

Adjust p H to

Evaporate solvent, take up residue in

ethe r

x

rec o rd polarog ram t o

Eva porat e s o lvent ,

1 0),

2 ml b u ffe r reco rd polarogram

take up residue in (pH

to dete rmine metabolites

d e te r m i ne COOH

Aqueous phase

Organic phase

r es i due in 2

Evapo rate solvent. take up m L formate buffe r

(pH 4 ) , record pol arogram to

fu

dete rmine l raze pa m

Figure

7_9

F low diagram for the polarographic determination of flurazepam and its metabolites in body fluids [39) . OH N-1 -hydroxy metabol ite; COOH N- 1 -acetic acid metabolite; NH N- 1 -dealkyl metabol ite; N H -3-0H N-1 -dealkyl-3-hydroxy metabolite. =

=

=

=

Application 20

Polarographic determination of nicotine [ 41 ] Sample:

Supporti ng electrolyte : Nicotine sta n d a r d : Dete rm i nati o n :

Tobacco, tobacco s m o k e . B ritton-Robinson buffer of pH 6 . Mix 40 m l each of 1 M H l 0 4 1 M C H l O O H a n d 1 M H 3 B 0 3 w i t h 425 m L 0.2 M NaOH a n d m a ke u p to the m a rk i n a 1 L flask. Di ssolve 1 00 mg n i coti n e i n p u re water a n d m a k e u p to 1 00 m l. A s n i cotine is l ig h t sensitive this solution should be made u p f r e q u e n tly . 1 I n tobacco . Place 1 g of toba cco i n a 1 00 m l vol u m etric flask with 50 m l p u r e water a n d 1 m l 2 M NaOH, s h a k e a n d let stan d for 1 2 h . Make up to th e m a r k with pure wate r and fil te r. Add 1 00 � L of filtrate to 2 5 m l of su pporti ng electro­ lyte, deaerate a n d record the D P - polarogram b e tw e e n - 1 . 0 V and -2 . 0 V: the pea k occurs at a b o u t - 1 .26 V (versus Ag/Ag C I , 3 M KC I ) . 2 I n to bacco s mo ke . Pass the s m o ke t h r oug h 50 m l su pporting el ectrolyte via a g lass frit (G 1 ) . De aerate 2 5 m l of this sol ution in the po l a ro gr aph i c cel l a n d record t h e DP polarogram as above . Calcu l ate the content by sta n d a rd a d d it i o n s . Q u oted determi nation l i m it is S �g l-1 .

The reduction of qui n o l i ne proceeds i n a similar way to that o f pyridine. The two-electron reduc­ tion is i rreversible, p roducing 1 ,2- or 1 ,4-dihydroquinoline in weakly acidic or basic solution. Acridine likewise is reduced by taking u p two electrons a n d gives two waves in the polaro gram.

1 98

Introduction to Voltammetric Analysis

Table 7.8

Polarographic half-wave potentials of some pyridines, quinolines and acridines

[14].

E1 12(V vs SCE)

Compound

Supporting electrolyte

Pyridine

Phosphate-citrate butTer

7.0

3-Acetylpyridine

BR buffer

2.0

-0.72, -0.87

6.0

-1 .07

1 0.0

- 1 .40

pH

-1 . 75

Pyridine-2-carboxylic acid

0. 1

M HCI

-0.89

Pyridine- 3 -carboxylic acid

0.1

M HCl

- 1 .08

Pyridine-4-carboxylic acid

0. 1

M HCI

-0.80

Pyridi ne- 2,3 -dicarboxylic acid

0. 1 M HCI

- 0 . 78

Pyridi ne-2,4-dicarboxylic acid

0 . 1 M HCI

Qui noline

0.2

8- Hydroxyquinoline

Acetate buffer

3.0

-0.89, -1 .08

Borate buffer

1 0.0

- 1 .48, - 1 .80

8.3

-0.79, - 1 .45

7.0

-0.65, - 1 .22

Acridine 1 -Aminoacridine

M Me4NOH in

-0.66

50% ethanol

Phosphate buffer in 50% ethanol

- 1 .50

Half-wave p o ten ti al s of some nitrogen h ete rocycle s are listed in Table 7.8. Q u in o l ine alkaloids such as the anti-malarial d ru g quinine, and the local anaesthetic dibucaine (or cinchocaine) (2-butoxy- N- ( 2 - ( d ie thyla m i n o ) ethyl ] -4-quinolinecarboxamide mono-hydrochloride) which un­ dergo two-electron reductions to the c o rre s po n di n g di hydroquinolines can r ea dily be determined polarographically. In an ac e t a t e buffered supporting electrolyte of pH 4.8, c inc h o ca i ne , give s two current peaks in the DP-polarogram. The first peak at -0.93 V is best suited for the concentration measurement. The second peak at about - 1 .25 V is distorted by im p u ri ties or hydrogen evolution. Because hydrogen waves may occur with these ni tro ge n bases it is best to use higher pH support­ ing electrolytes for their a na lys i s if p oss ibl e . Another problem that may occur is interference aris­ ing from the ad so rp t i o n of the analyte on the mercury electrode which may le a d to n o n - l i ne ar iP versus c pl o ts . Application 2 1

Polarographic determination o f ci n chocaine (dibucaine) i n pharmaceutical preparations [ 7 , 42]

Sample: Pre-treatment:

Su pporting e lectrolyte:

O i ntments, i njection sol ution . 1 I n j ect io n solution - none. 2 Ointments - v igo ro usl y stir 1 g of sample with 20 m l 1 M H C I for 1 5 min at 6 5°C to e xt ra ct the cinchocaine. Cool to room temperature, add 1 5 m l pure water, tra n sfer the solution to a centrifuge tube and centrifuge for 20 m i n at 7 500- 1 0000 rpm. Cool to 5°C, carefu l l y tra nsfer the aqueous phase with a p ipette to a 1 00 ml vo l u m et ri c flask. Vigorously rinse the pi p ett e and ce n t r ifug e tube (with 1 0 m l 1 M H C I), c e n tr i fu ge the washings as above. Withdraw the aqueous phase, comb ine it with the fi rst extract a n d make up to the m ar k with d i stilled water. A slightly turbid s o l u t ion will not affect the determination . Fi lter only if the solution is markedly turbid. Acetate bu ffe r , pH 4. 7-4 . 8 .

Applications: organic species

S ta n da rd s :

Dete rm i nation :

Disso lve 5 0 0 mg d ri e d cin choca i n e hyd roch loride i n p u re water, make u p t o the mark i n a 500 m l vo l u m e t r i c fl ask. T h i s stock solution contai n s 904 m g L-1 c i n ­ c h oca i ne b a s e . The co n ce nt ra tion re ma i n s u n c ha n g e d ove r 4 w ee k s if t h e solu­ tion is stored i n the dark i n a refr i g e ra t o r . A wo r k i n g sta ndard is made b y d i l ut i n g 50 m l of t he stock w i t h p u re wate r t o 2 0 0 m l i n a vo l u metric fl a s k . This 1 conta i ns 2 2 6 m g L-1 cinchoca i n e (or 2 50 m g L- cin c hoc a i n e hydrochlori de) and should be sto r e d i n a cool dark place . Pl ace 1 m l o f o i nt me nt extract ( o r a su i t a b le vol u m e o f i n j e ct i o n solution) i nto t h e p o l a ro g ra p h i c ce l l wit h 1 9 ml su pporti ng e l e ctrolyte, deaerate a n d record t h e DP-po l a ro g ra m from -0 . 7 0 V to - 1 . 2 0 V. T h e cu rre nt p e a k occurs at ca . -0 . 9 3 V (versus Ag/AgC I , 3 M K C I ) . U se sta n d a rd additions to determ ine the

conce ntrati o n . Notes:

1 99

1 I n o rder t o rem a i n with i n t h e l inear iP ve rsu s

c range, t h e tota l a m o u n t of cin­

choca i n base i n t h e 2 0 m l of solution i n the cel l ( i n c l u d i n g that i ntro d u ced by

the sta n d a rd additions) s h o u l d not exceed 3 4 0 �g (3 7 5 �g ci n c h o c a i n hyd ro­ c h l oride). 2

T h e dete r m i nation l i m it i s 2.8 �g c i n c h o c a i n base in 2 0 m l. ( 1 . 4 m g

L-1).

Quinine was used for a l o n g time a s a n anti-malarial dru g and i s now u sed a s a decongestant in numerous influenza remedies as well as a b itter substance in drinks such as tonic water. Q u in i n e in these products c a n be determined using DP-polarography. With a buffer at pH 7 as su p p o r tin g el e ctrol yte , the current peak occurs at about - 1 .03 V (versus Ag/AgCl, 3 M KCl) [43 ] . Other c om poun ds co n t a inin g nitrogen in a ring system, such as p i p e ri d i n e , indole and phe ­ nothiazine, are n o t reduced at a mercury el e ct rode . Some of these, like piperidine, can be oxidised voltammetrically and the resulting anodic current peak can be used for their determination. Heterocyclic compounds with two an d three ni t ro ge n atoms in the ri n g, such as imidazole, pyrimidine, pyrazi n e and triazine, a re polarographically active and a re reduced to the corre­ sp on d ing tetrah ydro derivatives. Only a single wave is observed in the pol a r o gram s and the half­ wave p oten t ial is pH dependent. V i t a m in B 1 ( th ia m in e ) is a pyrimidine derivative and is present

+

+

2 H . 2e .,.

Tetrahydropyrimidine

in vitamin p rep arati o ns as the hydrochloride (1) or m o n o n i t r a t e (II) salt. The thiamine content can be de te rm i n ed at the f.lg level.

(I)

(II)

200

I ntrod uction to Voltammetric Analysis

Application 22

Polarographic determination of thiamine (v itamin B 1 ) [ 44, 45]

Sample: Pre-treatment:

Supporting electrolyte:

Th iamine sta ndard:

Determ inatio n :

N otes:

Vita m i n tablets a nd solutions. Weigh 1 0 tablets and grind to a powder. Wei g h 1 / 1 Oth of the powder (average weight of a tablet) i n to a beaker and stir with 30 ml 0.0 1 M NaOH for 20 m i n . F i lter i nto a 1 00 m l vol u metric flask and make up t o t h e m a r k . Vita m in solu­ tions can be a n alysed d i rectly. Disso lve 4 . 1 g an hydrous sod i u m acetate i n ca. 400 m l disti lled water, add 2 . 86 m l g l acial acetic acid and d i l ute to a bout 950 m l with d isti l led water. Adj ust to pH 6.4 to 6 . 6 with 2 . 5 M NaOH a n d make up to 1 L. For a 1 g L_ , stock solution of the th i a m i ne cation (C12H 1 7N40St, d issolve 636 mg of the hydroch loride (or 6 1 7 mg of the mononitrate) i n 500 m l of the supporting electro lyte . This solution is sta ble for a l o n g time if stored in a refrig­ e rator. M a ke u p sta ndard solutions daily by appropriate dilution with the sup­ porting e lectrolyte. Place 1 8 ml of supporting electrolyte and 0.4 ml of sam ple sol ution in the cell and deaerate with n itrogen for 5 m i n . Add 0.8 m l 1 % triton X- 1 00 and deaerate aga i n . Record the DP-polarogram using a DME between - 1 . 1 0 V and -1 . 50 V . The current peak of thiamine occurs at ca . - 1 .38 V (versus Ag/AgCI, 3 M KCI ) . Use standard additions to determine the concentration. I n order to remain in the l i near iP versus c range the a mount of thiamine i n the cel l should not exceed 5 !Jg. 1 The S M DE can not be used as work i n g electrode for this determ ination . 2 N i coti n a m ide and Fe( l l ) i nterfere with th is determi nation .

Another group of polarographically active nitrogen heterocyclic compounds are the pteridines which contain four ring nitrogens. Folic acid ( pteroylglutamic acid) and riboflavin (a benzo [ g ] pteridine) which are both members of the vitamin B group are pteridines and can be determined polarographically. Folic acid is reversibly reduced in a single 4-elcctron step to tctrahydrofolic acid while riboflavin is reversibly reduced to dihydroriboflavi n in a single two­ electron step.

+

Folic acid H

� � y� A Jl _)c OH

N

2NH

H

H2N

N

I

N

H

H

HOOC

Tetrahyd rofolic acid

'Q I

h

I

C

/0 /

, NH

� CH I

COOH

-

4e , 4 H

+

Appl ications: organic species

201

Application 23

Polarographic determination of folic acid [ 46] Sample:

P re-treat men t :

S u p po rt i n g e l ectrolyte :

F o l i c acid sta n d a rd :

I nject i o n sol u t i o n , vita m i n ta b l ets .

I nject i o n so l ution s ca n be a n a lysed d i rectly afte r a dj u sti n g the pH to 8 . 0 with N a O H . G ri n d 1 0

vitamin

t a b l ets to a powd e r . Add 200 m g of the powd e r to

3 0 ml wate r in a bea ke r , adj u st the p H to 8.0 with 0 . 1 M N a O H and st i r fo r 1 5 m i n . C heck the pH a n d if ne cessa ry re-adj ust to 8 . 0 . F i lter t h e s o l u t i o n i nto a

1 00 m l vol u metric fl a s k a n d m a k e up to the m a r k w i t h a l ka l i n e wate r of pH 8 .

D i s s o l ve 6.2 g bori c acid i n 1 00 m l p u re water with 2 g N a O H . D i l ute t o 9 5 0 m l w i t h p u re wate r, a dj u st to p H 1 1 . 1 to 1 1 . 2 with

vo l u m etri c f l a s k a n d

2M

N a O H , tra n sfer it to a 1 L

m a k e up to t h e m a r k with p u re wate r .

P l ace 1 2 7 . 5 mg 9 8 % fol i c a c i d i n a b e a k e r w i t h 8 0 m l p u re wate r a n d wh i le

sti rri n g a d d 0 . 1 M N a O H u n t i l the sol uti o n i s cl ea r a n d pH 8 . 0 . D i l ute to 2 5 0 m l i n a vo l u metric flask, t ra n sfe r t o a b rown bott l e a n d store i n a refrig erato r . T h i s stock sol ution s h o u l d be m a d e u p d a i ly. D i l ute a s req u i red to obta i n t h e sta n ­ Dete rm i n at i on :

d a rd sol u t i o n s . Add 0 . 5 m l of s a m p l e s o l u t i o n to 1 9 . 5 m l of s u p p o rt i n g e l e ctrolyte i n t h e pol a ro g r a p h i c ce l l a n d d e a e rate. Record t h e D P - p o l a rogram between -0 . 7 V a n d - 1 . 2 V. Use sta n d a rd a d d itions to d ete rm i n e the fo l i c acid conce nt rati o n

from t h e c u rrent pea k at ca . -0 . 9 7 V ( ve rsus Ag/Ag C I , 3 M KC I ) . E n su re that the

conce n trati o n of fo l ic acid rema i n s with i n the l i ne a r iP v ersu s c ra n ge ( 1 . 5 �g to 1 7 5 �g in 20 m l) .

Riboflavin c a n be determined i n a similar manner [ 4 7 ) .

Application 24

Polarographic determination of riboflavin (vitamin B2)

Sample: P re-treatment:

Vita m i n p re p a rati ons, s o l u ti o n s and ta b l et s . S o l uti o n s ca n be a n a lysed d i rectly . Wei g h 1 0 tablets a n d g r i n d to a powde r. Wei g h 1 / 1 Oth of the powd e r i n to a 1 2 5 m i n i u m fo i l . Add 8 m l 0 . 2

M

ml

E rl e n meyer flask covered with a l u ­

KO H a n d pass nitrogen t h r o u g h t h e s o l u t i o n wh i l e

sti rri n g for 1 5 m i n . F i lter i nto a 1 00 m l vo l u m et r i c flask fi l led with n itrogen a n d m a k e u p to the m a rk wi th oxyg e n -free p u re water. C over t h e f l a s k w i t h a l u m i n ­ S u ppo rti n g e l ect ro lyte : Ri boflav i n sta n d a rd :

i u m fo i l a n d store i n the refri g erato r .

Di ssolve 3 . 73 g K C I a n d 1 6 . 5 g K2C 0 3 . 1 . 5 H 2 0 i n 1

L oxyge n -free p u re wate r .

D i ssolve 1 0 m g i n 8 m l 0 . 2 M KOH b y b u b b l i n g n i troge n t h ro u g h t h e s o l u t i o n .

U s i ng oxyg e n -free p u re water, r i n s e t h e s o l u t i o n i nto a 1 00 m l vo l u metric flask covered i n a l u m i n i u m fo i l , m a ke u p to the m a rk and sto re i n a refrig e rato r.

Determ i n at i o n :

Deae rate 1 9 m l s u p p o rt i n g electro l yte i n a d a r k g l ass p o l a ro g ra p h i c cel l with

n itroge n for 5 m i n , a d d 1 ml s a m p l e s o l uti o n and deaerate fu rther. U s i n g a

D M E , reco rd the D P- p o l a r o g r a m betwee n -0 . 3 0 V a n d -0 . 7 0 V. The c u rre nt

p e a k occu rs at -0 . 5 5 V (ve rs u s Ag/Ag C I , 3 M K C I ) . The d eterm i n at i o n l i m it is ca .

N otes

1 . 3 mg L- 1 sa m pl e vol u m e . W h e n dete r m i n i n g t h e co n centration by sta n d a rd 1 a dd itio ns , the l i nea rity r a n g e of 1 - 1 5 mg L- m u st be born i n m i n d . 1 Al ka l i n e sol u t i o n s o f ri bofl a v i n a re ra p id ly d ecom posed by atmospheric oxy­

g e n , l i g ht, a n d heat. The refo re it i s necessary to work u nder n itrogen in d a r k ­ e n e d vesse l s .

2 Trace levels o f ri bofl avi n ca n b e determ i ned by a dsorptive stri p p i n g volta m me­

try [48 ] .

202

Introduction to Voltammetric Analysis

E - 0 . 04 V P

Cyste ine

Cyst i n e EP - 0 . 4 V

- 0. 1

Figure 7 . 1 0

- 0.4

-

E [V ] (vs. Ag/AgCI , 3 M KCI) 0.8

+ 0 .2

0

- 0.2

E [V] (vs. Ag/AgCI, 3 M KC I)

DP-polarographic determination of cystine a n d cysteine i n 0. 1 M H C I04 [49) . Each standard addition 1 00 119 cystine or 5 0 119 cysteine. Sample contained 5.6 m 9 l-1 c yst i n e and 2.3 m9 l-1 cysteine.

Organic sulfur compounds such as the disulfides, sulfonamides, aromatic sulfonic ac i d esters, sul­ fones, sulfoxides, thiobenzophenone, thioethers , and thiocyanates c a n be p o l arogr aphically reduced. A se l ect i on of polarographically ac t i ve sulfur compounds is listed in Table 7.9. The polarograp h i c reduction of disulfides results in s plitti ng the - S - S - bond to produce th e c o r responding t hi o l s :

The determination of cyst i ne and the ox.idised fo rm of glutathione is based on this reaction . When the de gradat i on of bio l o gical s a m p l es leads to bo th cystin e and cysteine being formed it is often n ecessary to determine the ratio of these two amino acids. This may be car ri e d out by recordi ng the DP-polarogram between -0. 1 V to -0. 8 V for cyst ine and between +0.2 V a nd -0.2 V ( versus Ag!AgCl, 3 M KCl) for cyste i ne using 0. 1 M HC104 as s u p porting electrolyte [49 ] . Typical D P ­ pol a rogram s arc shown in Figure 7 . 1 0 . Sulfones and sulfonamides can only be reduced polarographically when the - S02 - NH - or - S02 - groups ar c conj ugated with a d o u ble bond. The reduction of sulfonamides may lead either to the splitting of the - S - N - bond or, in the p r esence of electronegative substituents in the benzene ring, to the splitting of the - C - S - bond [ 50] .

Appl ications: organic species

Tab l e 7 . 9

Polarograph i c h a l f- wave potent ials of selected orga nic s u l fu r compounds [ 1 4 ] .

Compound

Supporting electrolyte

Methyl benzenes ulfonate

0. 1 M TEAl in 1 0% ethanol

E112

( V v s NCE)

-2.06

Phenyl benzenesulfonate Diethyldisulfide

203

- 1 .96

0.025 M TBAOH in water

+ isopropanol +

- 1 .82

methanol ( I : 2 : 2 ) (pH 1 2 . 3 )

Di- (n- propyl ) disulfide

- 1 .84

Di- (isobutyl ) - disulfide

- 1 . 84

Di-( tert-butyl) -d isulfide

-2.04

Cystine

D iphenyld i s u lfid e

Acetate buffer, pH 3 . 8

--0.72

Phosphate buffer, pH 7.0

--0 .83

NH3 - NH/:I

- 1 .05

Phosph ate buffer i n 50% ethanol, p H 7 . 0

--0.50

2,2' - D imethyldiphenyldis ulfide

pH 7 . 0

--0.48

3 , 3 ' -Dimethyldiphenyldisulfide

pH 7 . 0

4,4' - D im ethyldiph cnyldisulfide

pH 7.0

--0 . 5 1

Dibenzyldisulfide

pH 1 2 .3

-

--0 . 5 1

1 .46

D i t h i o gl ycol ic acid

Phosph ate buffer

pH 3.0

--0.4 1

Glutathione (oxidised form)

Soren sen buffer, 0 . 1 M KCl

pH 9.45

--0 . 7 3

Di p h e n yl s u l foxi d e

0 . 1 M TEAl in 50% ethanol

Methylphenylsulfoxide

0 . 1 M TEABr in 50% ethanol

-2. 1 8

Diphenylsulfone

0. 1 M TEAl in 50% e thanol

-2.24,

Methylphenylsulfone

0. 1 M TEABr in 50% ethanol

-2. 1 4

-2.24,

-

2 . 66

2 . 64

--0. 5 5

Cysteine Decanthiol

0.0 1 M H2S04 in 70% ethanol

-0.49

Thiophenol

0 . 0 1 M H2S04

--0.34

D i mercaptopropanol

Acetate buffer,

p H 4.64

--0.49

Na-Diethyldithiocarbamate

-0 .44

Thioacetamide

-0.09

Thiourea

-

0 . 1 M HC101

+

2H

+0 . 1 0

+

Sulfonamides give well formed waves in aprotic media which have been used for the determina­ tion of sulfonamide dr u gs in medications [ 1 9 ] . Thiobenzophenone is reduced to the corresponding thiol i n a two-electron process. Likewise the reduction of aromatic sulfonic acids is a two-electron process producing the corresponding

204

I ntroduction to Voltammetric Analysis

sulfinic acid. Benzyl and phenyl thiocyanates give polarographic signals which have been used analyt i cally Alkaloids, proteins and many organic sulfur compounds reduce the overvoltage of the hydro­ gen reaction on the mercury drop electrode and are determined by means of the resulting catalytic hydrogen wave. In native protein molecules, the thiol and disulfide groups are masked and there­ fore do not catalyse this electrode process. When the pro t ei n is denatured by the action of bases, acids, or U .V. radiation, etc., these groups become exposed and the catalytic waves appear. This phenom enon has been used to determ ine single-stranded DNA in the presence of closed circular DNA in pl asmid [ 5 1 ] . Organic compounds with oxidisable functional groups (see Table 7. 1 ) include catechola­ mines, sulfonamides, phenothiazines, dopami nes, estrogens and other hormones, tocopherols and various groups of insecticides. These may be successfully determined in solid and liquid bio­ logical samples, foods, plants and environmental and potable water samples in the m g L- 1 range using DPV with a carbon, gold or platinum electrode. For the lower concentration ranges anodic stripping after adsorptive accumulation, or cathodic stripping after oxidative accumulation, is preferred (§. 7 . 2 ) . Electrochemically in active compounds may be determined voltammetrically o r polarographi­ cally by first converting them to an e lectroactive derivative. Nitration, nitrosation, oxidation, hydrolysis or some other preliminary chemical reaction is used to form the derivative. Examples of this are the polarographic determination of styrene monomer in polystyrene and mixed poly­ mers after converting the styrene to the pseudo-nitrosite with sodium nitrite [ 52 ] , alkylbenzene­ sulfonates in natural water samples after extraction and nitration of the aromatic ring [ 5 3 ] and the determination of beta-receptor blocking agents in tablets after conversion to the correspond­ ing N-nitroso derivatives [ 54 ] . .

Application 25

Determination of free styrene in polystyrene and mixed polymers [52]

Sample: Pre-treatment Styrene standard :

Determination:

Polystyrene (e . g yog h u rt bucket) . Break the sample into sma l l pieces, wei g h out about 1 g, dissolve it i n 1 5 ml dioxa n, mix with 1 1 0 m l acetic acid and filter. Dissolve 1 00 m g styrene i n 3 m l d ioxa n in a 1 00 ml vol u metric flask, add 22 ml acetic acid and 1 . 2 5 m l 50% N a N 0 2 soluti o n . Al low to react for 20 m i n at room temperatu re, add 20 m l 50% N a O O C C H 3 sol ution and m a k e u p t o the mark with p u re water. This solution, conta i n i n g 1 g L- 1 styrene, is sta ble for a bout 1 month . To 1 0 m l of the fi l tered sam p l e solution in the polarographic cel l add 0. 5 m l 5 0 % N a N 0 2 • A l l ow t o react f o r 20 m i n and t h e n add 1 0 ml pure water a n d 5 m l 5 0 % NaOOC C H 3 . Deaerate t h e solution w i t h nitrogen and record the DP­ polarogram between 0 . 0 V and -0 .4 V using an S M DE. From the cu rrent peak at -0 . 24 V (versus Ag/AgC I , 3 M KCI) determ i ne the concentration of styrene by stan d a rd additions . The determi nation l i m it is ca . 5 mg kg-1 ( 5 ppm).

If the electrochemically inactive analyte is a surfactant then it may be determined by tensammetry [ 5 5 ] . The peak observed in the a.c. polarogram as a result of the changes in the double layer capac­ itance when the analyte is adsorbed on the electrode surface is useful analytically in favourable cases and is quite sensitive. Polarography assumes an important role in the analysis of organometallic compo unds in envi ­ ronmental samples. In contrast to the spectroscopic methods (AAS, ICP-MS, etc.) which only give the total metal content in the sample, polarography is capable of characterising and deter­ mining the various chemical forms of the organometallic compounds as they exist in the sample

Appl ications: organic species

Table

7. 1 0

205

Peak po tentials of the first reduction step in the differen tial p ulse polarograms of selected organometal l i c compounds [ 56 ] .

Compound

Coumpound

� (V ) -

CH3Hg... CH3 C H2Hg+

(CH3)3Sb H

0. 35

( CoHs l l H g

( C6H , ) ;Sb 1 +

-0. 5 1

CH , ( C6H 5 ) 3Sb 1

( C H 3 ) 2 Sn 2 '

( CH)3Sn•

-0 . 8 0

-0.89

( C6H 5 ) 4Sb"'

-0.87

( CH , ) ,Pb+

-1.14

( C6H5)4Pb

-0.44

( C6H5 ) 2Tl '

-0. 7 7

-0.49

( C6H5 )3Sb

- 1 .07

CH3Sn 3-t-

-0 . 94

-0.33

-0.65

( C,.H3 ) Hg•

EP (V)

-0. 5 7

-O.X9

( C,H5 ) , Pb'

-0 . 50

( speciation) . Because there can be great differences in the toxicity of particular compounds formed by a given metal [ 2 8 ] , it is important to know the concentrations of these various species. The polarographic peak potentials of a few representative organometal lic compounds are listed in Table 7 . 1 0 . Organomercury compounds of the type RHgX generally are reduced in two steps via the following proposed mechanism:

pH < 7

RHg + + e . �

RH + Hg

RHg " + e-

J, (RHg)2

pH > 7

J, R 2 Hg + H g

RHg-

-"-" • H.:... -�

RH + Hg

Thiomersal (thimerosal, sodium-2 - (ethylmercurithio) -benzoate) , used in contact lens rinses and in various medications such as eye and nasal d rops, is readily determined polarographically, often directly without any pre-treatment. The electrode reaction on which the determination is based is [ 1 9 ] :

�COONa

�

S H g CH 2 CH3

-

+ e , +

H

+

�

CtCOONa �

SH

+

Organo- arsen ic, antimony and tin compounds can also be polarographically determined [ 93 ] . Mono-, di- and tri-alkyl tin compounds either singly o r i n admixture can b e analysed in the �g range by DP-polarography [ 57 ] . Trialkyllead( IV) compounds are reduced i n two one-electron steps whereas diethyllead( IV) is reduced in a single step process [ 1 4 ] . By mean s of OF-pola rography, Pb (II) can be determined in the presence of the organically bound trimethyllead( IV) as shown in Figu re 7. 1 1 [ 58 J .

206

I ntroduction to Voltam metric Analysis

0

- 0. 5

-

- 1 .0

1 .5

E [V] (vs. Ag/AgCI, 3 M KCI) F igure 7 . 1 1

DP-polarogra m of 1 . 0 sea water [58] .

x

1 o-5 M (CH3hPb+ a n d 1 .0

x

10

·5

M Pb2+ in fi ltered (0.45 1Jm membra ne)

7 . 2 STRIPPING METHODS The h igh sensitivity of adsorptive stripping voltammetry h as resulted in it becoming the preferred technique for the analysis of many organ i c compounds with electroactive groups at the trace and ultra-trace level in a wide variety of matrices. In m any cases it is the simplest technique capable of measuring the low concentrations dema nded by research groups or the environmental protection regulations. Pesticides in waters, soils and foodstuffs; toxic chemicals in industrial effluents, sew­ age, surface waters and sediments; medical preparations and pharmaceuticals in body fluids; and surfactants can now be readily determined in the sub-micromolar and n anomolar concentration ranges [ 59-62 ] . Other compounds which can be accumulated by adsorption and then determined voltammetrically by cathodic or anodic stripping i nclude dyestuffs [ 63 J ' benzodiazepines r 64, 65 ] , tetracycline [ 66 ] and i ndole [ 67 ] . I n many cases the cel l solution cond itions (solvent composition, supporting electrolyte, pH, etc . ) developed, and the electrode used ( usually a DME), for the determination of an analyte by differential pulse polarography at the 1 - 1 00 mg L- 1 level can be used for its determination at the ultra-trace level by adsorptive stripping voltammetry by simply changing the electrode to an HMDE, MTFE, GC, Pt, Au or modified electrode and including the accumulation step prior to the differential pulse cathodic or anodic voltage sweep. However care should be taken. While some adsorbable impurities in the sample may not interfere significantly with the DP-polaro­ graphic determination, they may interfere with the adsorptive stripping voltammetric determ ina­ tion because the analyte is present at much lower concentrations when using AdSV. The importance of AdSV in the trace analysis of adsorbable organic molecules has been summarised [ 68 ] . Potentiometric stripping has been used as an alternative to voltammetric stripping for the determination of adsorbed proteins [ 69] . Several compounds such as the triazines simazin and ametryn , the organophosphates guthion, diazinon, thiram and dimethoate, the nitro compounds DNOC and dinoseb can be determined in low organic content aqueous samples without any pre-treatment. For instance, the determination limit for DNOC in tap water is ca. 0. 1 flg L- 1 and in slightly contaminated river water ca. 1 flg L- 1 • The extent that impurities interfere may to some extent be overcome by chang­ ing the accumulation potential and/or accumulation time and using standard additions to evalu­ ate the concentration.

Appl ications: organic species

207

However, what is more common is that when a sample contains species in addition to the analyte which are adsorbed onto the electrode these substances interfere with the analysis, partic­ ularly if t hey are more strongly adsorbed or present in much higher concentrations than the ana­ lyte. The interfering species are preferentially adsorbed and prevent accumulation of the an alyte. In such cases the analyte must be separated from the sample matrix and the interfering species by high performance liquid chromatography, by solvent extracti on or by means of a suitable solid phase extractant. Substances which have been determined in the lower J.lg range by AdSV after separation from a solid or liquid matrix by solvent extraction include phosalone, carbofos, methamitron and isomethiozone in soil samples. The determination of the pesticide fenchlorazol- ethyl ( cthyl- 1 (2,4-dichlorop henyl ) - 5 - trichloromethyl - ( I H ) - 1 ,2,4-triazol- 3 - carboxylate) i s a n example in which solid phase extraction i s used to both extract and enrich the analyte ( 70 ] .

L-1

Application 26

Determination of fenchlorazol-ethyl by adsorptive stripping voltammetry [70]

Sa mple: Pre-treatment:

Solutions: Determ ination :

Dri n k i n g water Add aceti c acid to 1 L of sample until the pH is 2 . 8. Pass sam p l e at 5 m l m i n-1 through a 6 m l sol i d phase extraction col u m n ( Baker octadecyl or Su pelco ca r­ bopack) pre-treated by wash i n g with 2 x 5 ml C H 3 0 H , 2 x 5 ml pure water and 2 x 5 ml acetic acid (pH 2.8). E l ute a n a lyte with 2 ml C H 3 0 H . 0 . 1 M NaOH . Di ssolve 2 5 mg fen c h lorazol-ethyl i n 2 5 0 m l C H30 H . D i l ute this stock solution as req u i red to obta i n worki n g sta ndard solution s . Place 2 0 m l 0 . 1 M N a O H i n t h e voltammetric cel l a n d deaerate with n itrogen for 5 min then add the 2 m l of e l u ate a n d deaerate for 5 m i n . Using as l a rge a d rop as possi ble, accu m u late fench lorazo l-ethyl on the H M D E at -0 . 1 V for 3-1 0 m i n with stirri n g . Record the D P-stri pping volta mmog ram f ro m -0 . 1 V to - 1 . 3 V. U se the peak at -0. 9 V (ve rsu s Ag/Ag C I , 3 M KCI) and sta ndard addi­ tions to determine the concentration of fench lorazol-ethyl ( F i g u re 7 . 1 2). Deter­ m i nation l i m it 0 . 2 �g L-1 .

I [nA]

c

,.---,-....,..--.,----,--...,--, a

- 0 .2

Figure 7 . 1 2

---

- 1 . 3 - 8.8 E [V] (vs. Ag/AgCI, 3 M KCI)

--

---

8

16

c [n g

mL-1]

AdSV stripping determ ination of fench lorazol-ethyl after solid phase extraction from 1 L d rinking water and 1 0 m i n accu mulation at -Q.1 V. Eval uation by standard add itions: (a) 20 m l 0 . 1 M NaOH + 2 ml eluate, (b) a + 8 �g L-1 fenchlorazol, (c) a + 1 6 �g L-1 fench lorazol.

Tab l e 7 . 1 1

Adsorptive st ripping voltammetric dete r m i n ation of pesticides in various samples [ 72 ] .

Substance

Sample

Ametryn

River water

Bensultap

C ar h o fos

D i a z i non

Potato, river water Soil

Pre-treatment

E." -

Tol uene extraction \-\later/acetone extraction

2 ,4- Dichlorophenoxyacetic acid

Dimethoate

River water

Dinobuton

River water

DNOC

Fenitrothion

(V)

0.7

River water River water

lsometiazin

Nit ralin

Soil

-0.72

- 1 .20

H M D F.

+0. 1

0.0

HMDE

-0.70

HMDE

0.0

Paraquat

Water

Complex with

Phosalon

Water

CH2Cl2 extraction

Soil

Water/acetone extraction

B(C6H5)4-

MTFE

HMDE

-0.44

HMDE

0.0

-0.40

Hg!Pt

0.0

-

0 .70

1 .7

1 .0

0.6 0. 1 21

3 0.5

0.0

-0.56

-0. 3 -0. 1 -0. 1 0. 5

-0. 7 1

-0.51

-0.70

0.78

-

HMDE HMDE HMDE HMDE MTFE

River water

-LOS

Solid phase extraction

-0.2

-0.6

HMDE

Water*

Solid phase extraction

0.0

+0.6

Au

-0 . 8

-0. 3

1 .5

0.9

0.2

1 .5

22 2

-0.75

Water, soil

Anodic .> tripping

1 .2

Hg

Simetryn

•

92

Hg

Thiram

Complex with fe( I I )

3.5

�. 0

0.2

-0.32

HMDE

2,4,5-trichlorophenoxyacctic aci

§:

-0.64

-0.5

-

0. 4 5

Hg

HMDE

N 0 ao

(J.Ig l-I; !Jg lcg-1 )

-0 . 3

-

Simazin

-0.46

HMDE

Determination limit

-0.2

River water CI I1Cl1 extraction

HMDE

-0. 1

2nd peak more sensitive

Guthion

-1 .02

-0.4

-0.2

-

Electro de

-0 .93

-0.3

Note

EP ( V)

-0.6

Hydrolyse at pH 1 2

River water

Ethylenedithiocarbamate

· · · --

4

0.4

0.3

1 .5

0

0r:::

::>

....

0 < 0

3 3

iif

�

;::; · )> ::> Q>

-< "'

;;; ·

Applications: organic species a

EP - 0 . 75 V

I

209

b

i [nA] 800

ro nA

600

- 0.9

0

Figure

7.1 3

E

0

10

20

30

40

50

60

70

c [j.Lg L-1 ]

[V]

Determ ination of thiourea (6 . 5 IJg L-1 ) by cathodic strippi n g voltammetry showi ng the voltammogram and non-linear calibration curve.

Those compounds that form slightly so l u ble co mp o u nds with me r cu r y or silver ( Table 7 . 1 ) a re determined polarographically via the anodic wave o r by s t ri ppi n g voltammetry after a n o dic accu­ mulation. Among this gro up are agrochemicals, many of which containing thiourea or other thio compounds, which are often determined by c ath o d i c stripping with l i m i ts of determination in the sub-micro m o la r range [ 7 1 ] . A selec ti o n of pesticides that can be determined by ca t h o d i c strip­ ping vol tammetry after anodic accumulation is listed in Table 7. 1 1 [ 72 ] . Other sulfur-containing compounds which readily form insoluble Hg( I I ) co mp onds o n the surface of an a n o dic ally p o l a rised electrode (equ a t ions 3.8 and 3.9) are u su all y determined a t very low levels by cathodically s t r i pp i ng the Hg(I I ) c o m po u nd from the electrode. A s e le c t i on of sulfur-contai n i n g compounds which may be determined by CSV is g iven in Table 7. 1 2. Application 2 7

Determination of thiourea by cathodic stripping voltammetry [ 7 1 , 73]

Sa m p l e : Solutions: Determ ination:

Aq u e o u s s a m p l e s,

pharmaceutica l products, u r i n e . 2 M NaOH i n pure water. D issolve 1 00 mg thiourea in 1 L p u re water. Dil ute this stock as req u i red to prepa re fresh standard solutions daily. For thiourea concentrations between 0. 2-2 mg L- l . Add 10 ml sam ple so l uti o n to 1 0 m l 2 M NaOH i n t h e v o lt a m m e tr i c ce l l a nd dea erate with n itroge n . U s i n g a D M E, record t h e D P-pola rogram f r o m -0 . 4 5 V to -0 . 1 5 V. T h e a n o d i c pea k due to thiourea occu rs at ca. -0 . 2 6 V (versus Ag/Ag C I , 3 M KCI). Determ i n e the concentration by sta ndard additions.

Table -

7.12

Selected sulfur-containing compou nds which m ay b e determ i n ed by cathodic st ripping voltammctry.

Compound

Supporting electrolyte

Cysteine

0. 1 M Na2Bp7

Penicillam inc (2,2 -dimethylcysteine)

0 . 1 M Na 2 B407

0.02 M NaCI

Electrode

HMDE

HMDE

E"" (V vs SCE)

� (V vs SCE)

Linear

+0.2

-0.49, -0.68

Linear

-D.S l

6 - Mercaptoxanthine

HMDE

+0.2

-D.72

2 - Merca ptopurine

HMDE

+0 . 2

-

HMDE

+0.2

-0 .3

oxidiscd form redu ced form Thioamides ( various)

2- Thiobarbiturates

BR - buffer

(mol L-1 )

-D.59

+0.2

Glutathione

Determination limit

+0.2

HMDE

2 - Mercaptoethanol

Stripping waveform

0 .48 ,

-

l .O X 1 0-R

2.0

X

1 0'8

+0.2

HMDE

+0.05

HMDE

-0.90 (Ads)

p H 4.78

BR - buffer

pH S Lau rylsulfonic acid

pH 8 (with NaO H )

Dodecylbenzenc-sulfonic acid

pH 8

HM D E

-0.70 (Ads )

-0.46, -0.52

-D.3 - 1 .3 -1.2

to

-D.06

X

1 0'8

Linear

2.0

Dif[ pulse

ppm

Diff. pulse

w·6-w·8

Diff. pulse

8-. 0 "'

5'

::::! . ,.., )> "' "'

� v; ·

-0. 2

HMDE

c.. <::

3 3 !!!.

pH 4.78

BR - buffer

3' a

< 0 ;::;"'

0.85 Linear

N ... 0

1 0·6 to 1 0·8

Appl ications: organic species

21 1

Column conditioning 2 x 2 ml water 2 x 2 ml methanol

�

Pass sample through the column Flowrate 4 m l m i n 1 -

Wash column 50 m l pure water

Dry column 20 min under water-jet

�

suction

Selective elution

/ For TBT x 700 f.l l methanol

3

E l u ate + 8

+

200 11l

m l acetate buffer (pH 4.5) in methanol

1 o-3 M t ropolone

AdSV determination of TBT Eacc - 0.60 V; Ep - 0 . 8 1 V

Figure 7 . 1 4

Notes:

' For DBT und MBT x 700 f.ll 1 0-3 M tropolone in methanol

3

E l u ate + ( p H 4.5)

8

ml acetate buffer

AdSV determination of DBT and MBT E acc - 0.5 V ; E - 0.65 V for M BT; P E - 0.75 V for D BT P

F low diagram of the method for the AdSV determination of tributyl-, di butyl· and monobutyl-tin as the tropolone complex after solid phase extraction.

For concentrations between 5-60 �g L- 1 . Add 1 0 ml sample to 1 0 m l 2 M NaOH i n the ce l l a n d deaerate with n itroge n . Acc u m u l ate thiourea on a n H M DE at -0 . 2 V fo r 1 20 s with stirri n g . A l l ow 30 s rest time at -0. 2 V. Record the DP­ stripping voltam mogra m from -0. 5 V to -0. 9 V. Use the cu rrent peak at ca . -0 . 7 5 V to d e t ermi n e the thiourea concentration from the non-l inear ca libration curve (Figure 7 . 1 3). Th is curve m ust be determ i ned using stan dard solutions. The largest possible mercury d rops should be used . C h l oride co n c en t rat i o ns u p t o 1 0 mg L-1 do n o t i nterfere with t h e stripping voltammetric determi nation . Sam ples with > 2 mg L- 1 thiourea s h o u l d be d i l uted for the DP polarogra p h ic ana lysis.

21 2

I ntroduction to Voltammetric Analysis

Organotin compounds have been used as anti -fouling agents in marine paints, as fungicides and as a stabi liser in PVC. The use of tributyltin (TBT) compounds as pigments in paints for ships has led to this compound and its metabolites dibutyltin ( DBT ) and monobutyltin (MBT) appeari ng in harbour and estuarine waters. The individual compounds can be determined by adsorptive stripping voltammetry in the presence of tropolone. While the peaks for DBT and MBT are well separated so that these two compounds can be determined in the presence of each other, the peak for TBT is too close to that of DBT for them to be measured in the same solution. Consequently a preliminary separation of TBT from its metabolites using solid phase extraction is needed. This preli minary extraction also separates the organotin compounds from possible interferents and results in a significant enrichment of the analyte and increase in sensitivity of the determ i nation. The analytical scheme is summarised in Figure 7 . 1 4 [ 74] . The determination limits are reported to be 2 1 0 ng L-1 for TBT, 30 ng L-1 for DBT and 40 ng L- 1 for M BT (as Sn) . The method is suitable for surface, sea and drinking waters. The HMDE is the electrode usually used for adsorbing organic compounds which are then reductively determ ined. Carbon or noble metal electrodes are used for the accumulation of mole­ cules with oxidisable functional groups. These include a number of pharmaceuticals, bio-mole­ cules and pesticides [ 33, 75 ] . A selection o f polarographic and voltammetric determinations o f organic compounds is given in Appendices 4 and 5. Applications to the determination of compounds in a range of matrices other than medical and biological samples are listed in Appendix 4 and applications in medical and biological analyses are listed in Appendix 5 .

7 . 3 AMPEROMETRIC M ETHODS The main uses of amperometric detectors for the determination of organic compounds are as amperometric biosensors and as amperometric flow-through detectors used in flow injection anal­ ysis and liquid ch romatography ( § 7.4). When the membrane o f an amperometric sensor is coated with a layer of a suitable enzyme, the sensor can be used to determine the concentration of species that react with that enzyme. These amperometric biosensors [ 76, 77] are usually very selective because of the highly specific nature of most enzyme reactions. A good example is the glucose sensor used in clinical investigations. This sensor is essentially the same as the oxygen sensor (Figure 6. 1 9 ) except that the membrane consists of three layers. The outer layer is a polymer layer which is permeable to glucose but impermeable to proteins and other blood constituents. The middle layer contains the immobilised glucose oxidase (GO) and the i n ner layer i s a membrane permeable to small molecules such as H202• When the sensor is dipped into a glucose solution , the glucose diffuses through the outer layer to the enzyme layer wh ere the fol l owing catalytic reaction occurs: glucose + 02

+

H20 --G'-.'0--7 H202 + gluconic acid

The hydrogen peroxide then diffuses through the i nner membrane to the anode surface where it is oxidised at + 0 . 7 V (vs SCE) according to the reaction

The current produced by this reaction is proportional to the glucose concentration. Because the enzymatic oxidation is selective, it is possible to determine the glucose con ­ tent in t h e presence o f oxygen . The glucose oxidase is immobilised on the oxygen-permeable

Applications: organic species

Table 7 . 1 3

Examples o f am peromctric biosensors

[77j .

Substrate

Enzyme

Produce

Measurement range ( mol L- 1 )

Choline

Choline-oxida�e

H Pz

0.5

Ethanol

Alcohol-oxidase

H zO z

Fomaldehyde-

NADH

2 . 2 X 1 0 -4 1 0-6

Glucose-oxidase

HPz

Glutaminase,

HPz

0-4 X 1 0-

For ma ld e hyde

Gl u cose

Gl ut am in e Hyp ox a nt hine Lactate

M al a te

Ol igosaccharides

-· ·

dehydrogenase

glutamate- oxidase

2

o-2 . s x w-2

Xa n th i n e-oxid ase

H zOz

(4- I BO) x

Lactate - oxidase

HzO z

( 1 -40)

Dehyd rogenases

NAD H

( 5- 1 00 )

Glycoamylase,

H zOz

( l-25)

glucose-oxidase

213

X

1 0 __,

X

X

10-•

1 0-6

1 0-4

membrane of an oxygen electrode. Since the formation of gluconic acid consumes oxygen , less oxygen diffuses t h rough the membrane into the oxygen electrode and the glucose con­ centration can be determined from the decrease in the current produced by the reduction of oxygen at -0.65 V ( versus S CE ) . T h e difference between the oxygen-enzyme electrode and the glucose ( H202 indicating) el ectrode i s essentially the p otent ia l applied to the working electrode. The oxygen-hydrogen peroxide electrode can also be used for the determination of other clinically impo r ta n t compounds as can be seen from t h e exam p l es of biosensors listed in Table 7. 1 3 . Other amperometric biosensors are based on the detection of a coenzyme such as NADH or NAD+, the reduced and oxidised forms of the coenzyme nicotinamide aden ine dinucleotide (Table 7. 1 3 ) . This coenzyme serves as a hydrogen carrier in many enzymatic redox reactions.

7.4 FLOW-TH ROUGH DETECTORS

Amperometric and voltammetric detection after HPLC separation has been used to detect easily oxidisable phenols, aromatic ami n es, indoles, phenothiazines, thiols and polyaromatic hydrocarbons and easily reducible nitro compounds and quinones (Table 7 . 1 4 ) . These methods have been used in environmental, pharmaceutical, biochemical and food analysis, as well as for determining hormone residues in soil and water samples and in plant and animal products [ 78-82 ] . The main application of HPLC with amperometric detection is i n the analysis o f phenols and aromatic amines such as the catecholamines which can be detected with high sensitivity in the lower pg region in body fluids and biological samples ( Figure 7. 1 5 ) . With an inter-digitated arrray electrode in a flow-through cell it is possible to determine dopamine down in the fg region ( 84 ] . The determination o f adrenalin, noradrenalin and dopamine is used in the diagnosis of, and is being considered as an i ndex in, neurological illness. The level of methoxyhydroxyphenylglycol ( MHPG - a metabolite of noradrenalin) in urine is used in the evaluation of depression. Other applications of HPLC with amperometric detection include the determination of antibiotics and estrogen in meat [ 9 1 , 92 ] , of phe nolic compounds, ascorbic acid and flavonoids in drinks and foods, of chloramphenicol in fish products [ 1 2 1 ] a n d of penicillin in milk and beef [ 1 22 ] .

214

I ntroduction t o Voltammetric Ana lysis

Ta b l e 7 . 1 4

Amperometric detection of organic compounds following HPLC sepa ration.

---- · · · ---

Detection potential

(V vs Ag!AgCI, 3 M KCI)

Compounds Aromatic hydroxy compounds

Reference

4 , 8 3 , 1'1 4

Catecholamincs

+0.6

Halogenated phenols

+0.6 to + 1 .2

85, 86, 87

Flavone

+1.1

88

A n t i ox i d a n t s

+ 1 .0

89, 90

Estrogen

+ 1 .2

9 1 , 92

Hydroxybenzo [a] pyrenc

+ 1 .3 5

93

Aniline

+ 0 . 8 t o + 1 .0

9 4 , 95

Benzidine and o ther diamines

+ 0 8 to + 1 .0

Sulfonamidcs

+ 1 .2

Aromatic amines

lndoles

Indolyl - 3 - compounds, 5 -hydroxyindole

.

96

97

+0.8 to + 1 . 2

98, 99

Phenothiazines

Chlorpromazine, thoridazin

+ 1 .2

82,

Cysteine, glutathione, penicillamine

+0.6 to + 0 . 8

1 0 1 , 1 02

Uric acid, adenine, caffeine

+0. 1

Theophylin, theobromine

+ 1 .4

1 03

+ 1 .4 5

1 04

1 00

Thioles

Purines

Hydrocarbons

Polycyclic aromatic hydrocarbons Vitamins

Vitamins A, C and E

+0.8 t o + 1 . 1

+1.3

1 05, 1 06

+0.7 to + 1 . 2

108, 1 09

Phenylureas

+ 0 . 8 to

1 1 0, 1 1 1 , 1 1 2

Carbamates

+0.9

Bentazon , phenoxycarboxylic acids

+ 1 . 25

Pentachlo rophenol

+ 1 .0

Bromoxynil, loxyn il

+ 1 .2

Vitamins

D2

and D�

1 07

Narcotics, Addictive drugs

morphi ne, ethylmorph i ne, acetyl morphine, diacetylmorphine,

Cocaine,

hallucinogens, anti-depressives, etc. Pesticides

Metomyl ,

Methomyloxime

Dithianon

+ 1 .3

-0.05

+ 1.3

to + 1 . 3

to + 1 . 3

1 1 1, 1 13, 1 1 4

115 1 16

1 1 7, 1 1 8

119

1 20

Applications: o rg a n i c species

21 5

Noradrenalin

I

Adre n alin Dopam ine

0

Figure 7 . 1 5

3

6

1 00

pA

9 t [min]

Chromatogram of noradrenal i n , adrena l i n and dopamine (200 pg each) detected by the Metrohm electrochemica l detector with a carbon paste electrode at +0.9 V.

Phenolic antioxidants, such as tert-butyl hydroxytoluene, tert-butyl hydroxyanisole, propyl gallate and tert-butyl hydroquinone, used as preservatives in foods are determined amperometri­ cally at +0.7 V (versus Ag/AgCl, 3 M KCl) using a modified nickel electrode after HPLC separa­ tion . The determination limit lies between 0. 1 and 0.6 mg L- 1 [ 1 23] . A phthalocyanine-modified cobalt electrode is used to determine various hydroxylamines after H PtC separation [ 1 24 ] .

REFERENCES I 2 3

4 5

6

7 8 9

10

II 12 13

14

16 15

P.M. Bersier and J. Bersier, CRC Critical Reviews in Analytical Chemistry, 1 985, 1 6, 1 4 . P. M. Bersier and J. Bersier, CRC Critical Reviews in Analyt ical c.:nemistry, 1 985, 1 6, 80. W. f. Smyth, CRC Cr itica l Reviews in Analytical Ch e mis t ry, 1 9 8 7 , 1 8, 1 5 5 . J. Wang, Electroanalytical Techniques in Clin ical Ch e m is t ry and Labo ratory Chem ist ry, Ver l a g Chcm ie, Weinheim, 1 981!. J.P. Hart, Electroanalysis of Biologically Importa n t Compounds, in Analytical Chemistry, H . G . Fogg, A.A. Barros and J.O. Cabral, Analyst, 1 986, 1 1 1 , 83 1 . J. Vo l ke , Bioelectrochemistry a n d Bioenergetics, 1 983, 1 0, 7 .

Substituem F!ffects in O rga n ic Polarography, Plenum, N ew York, 1 967. Organic Polarography, I n tcrscie nce, New York, 1 969. M . von Stackelberg and W. Stracke, Z. Elekt roch emie 1 949, 53, 1 1 8 . J. van Beemen and ] . Delcy, ]. Analytical B i oc h emis t ry 1 97 1 , 44, 254. S . Wawzonek and T. W.Mdntyre , J. Electroanalytical Chem istry, 1 966, 1 2, 544. S . Wawzo nek, Analytical Chemistry, 1 952, 24, 32.

Ellis

Harwood Series.

P. Zuman,

P. Zuman and C . L. l'erri n ,

,

,

G . Henze and R. N eeb, Elektrochem ische Analytik, Springer Verlag, B e rli n , 1 986.

G . K . Simpson and ]. Traill , lliochemical /. , 1 946, 40, 1 1 6. P. M . Bersier, Hagers Handbuch der Pharmazeu tisclien Praxis 2: 500 Me th o de n , Polarographische, Volta mmet rische and Twsammetrische Analyse, E. Nii rnberg and P. Surman n , Editors, 1 99 1 .

216 17 18 19

20 21 22 23 24 25 26 27

28

29 30 31 32

I ntroduction t o Voltammetric Ana lysis Met ro hm Ap p lic a t i o n Bu lletin No. 1 9 6 1 2 e a n d M et ro h m Ap p l i c at ion Note No. V 67. P. Kabasakalian a n d J , M cG l ot te n , Analytical Ch em ist ry, 1 959, 3 1 , 1 09 1 . P. M . Bersier and }. Bersier, Wilson and Wilson's Co mp reh en s ive A n a lytica l Chemistry, Vol . XX.V ll, G. Svehla, Editor, Elsevier, Ch. 4, Analytical Vo l tam m e t r y in P h a r m a cy, p. I 59. A. Lec h i e n , P. Valen ta , H.W. N ii r n be rg , and G.J. Pa t r i ac h e , Fresenius Z. A n a lytisch e Chemie, 1 982, 3 1 1 , 1 0 5 . S . Kozar, A . Bujak, J , Ed e r- Trifunovic and G . Kn iew ald , Fresenius Z. A n aly t isc h e Chemie, 1 988, 329, 760. Metrohm Application B ul l e t i n No. 98 I 1 d and Metrohm Application Note No. V 56. A. Voulgaropolous, P. Va le n t a and H.W. N ii rn b e rg , Fresenius Z. Analytische Ch e m ie, 1 984, 3 1 7, 367. H . Hriischwei l er a nd G.C. Mi n ko ff, A naly tica Chimica Acta, 1 955, 12, 1 85. J.C. Vire a nd G . } . Pa t r i a rch e , A n alys is, 1 978, 6, 395. P.M. Bersier, Vo l ta mm e tr isch e, Po larograph ische und Temammetrische A n a lytik Orga n isch er und Metallorgan ischer Stoffe, Analytiker '1aschenbuch 1 7, Springer-Verlag, Be rl i n , 1 988. P. Surmann, Quan titative A na lyse v on Arbeitstoffen und Z u b ere i tu ngen , Wissenschaft Verlagsgesellschaft, St u tt g a rt , 1 9117. P.M. B e rs ie r and }. Bersier, Wilson and Wilson's Comprehensive A n a lytica l Che m is try, Vol. XX.Vll, G. Svehla, Editor, El s e v ier, Ch. 4, An a ly t i ca l Voltammetry in En v i romne ntal Science, fl. O rga n ic a r1d Organometallic Species, p. 38 1 . M . Geissler, Polarog rap ische An a lys e, Akadern ische Verlagsgesellschaft, Geest and Po r t i g , K.-G. Leipzig, 1 980. J.S. Burmicz and W.F. Smyth, Analytical Proceedings, [ London ] , 1 9110, 1 7, 2114. H .M . Pyly p iw and G.W. H ar r i n g ton , A nalytical Chemist ry, 1 9 8 1 , 53, 2 3 56. Hlect roche m ist ry in Ca n ce r Research, A n a ly tica l Pro ceedi ngs, [ London ] , 1 980, 1 7, 278, (Conference N.A. La n n i ga n , J.R. J ohnson and R.H . l'ryce-Jones, ]. Pharmacy and Pharmacology, 1 98 1 , 33, 9P.

proceedings).

33 34 35

36 37 38

39 40

41 42 43 44

46 47 411 49

45

50 51 52

53

54 55

56

57 58 59 60

61 62 63 64 65 66 67 68 69

S. S i l ves t r i , Pharmaceu tica Acta Helvetiae, 1 975, 50,

304.

10, 25. W.F. S myth , M .R. Smyth, }.A. Groves and S.B. Ta n, A n a lyst, 1 978, 1 03, 497. E. Jacobsen and T.V. Jacobsen, A n a lytica Ch im ica Acta, 1 9 7 2 , 60 , 472. M.R. Smyth and }.G. O s te r yo ung , A na ly tica l Chem istry, 1 9 78 , 50, 1 632. W. F. Smyth and J.A. Groves, Anulytica Ch im ica Acta, 1 98 1 , 123, 1 75 . P.M. Hersier and J, Be rs ie r, Trends in A na lyt iml C hem is t ry, 1 986, 5, 97. Metrohm Ap p l i ca t i o n Bulletin No. 5 7 I I d. Metrohm Application Bulletin No. 2 5 1 1 1 d. G . Sontag and G. Kainz, Mik roc h i m i ca. Acta, Wein, 1 977, 2, 425. Metrohm Ap p l i c at i o n Hulletin No. 2 1 8 I 1 d. T. Vc rg are , D. M ar ti n and } . Vera, A n alytica Chimica Ac ta, 1980, 1 20, 3 4 7 . F. . Jacobsen and M. Bj o r n se n , A�1alytica Chimica A cta, 1 978, 96, 345. Metroh m A p pl i c a t i o n flul l etin No. 2 1 9 1 1 d. }. Wang, D. B. Luo, P. A . M . Farias a n d J.S. Mahmoud, A�1alytical Chemistry, 1 985, 57, 1 58. H . Oelschlager, Bioelectrochemistry and Bioenergetics, 1 98 3 ,

Metrohm Appl ication flull etin N o . 1 9 1 1 1 d. R . M . S m yt h and W.F. Smyth, A 11alyst, 1 978, 1 03, 529. P. Boublikova, M . Voj t is ko v a and E . Pa l e c e k , Analytiml T.etters, 1 98 7 , 20, 2 7 5 . Metrohm Application B u l l et i n No. 1 36 I 1 e a n d Metrohm Application No t e No. V 64 . J.P. H ar t, W. F. Smyth a n d B.J. Birch, A na lys t, 1 979, 1 04, 853. M . A . Ko ra ny a n d H . R i e de l , Fresenius Z. Analytische Chemie, 1 9113, 3 1 4, 678. P.M. Be rs ie r and J, Bersier, A n a lys t, 1 988, 1 1 3 , 3. W.A. Ma c Cre h a n , R.A. D u r st and J . M . B e lla m a , An aly tical Letters, 1 977, 10, 1 1 75 . }. Schw arz , G. Henze a n d F. G. T ho m a s , Freseni us ' ]. A n a ly t ical Chemistry, 1 995, 3 5 2 , 474. A.M. B o n d , } . It B r a db u r y, G . N . Howell, H .A. Hudson, P.J. Hanna a n d S. S t ro t her, ]. Elect ro a n a ly t ica l Chem istry, 1 983, 1 54, 2 1 7. W.F. Smyth a n d }.S. B urmi cz, O rga ni c S tripping Analysis, Met ro h m , Herisau, 1 989. R. Ka l vo d a , A na ly t i ca C/t imica Ac ta , 1 982, 1 38, 1 1 . M . Pedrero, F.J. M a n u e l d e Villena, } . M . Pin garron and L. M . Polo, Electroanalysis, 1 9 9 1 , 3, 4 1 9 . C. Li , B.D. James and R.J. M ag ee, Electroa nalysis, 1 990, 2, 63. }. Barek, A.G. foogg, J.C. Moreira, M . V. B . Zanoni and }. Zima, A naly t ica Chimica Acta, 1 996, 320, 3 1 . L. Hernandez, A. Zapardiel, }. A.P. Lopez and E. Bermejo, Analyst, 1 987, 1 1 2, 1 1 49. L. H e rn a n d ez, A. Zapardiel, J.A. P. Lopez a n d E. B e r m ejo , udam ta, 1 91111, 35, 287. J, Wa ng , T. Peng and M . S . Lin, Bioelectrochemis t ry and B i o en erge tics, 1 986, 15, 1 47. N . E. Z o u l i s , D.P. N i co l el i s and C.E. Efstathiou, Analyst, 1 990, 1 1 5, 29 1 . R Kalvoda, Pure and Applied Chemistry, 1 987, 59, 7 1 5. M . J . H on e yc h urc h a n d M . J. Ridd, Electromwlysis, 1 996, 8, 654.

Applications: organic species

70 71

72 73 74 75 76 77

78

A. M eye r and G. H e n ze , Fresen ius' f. A n a ly t i ca l Ch e m is t ry, 1 994, 350, ! 5 0 . V. Stara and M . Ko p an i c a, A n a ly ti ca Ch im ica Acta, 1 984, 1 59 , 1 0 5 . A. Meyer and G. H en ze , GI T l:'a ch zei ts chr ift fur da s Labora torium, 1 99 5 , 39, 1 0 5 1 . Metro h m Application Bulletin No. 1 92/ 1 d . ). S c hwar z , G. Henze and F. G. Thomas, Fres eniu s ' f. Analytical Ch e m is t ry, 1 995, 3 5 2 , 4 7 9 . A.Z. Ab u Zuhri and W. Vo e l te r, Fresenius' f. Analytical Ch e m is t ry, 1 998, 360, 1 . F.. A . H . Hal l , Iliosensoren, Springer-Verlag, 1 995. K. Cam m a n n , U. Lemke, A . R o h en , ). Sander, H. Wi l k e n and B. W i n te r, Angewa ndte Chem ie, 1 99 1 , 1 03, 5 1 9 . K. St u l i k and V. Pacakova, Electroanalytical Mea s u reme n ts itr Flo wi trg Liquids, Ellis Ho rwo od , C h i c h e s te r, W. Buchberger, Elek t ro ch e m isch e Analyseverfahren, Spektrum Akademischcr Ve rl a g , Berlin, 1 998.

1 98 7 .

79 80 81 82

83 84

85

86

211

Electroanalysis of Bio log ica lly Imp o r ta n t Compounds, Ell i s

D . M . Radzik a nd S.M. Lu n te, CRC Critical Reviews i n A n a ly ti ca l Ch e mis try, 1 989, 20, 3 1 7 .

J . P. Hart,

H o rwood, C i c h e st er,

1 990.

G. Mu sc h , M .

D e S m et and D.L. Massart, f. Ch ro m a tog rap hy, 1 985, 348, 97. F. Cheng, N. Lin , ) . Ku o, L. Cheng, F. Ch ang and L . C h i a , Electro analysis, 1 9 9 4 , 6 , 87 l . 0. Niwa, H . Tab e i , B . P. Solomon, F. X i e a n d P.T. K i s s i n ger, f. Chro m a tog raphy, 1 99 5 , B 670, 2 1 . E. Pocurull, G . Sanchez, F. Borrull and R . M . Ma rce, f. Ch ro m a togra p hy, 1 995, A 696, 3 1 . N . Cardellicchio, S. Cavelli, V. Piangerelli, S. Giandomenico and P. Ragone, Fresenius' f. A naly tical Ch e mis t ry,

B. Paterson, C.E. Cowie and P. E. Jackson, f. Ch ro ma tograp hy, 1 996, A 73 1 , 9 5 . S . M . Lunte, K.D. Blankenship a nd S . A . Read, Analyst, 1 9 8 8 , 1 1 3, 99.

1 997, 358, 749.

87 88 89 90 91

92 93 94 95

96 97 98

W.P. Kin g , K.T. J o s e p h and P.T. K i s s i n ge r, f. A sso c i a t ion of Offi c ia l A n a ly t i ca l Chem ists, 1 980, 63, 1 3 7 . ) . Uil ig e r and F. S a u cy, Z. Lebensmittel- Untersch uch ung rmd -Fo rs ch u ng, 1 980, 1 70, 4 1 3 . K . Shimada, F. Xie an d T. N a mb a r a , ] . Chrom a tography, 1 982, 232, 1 3 . K. Sh i m ada , M. Hara and T. Nambara, Chem ical and Pharmaceu t ica l Bulletin, 1 98 5 , 33, 685. H . - P. Nirma ier, E. Fi s c h e r and G . Henze, E lea ro a n a lys is, 1 998, 1 0, 1 8 7. M .C. Hen n i o n , P. S u br a , V. Coquart a n d R. Rosset, Frese n iu s ' f. An a ly ti ca l Ch e m i st ry, 1 99 1 , 339, 4 8 8 . M . S . Va rney a n d M.R. Preston, f. Clr ro ma tography, 1 98 5 , 348, 265. P. Trippel- Schulte, ). Zeiske and A. Ke t t r u p , Chromatographia, 1 986, 22, 1 3 8 . M . A . Alawi a n d H . A . Russel, Ch romatographia, 1 98 1 , 1 4, 704. G.M. Anderson, ).G. Yo u n g , D.K. Ba t te r, S.N. Yo ung, D.J. C o h e n and B.A. Shaywitz, f. Chroma tography, 1 98 1 ,

223, 3 1 5. 99 H.). Humbert, ). Denouel and H . P. Keller, f. Chromatography, 1 987, 422, 205. 1 0 0 K. Murakami, T lJ"eno, ) . H ij ka ta , K. Shirasawa and T. Muto, ]. Chro m a tography, 1 982, 227, 1 03. 1 0 1 C . G . Hon egger, H. Langem a n n , W. K r e n g e r a n d A . Kem pf, ]. Ch ro m a tog rap hy, 1 98 9 , 487, 463. 1 0 2 N . C. S m ith , M . Du nnett and P. C. M ills, ]. Chroma tography, 1 995, B 673, 35. 103 A. Meyer, Ngiruwonsangga and G. Henze, Fres en i u s ' /. A n a ly tical Ch em is try, 1 9 9 6 , 3 5 6 , 2 8 4 . 1 04 ) . Schiewe, A.M. Bond and G. H e nze , Umweltdiagnost ik mit Mi k rosyst eme n , G. Henze, M. Kohler and J . P. Lay, Editors, Wiley-VCH, 1 999, p. 1 2 1 . 1 0 5 M . E. Murphy and J.P. Keher, f. Chromatography, 1 98 7 , 42 1 , 7 1 . 1 06 ).). Hagen, K.A. Washco and C.A. Monnig, f. Ch ro m a tograp hy, 1 996, B 677, 2 2 5 . 1 07 J . P. Hart, M . D. Nor m a n a n d C . ) . L a cey, A n a lyst, 1 992, 1 1 7, 1 44 1 . 1 08 j . O. Svensson , Q.Y. Yue and ) . S awe , ]. Ch ro m a tograp hy, 1 995, B 674, 49. 1 09 G . Achilli, G . P. Cellerino, G.V. Melzi d'Eril and F. Tagliaro, ]. Chroma tography, 1 996, A 729, 273. 1 1 0 M . W. F. Nielen, G . Koomen, R. Frei and U.A.T. Brinkman, f. L iq ui d Ch r o m a t ography, 1 985, 8, 3 1 5 I l l Q.G. von Nehring, J.W. Hightower an d ).L. Anderson, Analytical Ch emistry, 1 986, 58, 2777. 1 1 2 J. H au s e n , A. M e ye r and G. I I e n ze , Fresenius' f. A n aly tical Chemistry, 1 99 3 , 3 46 , 76 1 . 1 1 3 J.L. A n d e r s o n and D.). C he s n ey, An a ly tica l Ch em i s t ry, 1 98 0 , 5 2 , 2 1 56 1 1 4 ) . L. Anderson, K.K. Whiten, j . U . B re wster, T. - Y. O u and K . W. N o n i d e z, A n alyt ica l Chem istry, 1 9 8 5 , 57, 1 3 66. 1 1 5 W. Schussler, Ch ro m a t ograp h ia , 1 990, 29, 24. 1 1 6 K.D. Mc M u r t re y, A. E. Holcomb, A.U. Ekwenchi and N.C. Fawcett, f. Liquid Ch roma tography, 1 984, 7, 953. 1 1 7 W. Buchbe rger, H. Mal issa an d K . Wi n s a u e r, Mikroch i m ica Acta, (Wien ) , l 984, I , 5 3 . 1 1 8 A. Meyer a n d G . Henze, Fresetr ius Z. Atr a ly tisch e Chem ie, 1 989, 3 3 2 , 8 9 8 . 1 1 9 M.A. Alaw i and H.A. Ru s s el , Fresen ius Z A n a ly tis c h e Ch e m i e, 1 98 1 , 309, 8. 1 20 W. Buchberger and K. Wi n sa ue r, Mikrochim ica Acta, ( Wien ) , 1 980, 2, 2 5 7 . 1 2 1 S. Mannino and ). Wa ng, Electroanalysis, 1 992, 4, 8 3 5 . 1 22 S . Li hl , A . Rehorek and M. Petz, ]. Chromatography, 1 996, A 729, 2 2 9 . 1 23 M . A . Ruiz, E. Garcia-Moreno, C. Ba rb a s and ).M. P i ng a r ro n, Electroatralysis, 1 9 9 9 , I I , 470. 1 24 X. Qi and R.P. Baldwin, Electroa n alysi s, 1 994, 6, 353.

Appendices

A selection of ap p l icati o ns of vo l ta m m et ry ( and chronopotentiometry) to the determination o f in o r gan i c s p ecies is p r e s e nt ed in Appendices 1 , 2 an d 3 . Exa m p l es of the determination of inor­ gani c a n a l ytes in wat er samples are lis t e d in App e ndi x 1 , i n medical a nd biological samples in Appendix 2 and in s a m p l e s from i nd u stry, agriculture, metallurgy and other areas in App e n di x 3 . A s election of voltammetric m eth o ds for the determination of o rgani c substances in s a m p l e s such as foodstuffs, waters, and c o m m e rc i a l p r od u cts is g i ve n in Ap pe n d ix 4 while ex a mp les of the determination o f o rgan i c substances in b i ologica l and medical sam p l e s by voltammetry are listed in Ap p e n dix 5 . Because voltammetric and chronopotentiometric methods can b e u s e d t o determine analyte concentrations from the g L- 1 down to the ng L-1 range, the applications i nc l u d e d make use of a va r iety of measurement te c h n i q u e s suitable for the range of a nal y te concentrations encountered in practice. However e m p h asi s is placed on the use of the adsorptive st ri p p i n g techniques devel­ o pe d over the last 1 5 years or so. These have the sens i t i v ity needed to measure th e concentrations of toxic metals, pes t ici d e s and other po ll u tants in environmental sampl es and foodstuffs at the trace and ultra-trace levels required by legislation and to measure the concentrations of toxins and of drugs and their m etabo l ite s in b i o l o g ic al fluids and tissues at the low levels required in clin­ ical a n d r es e a rch s t u d i es . Metal ions a re simply listed as th e element, without c h a rge , when they norm all y occur in a single valance state ( no distinction is m ade between a metal in its zero valance state in an alloy and in its common va lanc e state in sol u ti o n ) . The oxidation s t a te of any analyte is o n ly i ndicated for those elements wh i ch c o mm o n ly occur in more than one valance state and fo r which a knowledge of the concentrations of the various valency forms in a s a mp l e is required. The ionic ch a rge is only shown on a n ions. Complex accumulation p ro cesse s in adsorp t ive stripping measurements are indicated. For example, in t he DPCSV determination of tin in the p re sence o f tropolone, d u ri n g the accu m ul a t ion step all the Sn in solution is first reduced to the m e ta l and dissolves in the mer­ cury electrode. The p o tenti a l is i n c r ease d to oxidise th e Sn back to S n ( I I ) which then forms a c o m ­ plex w i t h t ro p o l o n e that is adsorbed on the mercury surface. This is l i st ed in column 7 as Red.;Ox.,Ads. El ectrode potentials a rc quoted re l at ive to the Ag/AgCl, 3 M cr refe r e nce electrode except where noted as follows where they are relative to: AAg/AgCl, sat.KCl; 8 Ag/AgCI, 1 M cr; E t. rAg/AgCl, 1 0 M CC Ag/AgCl, 0.5 M Cr; Ag/AgBr, 0. 1 M TEABr; E S a t u r ate d calomel electrode; H + NN or mal ca l o m e l electrode; Ag/0.0 1 M Ag in a ce t on i tri le ; Standard hydrogen electrode. For those m e as ure m ent techniques which p r o du c e a s i g m o i d a l wave rather than a peak in th e i- E si gn a l ( DCP or LSV using a micro electrode ) , the El /2 value for the wa v e is listed u n d e r EP. Since there i s no unifo r m i ty in reporting t h e limit of d e tec t ion ( LOD) these sh ould be taken as guides only. The va l u es listed are those quoted by the author i r r e sp ecti ve of how it has been deter­ m i ne d . When no LOD has been quoted, the l o we r limit of the linear range of t h e iP versus concen218

Appendices

21 9

t ra ti on plot is listed or the compilers of the table have estimated a LOD from t h e voltam mograms in the original article if possibl e Si gn ific an t i nterferences are listed if mentioned in the original article: these may not be the only substances which interfere with a given determination. A list of abbreviations used throughout Ap p e n d i c e s 1-5 is given in Appendix 6. .

Appendix 5 Element

Organic species-biological and medical samples

Guanine

Halazepam

Matrix

Pre-treatment

Working electrode

·--- ·

Ac.cumulation process

HMDE

Adsorption

0.0

DPCSV

-0.2 1

Adsorption

-0. 1 5A

DPCSV

-0.4

Ac-pH 4.8, 10% CH,OH

HMDE

Adsorption

-0.55

DPCSV

-0.90

25 (urine)

Bor-pH 9.2,Cu2'

HMDE

Adsorption

-0. 1 5

DPCSV

-0.27

0 . 78

0. 1 KNC\·pH 10 in 75% CH ,OH

DME Adsorption

-0 . 4A

Urine, ta blets

SEx

CH,OH

0.01 M NaOH in 75% C,H,OH

Phos-pH 7.4

Std.soln.

HMDE

SMDE HMDE

Red.;Ads.

Adsorption

Red .;Ads.

Loprazolam

Std.soln.

0 . 04 M NH4Cl

SMDE

Methylnicotinamide

Std.soln.

Na,co,

GCE

Midazo1am

Phar.

BR-pH 5.0

HMDE

Adso rption

Mitomycin C

Clinical

Bor-pH 1 0.2

HMDE

Adsorption

SPEx

Urine

0. 1 M HC\04

Monensin

Std.soln.

0.2 M KF

SMDE

Naftazone

Std.soln.

NaCl04-pH 3.0

HMDE

Std.soln.

0.05 M NaOH

HMDE

Blood, phar.

Urine,blood

SPEx ZnSO,,C ,H,OH

CPE

Adso rpti o n

Adsorption

Adsorption

Ox. to Hg salt

BR-pH 4.4

HMDE

Phos-pH 12

HMDE

Adsorption

Adsorption

251

-0.45

DPCSV

-0.65

1.9

252

--0.2A

LSCSV

-0.53'

A

+0jA

DifCPS

-0.63

swcsv

'

0 . 67

'

ACASV

+0.75

-1.1'

AST

- 1 . 3 - - 1 .5

o.o

'

swcsv

-0.05A

swcsv

Albumin, SUDS

254

2.2

'

-O. l '

100

253

Gelatin

255

0.015

2 56

0.0 1 1

256

DPCSV

-0.43 '

4. 1

257

-0.58'

DPCSV

-0.76

6.7

258

DPCSV

-0 .68

0. 10

259

DPCSV

-0.86

1.0

260

LSCSV

' -0.76

0.8

261

DPP

-0.38

100

262

-0 . 2'

U rine

SPEx

O. l M HCIO,

HMDE

Vacci ne

NaNO,

Phos-pH 6.8

Pt

Phos-pH 7.0

HMDF.

Adsorption

248

1 04

-0.5

HMDF.

248

- 1 .55

Adsorption

AmCl-pH 8.0

5000 2.6

DPP

HMDE

Dilute

247

250

NaCI04-pH 2.0

Culture

Tryptophan

0.15

Std.sol.n.

Pyocyanin

' -0.6

246

-0.8'

-0.25

0"}1etracydine

Red.;Ads.

- 1 .40'

- 1 .43 '

Purine!!,catechol 162 - -------

249

-0.5

Std.soln.

0.03

249

Adsorption

Pro teins

245

0.076

'

Reference

1 10

HMDE

�-Propiolactone

Dill'SA

Interferences

23

BR-pH 2.0

Adsorption

DPCSV

+0.05'

Albumin

acid

DPP

-0.9'

Oxazepam

Pipemidic

· · ··--

BR-pH 4.8, Cu1'

Pharm.

Niguldipine

Oeuction limit (J1gL.1)

tj. (V)

Bor-pH 8.5,Cu''

llydroxyprogesterone

:-lifedipine

Measurment technique

Water

Urine

Mitoxantrone

E_ (v)

Std.soln.

Histidine

Insulin

Supporting electrolyte

(continued)

-0.5

'

-0.9'

-0.5''

Di£PSA LSASV

-0.2A

A

-0. 59/-0.62 -0. 1 7

'

'

2 ' 1 0 -10

Each other

263

0.42

Surfaclants

264

)> "C "C rtl :::J c.. )( ' �

N w "'

Appendix

5

Organic species-biological and medical samples

Element

Matrix Tiss u e

Ranitidine

Phar.

Riboflavin

Pre-treatment SEx

0.001 M HC1

Phar. Sh

(continued)

Working electrocle

Accumulation process

BR-pH 2.3

HMDE

Adsorption

SM DE

Adsorption

0.2 '

Ac-pH 5.0

HMDE

Acio;orption

-0. 1

0.01 M KCl

HMDE

Adsorption

-0.70

Su.pporting electtolyte 0.001 M N a OH

E.. (V)

Measurment technique

-0.56

DPCSV

' -0.56

PSACCSV '

E, (V)

DPCSV

DPCSV

-0.29

-

1 . 35'

Detection limit (fiBL-1) 0.22

0.36

Rufloxacin

Urine,blood

Streptomycin

Urine

O.oi M NaOH

s�mE

Adsorption

-1.2

LSCSV

- 1 .58

0.4 1

Water, Vet.

0. 1 M HCI

HMDE

Acio;orption

-0.30

SWCSV

-0.60

12

BR-pH 3.0

HMDE

Adsorption

-0.5

DPCSV

-0.77

17

SMDE

Adsorption --- ---

-0.8 '

DPCSV

Sulfadimetoxal

Tcmazepam

Testosterone

- ---

--- ·

Urine

Phar.

---

---

SEx · --- ·

0.005 !\-1 NaOH --- --- . . ---

---

·---

---

3.7

-J .35A 0.086 ___ . . ___ ---

Interferences

265

Other tlavins

266, 267

Plasma matrix Zn1-t

Creatinine,

Reference

uri< acid

243 268

269 270 271

Proteins

272

Other tctraq'Clines

273

--- --- · --- ·---

Tetracycline

Std.soln .

Bor-pH 5.5

HMDE

Adsorption

-0.6'

LSCSV

- 1 .28 '

O.Y

Theophylline

Phar.

RR-pH 7.5

HMDE

Adsorption

0 .0

DPCSV

-0.38

0.6

Cu(ll)

274

6- Thioguanine

Std.soln.

HMDE

DPV

-0.98'

3.3

Other thiols

275

Phos-pH 7.0

Pt

DPP

-0.64

1 04

276

0 . 1 M H2S04 in C2H10H:C,H,. J : 1

GCE

CP

+0.66/0.83

1 0'

277

BR-pH 2.5

HMDF.

Adsorption

-0.9

DPCSV

-1 .32

so

278

Phos-pH 9.0

GCE

Adsorption

0.0

DPASV

+0.74

4.1

Metabolites

279

HCI-pH 0.9

HMDE

Adsorption

+0.05

DPCW

-0.28

96

Proteins with S-S

280

CPE

Adsorption

Ox.;Ads.

+0.3

DP!I.SV

+ ! .01

0.2

-0. 1

LSCSV

-0.22

-0.64

Thiomersal

Eye drop s

Tncopherals

Veg.oil

Torasemide

Urine

Trimipraminc Trypsin

TC)rtophan Vitamin K I

Vitamin K3 ( M enadione;

Urine Std.soln. Serum Std.soln. Plasma Std.soln.

TIC

SEx SPEx

0.! M H 2S04

Ac-pH 3.5

SEx

0.5 M NaCIO,,pH 5

CuM FE

Ac-pl I 5, 50% ale

CPE

Adsorption

oc

LSCSV

0.3 M HCIO,

SMDE

Red.;Ads.

-0 . 1

SWASV

+0.1

2.0

CJ', CNS , r,

Pb"

281 23

130

282

0.022

283

� :;.... a

Q. c:

a 0 ::>

g

� 6f 3 3 �

::::! . n

)>

::::1 Qj

� v; ·

Appendix 6 Appendix 6

Abbreviations used in Appendices 1 -5 .

Abs

Ab sor pt io n

Ac

Acetate buffer

Ad s

Adsorption

AD

Acid d ig es tion

ale

Alcohol ( ethanol )

Am

Ammonium buffer

A mAc

Ammonium acetate buffer

AmCl

Am monium ch l o r id e buffer

Am Cit

Ammonium citrate buffer

AmFor/Phos

Ammonium formate plus phosphate mixed buffer

AN

Acetonitrile

APDC

Amm o n i u m py rrol i di n ed i th ioc a rb a m at e

aq

aqueous

BES

N,N bis ( 2 - hyd roxyethyl ) - 2 -aminoethane- sulfonic acid

B eryllon lil

4- [ ( 4-diethylamin o - 2 - hydroxyphenyl) azo] - 5 - hydroxynaphthalene-2,4-disulfonic acid

BEx

Back e x t racti o n

2-BIBH

2- Benzylideneiminobenzohydroxamic a c i d

Bor

Borate buffer

Br-PADAP

2- ( 5 - Bromo- 2 -pyridylazo ) - 5-diethyl -aminophenol

Br- PADN

4 - ( -2- ( 5 - Bromo pyridyl a zo ) - 1 , 3 ,- d ihydroxyn ap h t h ale n e

BR

Britton-Rob inson buffer

Br-TUO

7- Bromo- 5- ( 2 - chlorophenyl) 1 ,3 -dihydro- 2H- thieno- 2,3 -e- 1 ,4-diazepam - 2 -one

c

C a t a lyti c peak

Cd,Zn MT

Cd,Zn metallothionein

Cit

2- ( N- cyc loh exyl am i n o )ethanesulfonic acid bu ffer

CA CHES

Chloranalic acid { 2 , 5 - D i chloro - 3,6-dihydroxy- 1 ,4 -benzoquinone)

Citrate buffer

Cupferron

Ammonium nitrosophenylhydroxylamine

DA,AD

Dry ashing followed by acid digestion

DAB

3 , 3 ' - D iaminobenzidine

DASA

UBr-PADAP DCM

Der

1, 2 - D i hydroxyan thraquinone-3-sulfo n i c acid

2 - ( 3 ,5 - Dibromo - 2 - pyridylazo ) - 5 - diethylaminophenol Dichloromethane

D c ri vat isati o n

Di a z

D iazotisation and coupling

DMF

Distillation

DMG

D i m e t hyl gl yox im e

D i st

D i m e t hyl for m a mid e

DMSO

Dimethylsulfoxide

D MT D

2 , 5 - Dimercapto- 1 , 3 , 4 - thiadiazole

DPC

4,6-Dinitro - 2 - hydroxytoluene

DPG

Diphenylguanidine

DNC

237

Diphenylcarbazide

238

I ntroduction to Voltammetric Analysis

Appendix 6

Abbreviations used in Appendices 1 -5.

(continued)

DTADP

3 - (4,5-Dimethyl -2 thiazolylazo)-2,6-diam i nopyridine

DTPA

Diethylenetriamine-pentaacetic acid

EBBR

Erichrome Blue Black R {4- { 2 - ( 5 -bromopyridyl)-azo ] - 1 ,3 -dihydroxynaphthalene}

EBT

Erichrome Black T

EDTA

Ethylenediaminctetraacetic acid

F.n

Ethylenediamine

Extn

Extraction

Ferron Ha

7 -lodo- 8- hydroxyquinol i nc-5 -sulfonic acid

HCS

Hydrocarbon solvents

HE PES HNB

Hydroxylammonium chloride N- 2 -hydroxyethylpiperazine- N'- 2-ethanesulfon ic acid

Hydroxynaphthol Blue

Hom

Homogenised

HOx

Oxalic acid

HSA

Human serum albumin

Hyd

Hydrolysis

IX

Ion exchange

MES

2 - ( N-morpholino )-ethanesulfonic acid

MESep

Molecular exclusion separation

MGK 326

Dipropylpyridine-2,5-dicarboxylate

Mod

modified

Morin

2 · ,3,4' ,5,7 -pentahydroxyflavone

MPAQ

I , 2-Cyclohexanedione dioxime

Nioxime 1 N 2N

oc

5 - [ ( 4- Methylphenyl) aw ] -8-aminoquinoline 1 -Nitro- 2- naphthol

Open circuit

OCP

o-Cresolphthalexone

OD,MW

Oxidative digestion in a microwave oven

OF

Oxidative fusion

o-PDA

I ,2-Diaminobenzine ( o-phenylenediamine)

Oxine

8-Hydroxyquinoline

Ox

Oxidation

OxPr

Oxidation pro duct

lpl

Plating or pre-concentration supporting electrolyte

PAR

4-(2 -Pyridylazoresorcinoi)

Phar

Pharmaceutical preparations

Phos

Phosphate buffer

Phos/cit

Phosphate-citrate buffer

Phth

Phthalate buffer

PIPES

Piperazine- N , N ' -bis(2-ethanesulfonic acid)

pl.

Plating

ppt.

Precipitati on

QT

2 - Quinolin ethiol

Appendix 6

Appendix 6

Abbreviations used in Appendices l-5.

(continued)

RE

Rare earths

Red

Reduction

SATP

Salicylideneamin o - 2 - thiophenol

SDDS

Solve n t ext ra ct i o n

Is)

SEx

Stripping electrolyte

Sodium do de cyl sulfate

SPEx

Solid phase extraction

SSA

Sulfo sa l ic ylic acid

Std. soln

Solochrome Violet RS 1 5 -Sulfo-2-hydroxybenzeneazo - 2 - naphthol)

SVRS

Standard s o l u tions

TBAH FP

Tetra b utylammo niu m hexafluorophosphate

TBP

Tri-n-butylphosphate

TEA

T EAB F4

Triethanolamine

TEABr

Tetraethylammonium bromide

TEAP

Thin l aye r ch ro m a togra p hy

TLC

TP

Tetraethylammonium tetrafl u orob o rate

Tetraethylammonium p e rchlo rate

u.v.

Thymol p h th a l exon e

Var.ind

Various industrial ap plications

Vet.

xo

U.V. digestion

Veterinary preparation Xylenol Orange

Electrodes AgRDE

S ilver rotating disc electrode

AgTFE

Au RD E

Silver thin film e l ect ro de

AuT FE

Gold thin film elec t rod e

CE

Carbon electrode

CPE-mod

Modified carbon paste electrode

Gold rotating disc electrode

CRDE

Carbon ro t at i n g disc e l ectro d e

Cu -MFE

Cop p e r based mercury film e l ectro d e

DME

Dropping mercury electrode

GCE

Glassy carbon electrode

GCE-mod

Modified glassy carbon electrode

GE

Gr a p h ite el ect ro d e

HMDE

Hanging m e rc u r y drop electrode

M FRDE

Mer cur y film rotating disc electrode

MTFE

Mercury thin film el ect ro de

film e lect ro d e

MTFE-mod

Modified mercu ry thin

SMDE

Static mercury drop electrode

WIGE

Wax im pr e gn a te d graphite elec t r od e

239

240

I ntroduction to Voltammetric Analysis

Appendix 6

Abb revi ations used in Appendi ces 1-5.

Measuring techniques

ACASV

ACCSV ACP

(contin ued)

-------

Alternating current anodi c stripping voltammetry Alternating current cathodic stripping vol ta nunetry Alternating current polarography

ACV

Alternating c urrent voltammetry

AST

Anodic stripping tensamm etry

CP

Chronopotcntiometry

CPS

Chronopotentiometric stripping

DCP

Direct current polarography

DerPolar

Derivative polarography

DerPSA

Derivative potentiometric stripping analysis

DifCPS

Differential chronopotentiometric stripping

DifPSA

Differential potentiometric stripping analysis

DPCSV

Differential pulse cathodic stripping voltammetry

DPASV

Differential pulse anodic stripping voltammetry

DPP

Differential pulse pol arography

DPV

Differential pulse voltammetry

LSASV LSCSV

Linear scan cathodic stripping voltammetry

LSV

Linear scan voltammetry

oswcsv

Osteryoung square wave cathodic stripping voltammetry

PSA

Potentiometric stripping analysis

PSACCSV

Phase-sensitive alternating current cathodic stripping vo ltam m etry

SSP

Single sweep polarography

SWASV

Square wave anodic stripping voltammetry

SWCSV

Square wave cathodic stripping voltam m etry

Linear scan anodic stripping voltammetry

-------

Appendix 7

Appendix 7

Internet addresses of

241

i n strument suppliers

AD Instruments

HEKA Electronik

www. adinstruments.com

www.heka.com

Arne!

KH Design and Development www. kh des i g n . d e mon . co. uk

www. amelsrl .com Axon

Keithly

www. axonet.com

www. ke it hly. co m

Bank Electronik

M et ro hm

www. bank-ic.de

www. m e t r o hm. c h

BA S( B i o a nalyt i c al System s)

P i n e I n stru me n t Co m pa ny www. p in e i n st. com

www. bioanalytical.com B i o Lo gi c

Pro D i gital ( PAR)

www. bio-logic.fr

www. p rodigital.com.au

Cypress Systems

Radiometer

WW\v. cy p res sh o m c .co m

www. radiometer.com

Eco c h e m i e www. e co ch e m i e . n l

Solartron www. solartron.com

Electrosynthesis C o mp any

Sycop el

www.electrosyuthesis.com

www. syso p e l . c om

Gamry Instruments

Warner Instrument Co rp orat i o n WW\V. warnerinst.com

www.gamry.com

REFERENCES 2

3

5

4

6 7

8 9 10 11

12

13 14 15 16 17 18

19 20

C.M.G. van den l:l e rg , K. Murphy and J.P. Riley, Analytica Ch i m i ca Acta, 1 986, 1 88, 1 77 . E. Stryjewska and S. Ru b el , f:lectroanalysis, 1 99 1 , 3, 995 . T. M . Florence, F.J. Mille r and H.E. Zittel, Analytical Ch e m is try, 1 966, 38, 1 065. J. Wang, J. Lu a n d R.

J. Zima and C.M.G. van

Se t i a dj i , Talanta, 1 993, 40, 3 5 1 . den Berg, At�alytica Chimica Acta, 1994, 289, 29 1 . U. G reu l a ch and G. H en ze, Analy ti ca Ch im ica Acta, 1 995, 306, 2 1 7. H. Huiliang, D. J ag n er and L. Renman, Analy tica Ch imica Acta, 1 988, 2 07 , 3 7 . F.G . Bodewig, P. Val e nta and H.W. Niirnberg, Fresenius Z. Analytische Chem ie, 1 982, 3 1 1 , 1 8 7. "[ V. N g h i and F. Vydra, /. Electroanalytical Ch e mis try, 1 975, 64, 1 6 3 . J. Wan g , P.A.M. l'arias and J.S. M ah m o ud , /. Electroanalytical Chemistry, 1 98 5 , 1 95, 1 65 . Metrnhm Ap p l i c a t ion B ulletin No. 74/2 e. H . Es ki l s son and D . J a gne r, An a ly tica Ch i m ica Acta, 1 982, 1 38, 2 7 . C.M.G. van den Berg, ]. Electroanalytical Chem istry, 1 986, 2 1 5 , I l l . K . Yokoi , M . M i zu m ach i and T. Ko id e , Analytical Science, 1 995, 1 1 , 2 5 7 . H.W. N iirn b e rg , P. Va l e n t a , L. M a r t , B. Raspo r and L. Sipos, Fres e n i u s Z. A n a ly tische Chem ie, 1 976, 2 8 2 , 357. D. J a g ne r, Analytical Chemistry, 1 978, 50, 1 924. j . Wang, P. A.M. Farias and J.S. Mahmoud, Analytica Ch im ica Acta, 1 98 5 , 1 7 1 , 2 1 5 .

Marjanovic and P.P. Do ki e , Electroanalysis,

1 997, 9 , 5 7 2 .

J.B. D o n a t and K.W. Br u la n d , Analytical Ch e m is try, 1 9 8 8 , 60, 240. Z.J. Suturovic, N.J.

and K.S. Siow,

1 99 1 , I, 89.

Ta lan ta, 1 9 9 6 , 43 , 2 5 5 .

Z-Q. Zang, Mikrochim ica Acta,

[Wien ] ,

H. Eskilsson, C. H a r a ldss o n and D. Ja gn e r, A na lyti ca Chimica Acta, 1 985, 1 75, 79.

21 22

Z. Gao

24

M.

J. Goli mowski,

322, 3 1 5.

26

N.A.

1 985, 42, 1 636;

27

Z. G a o and K.S. Siow, Electroa nalysis, 1 996 , 8, 602.

23

25

Kub l ik , A n a lyt ica Chimica Acta, 1 986, 1 85 , 209.

A. Romanus, H . Muller and D. K i rsch, Fresenius' ]. Analytical Chem is t ry, 1 99 1 , 340, 363.

Donten a n d Z.

P. Valenta a nd H . W. Niirnberg, Fresenius' /. Analytical Cl1emistry, 1 99 1 , Malakhova, A.V. Chernysheva and Kh.Z. Branica, Zh umal Analiticheskoi Kh imii,

Electroanalysis, 1 99 1 , 3 , 803.

242 28 29 30

31 32 33 34

35

36 37 38 39

40 41 42 43

44 45 46 47 48

49 50 51

52 53 54 55 56 57

59

58

60 61 62 63 64 65 66

67 68 69 70

72

71

73

74 75 76 77

78

80 81

79

82 83 84 85 86 87

I ntroduction to Voltammetric Analysis M . Esteban, C. A r in o , I . Ruisanchez, M.S. Larrechi a n d F.S. R u is , Analytica Chimica Acta, 1 994, 285, 193. S.T. Cro s m u n a n d T. R . M u e l l e r, Analytica Ch imica Acta, 1 975, 75, 1 99. C.M.G. van d e n B e r g , Analytica Chimica Acta, 1 984, 164, 1 9 5 . A . Bobrowski, Talanta, 1 989, 3 6 , 1 1 23 . M . P. N ew to n and C.M.G. v a n d e n B e r g , Analytica Ch imica A cta , 1 987, 1 99, 59. R.D. Riso , P. Monbet a n d P. Le Corre, A 11alyst, 1 997, 1 22, 1 593. C.M.G. van den Berg, M. N i m mo , D. Abollino and E. Mentas i , Electroanalysis, 1 99 1 , 3, 472 . J, Golimowski, A nalytical Lett�'Ts, 1 989, 2 2 , 48 1 . J. Wang and J. Zadeii, A n a ly tica Chim ica Acta, 1 986, 185, 229. J.M. P i n ni l l a , L. Hern andez and A.J. Conesa, Analytica Chimica Acta, 1 996, 3 1 9, 2 5 . D. Jagner, Analytica Chimica Acta, 1 979, 1 05 , 3 3 . A. Moe l l er, F. Scholz and M . L ov r ic , Electroanalysis, 1 995, 7, 987. J. Wang a n d J.S. Mahmoud, Ana ly ti ca Chim ica Acta, 1 986, 1 82, 147. C.M.G. va n den B e rg , Analytical Ch emis try, 1 98 5 , 57, 1 5 3 2 . C. H u a , D. J a g ner and L. Renman, Analytica Chimica Acta, 1 987, 1 92, 1 03 . S. H . Khan and C.M.G. v a n den Berg, Marine Ch e mis try, 1 989, 2 7 , 3 1 . M . Karakaplan, S . Gucer a n d G . Henze, Fresenius' /.Analytical Chemistry, 1 992, 342, 1 86. H. Sawamoto, Bunseki Kagaku, 1 988, 37, 3 1 2 . P. A . M . Farias, A.K. Ohara, I . Takase an d S.L Ferreira, Talanta, 1 993, 40, 1 1 67. K. Markusova. L Kok u t ova and J. Dzurov, Electroanalysis, 1 996, 8, 582. Q. Wu a nd G . E . Batley, A n a lytica Chim ica Acta, 1 995, 309, 9 5 . J. Wa n g a n d K. Varughese, Analytica Chi mica Acta, 1 987, 1 99, 1 85. G . Lundquist a n d }.A. Cox, Analytical Chem istry, 1 974, 46, 360. C.M.G. van den Berg a n d G . S . Jacinto, Analytica Chimica Acta, 1 988, 2 1 1 , 1 29. J.K. Christensen, L. K r yger, J. M o r tens e n and J. Rasmussen, Analytica Chimica Acta, 1 980, 1 2 1 , 7 1 . G. Capodaglio, C.M.G. van den Be rg and G. Scarponi, /. Electroanalytical Ch em is try, 1 987, 235, 275. W. Wag ne r, S. Sa nd er and G. Henze, Fresenius' /. Analytical Che m is try, 1 996, 354, I I . C.M.G. van den Be rg and S.H. Kh a n , A nalytica Chimica Acta, 1 990, 23 1 , 2 2 1 . P. B re ye r and B.P. G i lb e rt , Analytica Ch i m ica Acta, 1 987, 20 1 , 33. D.A. Griffin, Analytical Chemistry, 1 969, 4 1 , 462. C.S.P. Ayer, P. Val e n ta and H.W . N iirnberg, Analytical Let ters, 1 98 1 , 1 4 , 92 1 . J, Wang and J. Zadei i, Talanta, 1 987, 34, 909. C.M.G. van den B erg , S.H. Khan and J.P. Riley, A nalytica Chimica Acta, 1 989, 222, 43 . C . McCrory- Joy a n d J.M. Rosamilia, A n aly tica Chimica Acta, 1 987, 1 42, 2 3 1 . M. Fri edrich and H. Ruf, J. F.lectroanalytiml Chemistry, 1 986, 1 98, 26 1 . J. Wan g and J , Lu, Electroanalysis, 1 994, 6, 405. H - Y. Yang and I-W. Sun, Analytica Chimica Acta, 1 998, 358, 285. R. Setiadj i , J. Wang a nd G. Santana-Rios, Tala n t a , 1 993, 40, 845. H . Li and C.M.G. van d e n Berg, A nalytica Chimica Acta, 1 989, 22 1 , 269. K. Yokoi and C .M . G . van den Berg, Analytica Chimica Acta, 1 99 1 , 245, 1 65 . J, Zhou an d R. Nceb, Fresenius' /. Analytical Chemistry, 1 9 9 0 , 338, 3 4 . ) . Wa ng and J. Lu, Analytica Chim i ca Acta, 1 993, 282, 329. S. S a n d e r, W. Wagner and G . Henze, Analytica Ch imica Acta, 1 995, 305, 1 54. S . Sa n de r and G . Henze, Frese n i us ' /. Analytical Chemistry, 1 994, 349, 654. P.A.M. Farias a n d A.K. Ohara, Electroanalysis, 1 99 1 , 3, 985. J, Wa ng a n d J. M . Zadei i , Ta lanta, 1 987, 34, 247. C. Hua, D. Jagner and L Rcnman, A n a lyt ica Ch i mica Acta, 1 987, 197, 265. C.M.G. van d en Berg and Z.Q. Huang, Analytical Ch e mis try, 1 984, 56, 2383. S . Sander and G. Henze, Fresenius' ]. Analytical Chemistry, 1 996, 356, 259. B. M agya r, H . R. Elsener and S. Wu n d e rli , Mikrochimica Acta, [ Wien ] , 1 990, 3 , 1 79. J. Wang an d J. Z a d eii , Talanta, 1 986, 33, 3 2 1 . G. G il la i n , G. Duyckaerts a n d A. D is tec he, Analytica Chimica A cta, 1 979, 1 06, 23. C.M.G. va n den Berg, Ta lan ta, 1 984, 3 1 , 1 069. D. )agner, M . Josef.� on and S . Westerlund, A nalytica Chimica Acta, 1 98 1 , 1 29, 1 53 . J . Wa n g , P. Tuzhi a n d K . Va mghe se , Talanta, 1 987, 34, 56 1 . M . Karpiuk, M . Pilo lowicz, E. Stryjewska and S . Rabel, Fresenius ' /. Analytical Chemistry, 1 995, 35 1 , 693. Metrohm Application Note No. V 38. C. L i , B.D. Ja m e s a n d R.J. Magee, Mikrochimica Acta, I Wi en ] , 1 989, 2, 149. S . B . Adeloj u , 'l� M . Yo ung, D. J a gn e r and G.E. Batley, A n alytica Chimica Acta, 1 999, 38 1 , 207. L. Thum us, Analytica Ch i m i ca Acta, 1 996, 3 1 8, 303.

Appendix 7 88 89

90 91

92 93 94

95 96 97 98 99

1 00 101 1 02 1 03 1 04 105

Z. Qing and H. Yu y in g , Talan ta, 1 987, 34, 5 5 5 .

Z43

E. Wan g , W. S u n and Y. Yang, A n alyt ical Chemistry, 1 98 4 , 56, 1 903.

J. Zh ao

and W. Jin, ]. Electroanalytical Ch e mistry,

1 989, 256,

181.

C . Li, B.D. James, j . Rumbli and R. I. Magee, Mikroch im ica Acta, [ Wien ] , 1 988, 3 , 1 7 5 . L. Almestrand, D . Jagn e r and L. Renman, A na ly t ica Ch im ica Acta, 1 987, 1 97, 7 1 . D. Jagner, M . Josefson and S. West e rl u n d, An a ly tica Chimica Acta, 1 98 1 , 1 28, 1 5 5 . R. He i n r i ch and J. A n ge r er, International ]. Environmen tal Analytical Ch emis try, 1 984, 1 6, 305.

S.B. Adel oj u , A.M . Bond a n d H.H. B r i ggs , A na lytica Chimica Acta, 1 9 8 4, 1 64, 1 8 1 . M.J.M. Gilbert, H . K.j. Powell and ].J. Fardy, A n a lytica Chimica Acta, 1 988, 207, 1 03 . M.G. Pa n nel i and A.N. Voulgaropoulos, Freseni us ' ].Analytical Ch e m is t ry, 1 99 1 , 34 1 , 7 1 6. Z. Zhao, X. Cai, J, Pai, Y. Zang and X. Z h o u , Electroanalysis, 1 99 1 , 3, 949. J. Z ha o, D. Sun a n d W. ) in, A na ly tica Ch imica Acta, 1 9 92 , 268, 293. L-G. Dan i el ss o n , D. Ja gn er, M . Josefson and S. Westerlund, A nalytica Ch i m ica Acta, 1 9 8 1 , 1 27, 1 4 7 . Z. Gao and K.S. Si o w, Talanta, 1 996, 43, 727. Metrohm Applicat i o n Note No. V 47. C. S ch l e i ch and G. He n ze, Fresen ius' f. Analytical Chemistry, 1 990, 338, 1 4 5 . ).K. Christensen and L. Kryger, A n a ly tica Chimica Acta, 1 980, 1 1 8, 5 3 . L. Calvo Blazquez, R.M. Garcia-Monco, A. G u ib e rte a n Cabanillas a n d A. Sanchez M is i ego, Fresenius Z. Analytische Chemie, 1 989, 334, 1 66.

M . Karakaplan and G. Henze, Electroanalysis, 1 99 3 , 5 , 623. 10 7 Prin ceton Applied Research, Application B r i e f A - 3 , 1 974. 108 B. P i h l a r, P. Vale n ta and H.W. N u rn b e r g, Fresenius Z. Analytische Ch e m ie, 1 98 1 , 307, 337. 1 09 Z. Zhang, Z. C he n g , S. C hen g and G . Yang, Tala n ta, 1 99 1 , 38, 1 487. 1 1 0 L. Duic, S. Szechter and S. Srinavasan, f. Electroanalytical Chemistry, 1 9 73, 41, 89. 1 06

I l l Metrohm Appl ication Note No. V 54. 1 1 2 0. Nygren, G.T. Vanghan, T.M. Fl o ren ce , G.M.P. Morrison, I.M. Wa r n e r and L.S. Dal e , Analytical Chemistry, K. Hoppstock, F. Al t , K. Ca m m a n n a n d G . Web e r, Fresenius Z. A nalytische Ch e rrl i e, 1 989, 335, 8 1 3. 1 989, 62, 1 63 7 .

113

l l 4 P. Breyer and B.P. G ilb ert , A na lytica Chimica Acta, 1 987, 20 1 , 23. C. Hua, D. J a gner and L. Renman, A nalytica Chimica A c t a , 1 987, 197, 2 5 7. 1 1 6 J. Wa n g , C. Sun and W. Jin, f. Electroanalytical Chemistry, 1 9 9 0 , 29 1 , 59. 1 1 7 A. Ciszewski, W. Wascak and W. Ciszewska, A n a lytica Chimica Acta, 1 997, 343, 225. 1 1 8 M . Cullen and S. La n ca shire , Analyst, 1 99 6 , 1 2 1 , 75 . 1 1 9 Metrohm Application No te No. V 40. 1 20 Metrohm Application N ote No. V 23. 1 2 1 D. j. Meyers and J. Ost e ryo u ng, Analytical Ch e m istry, 1 9 73, 45 , 267. 1 22 Metrohm Application Note No. V 37. 123 Metrohm Application Bulletin No. 1 4 7. 1 24 Metrohm App l i c at i on Note No. V 5 1 . 1 2 5 B . H o ye r, H.J. Skov and L. K r yge r, A nalytica Ch imica Acta, 1 98 6, 1 88, 205. 1 26 D. Sancho, M. Ve ga r, L. Deban, R. Pardo and G. Gonzalez, Analyst, 1 997, 122, 727. 1 2 7 M . M . P alre ch a , Fresenius ' ]. Analytical Che m istry, 1 995, 35 1 , 80 0 . 1 2 8 Metro hm Application Bulletin No . 1 1 4/ 1 e. 1 2 9 S. Tanaka, K. Sugawara and M. Taga, Fresen ius' f. Analytical Chemistry, 1 990, 338, 898. 1 30 Metrohm A p pli c a ti on Note No. V 24. 1 3 1 P. Pheiffer Madsen, I . Dr u b a ek and J, Sorensen, A naly ti ca Ch imica Acta, 1982, 1 5 1 , 479. 1 3 2 D.G. B a lli n ge r and T.A. Ha rtlage, Water a n d Sewage Wo r ks, 1 962, 1 09, 338. 1 33 j. Lu, W. lin, S. Wa n g and T. Sun, f. Electroanalytical Ch e m is try, 1 990, 29 1 , 49. 1 3 4 Me t ro hm A p pl i c at ion Bulletin No. 3 7 e 1 35 Me t ro hm Application Note No. V 5 2 . 1 36 j. Wang a n d S. Mannino, An a lys t, 1 989, 1 1 4, 643. 1 3 7 C.Q. Sun, Q. Gao and L.L. L i u , Ta la n ta, 1 995, 42, 8 8 1 . 1 15

1 38 139

K. H a seb e , S. H i k i m a , T. Kakizaki, T. lwashimizu a n d K. Aoki, Analyst, 1 990, 1 1 5 , 4 1 3 .

M c t ro h m Ap p l ica t io n Note No. V 63.

Me t roh m Ap plicatio n Notes No. V 36 an d V 4 2 . Note No V8. 1 42 C. Sun, J, Wan g , W. Hu and X. M a o , ]. Electroa11alytica/ Chemistry, 1 99 1 , 306, 252. 1 4 3 P.A.M. Farias, C.M.L. Martins, A . K . Ohara and J.S. Gold, Analytica Chimica Acta, 1 994, 293, 29. 1 44 E. Cziollok and R. Neeb, Fresenius Z.Analytische Chemie, 1 989, 335, 860. 1 45 J. Al a ry, j. Esdassan and j. Van da e l e, Analyst, 1 986, I l l , 593. 1 40 141

Metrohm A p p l i c a tio n

244

I ntroduction to Voltammetric Analysis

1 46 Metrohm Application Bulletin No. 1 3 2/ 1 e. 147 A. Meyer and R. Neeb, Fresen ius' ]. Ana ly tical Ch e m is try, 1 995, 32 1 , 235. 1 4 8 R. Mr:djak, A . M . Bond, T. ) . Cardwell, R.W. Cattrall, R.W. Knight, O . M . G . Newman a n d B.R. Champion, Analyst, 1 994, 1 1 9, 1 057. 149 D . D . Gilbert, A n a ly ti cal Chem istry, 1 96 5 , 37, 1 1 02. 1 50 C. Wei and G.M. Morrison, Scie nce of th e Total Environment, 1 994, 1 46/ 147, 1 69. 1 5 1 Mctrohm Application Bulletin N o . 220/2 e . 1 52 K. Hoppstock a n d M . Michalitz, A nalytica Chimica Acta, 1 997, 350, 1 3 5. 1 53 Metrohm Application Note No. V 1 5. 1 54 A.M. Bond, S. Kratsis and O.M.G. Newman, Electroanalysis, 1 997, 9, 6!11 . 1 55 Metroh m Application Note No. V 1 7. 1 56 G. Weber, An a ly tica Chimica Acta, 1 986, 1 86, 49. 1 5 7 U. Greulach, G. Henze and G . Kaiser, Fresenius' /. Analytical Chemistry, 1 994, 349, 688. 1 5 8 N .A. EI- Maali and D. EI- Hady, Electroanalysis, 1 999, 1 1 , 20 1 . 1 59 Me t ro hm Application Note No. V 6 5 . 1 60 H - L. Ye and Y- H. He, Tala nta, 1 984, 3 1 , 6 3 8 . 161 ) . Barek, ). Matejka and ). Z i m a , Collection of Czechoslovak Chemical Com m u n ications, 1 994, 59, 294. 1 62 B.C. Houseman, C.M.G. van den Berg and J.P. Riley, An a lyt i ca Chimica Acta, 1 987, 200, 29 1 . 1 63 ). A . Rodrigues and A.A. Barros, Talanta, 1 995, 42, 9 1 5. 1 64 ) . Skopalova and M. Kotoucek, Fres e nius ' ]. A na lyt ical Chemistry, 1 99 5 , 35 1 , 650. 165 P. Hernandez, E. Lo ren zo and L. Hernandez, An a ly tica Chimica Acta, 1 990, 238, 383. 1 66 M . Li, N. Hu, a n d S. Lin, 1alan ta, 1 995, 42, 1 389. 1 6 7 V. Stara and M . Ko p a n i c a, Collection of Czechoslovak Chem ical Com m unications, 1 985, 50, 42. 1 68 J, Schwarz, G. Henze a n d F.G. Thomas, Fres en i us ' ]. Analytical Chemistry, 1 99 5 , 352, 474. 1 69 J, Schwarz, G . Henze and F.G. Thomas, Fresen ius ' ]. Analytical Chem istry, 1 99 5 , 352, 479. 1 70 H . Kitamura, A. Sugimae and M. Nakamoto, Bulletin of the Chemical Society, japan, 1 985, 58, 264 1 . 1 7 1 Z . Tocksteinova and M . Kopanica, Analytica Chimica Acta, 1 987, 1 99, 77. 1 7 2 E. Bermejo, E. Arauzo and L. H e rn a n d e z, Fresenius' ]. A n a ly t ica l Chemistry, 1 994, 350, 620. 1 73 A . Meyer and G. Henze, Frese n i u s Z. Analytische Chemie, 1 987, 327, 1 23. 17 4 Metrohm Application Note No. V 62. 1 75 S.B. A d eloj u and S.J. Shaw, Electroanalysis, 1 994, 6, 645. 1 76 P. Gratteri, S. Pinzauti, E. La Porta, G. Papeschi, V. Cavrini and G. Santoni, Analyst, 1 99 1 , 1 1 6, 725. 1 77 Metro h m Application Note No. V 3 L 1 78 A. Meyer and G. Henze, Freserzius Z. Analytische Chem ie, 1 98 8 , 329, 764. 1 79 ) . Barek, H . Ebertova, V. Mej strik and J. Zima, Collect ion of Czechoslovak Chemical Com m u n ications, 1 994, 59, 1 76 1 . 1 80 M . Pedrero, B . Casado, F.J. Manuel de Villena and j . M . Pingarron, Fresenius' ]. An a lytical Ch e mi s t ry, 1 994, 349, 546. 1 8 1 R.M. Alonso, B. Gallo and A. G . Fogg, An a lyst, 1 98 7 , 1 1 2, 1 6 1 L 1 8 2 A. Meyer and G. Henze, Fres e n ius ' ]. A n alyt ica l Chemistry, 1 994, 350, 1 50. 1 83 Metrohm Application Bulletin No. 1 96/2 e. 1 84 P.J. Elving, A.J. Mart i n and I . Ro s en th a l , A na lytical Chem istry, 1 953, 25, 1 082. 185 M. Jesus Portella, RJ. Barrio, M.A. Goicolea and Z. Gomez de Balugera, Analyst, 1 994 . , 1 1 9, 2 1 83 . 1 86 G. Scarano a n d E. M o relli , Analytica Ch i m ica Acta, 1 994, 296, 277. 1 8 7 C. Olmedo, L. Deban, D. Vazquez, R. Pardo and S . Palmero, Electroanalysis, 1 994, 6, 694 . 1 8 8 M.A. Goicolea, ).F. Arran z, R.). Barrio and Z. Gomez de R alugera, Fresenius' j. Analytical Chemistry, 1 9 9 1 , 339, 1 66. 1 89 M. Pedrero, V. Ca lvo , F.J. Manuel de Villena, J.M. Pingarron and L.M. Polo, Analyst, 1 993, 1 1 8, 1 405 1 90 C. faller, A. Meyer and G. Hen1.e, Fresenius' ]. Analytical Chemistry, 1 996, 356, 279. 1 9 1 E. Ca sa ssas, M. Esteban and C. Arino, /. Electroanalytical Chemistry, 1 986, 2 1 5 , 1 03. 1 92 D.C. Coomber, D.). Tucker and A . M . Bond, Analyst, 1 997, 122, 1 587. 193 E. Reichart and D. Obcndorf, Analytica Chimica Acta, 1 998, 360, 1 79. 194 M. Kotoucek and M . Op ravilova , A na lytica Chimiw Acta, 1 996, 329, 73. 1 95 L. Hernandez, P. Hernandez, ). Vicente and M.H. Blanco, Analytica Ch imica Acta, 1 997, 356, 239. 1 96 A.M. Fa r r i ngto n and J . M . Slater, Analyst, 1 99 7 , 1 22, 59 3 . 1 9 7 ) . C . Moreira, C . E Law and A.G. Fogg, Analyst, 1 98 9 , 1 1 4, 1 607. 1 98 Metrohm Application Note No. V 64. 1 99 S. Sander and G . Henze, Electroanalysis, 1 997, 9, 243. 200 M . R. Smy th a n d ).G. Osteryoung, Analytical Ch e m is try, 1 977, 49, 23 1 0. 20 1 V. Stara and M. Kopanica, A n a lytica Chimica Acta, 1 984, 1 59, 1 05.

Appendix 7

245

202 J. Barek, A. G. Fogg, J,C. Mo r e i ra, M. Zanoni, B. Valnice and ). Zima, A n a ly tica Ch i mica Acta, 1 996, 320, 3 1 .

203 A. G . rogg, M.V. B . Zanoni, A . R. H.M. Yusif, R . A h m ed , J. Barek a n d J , Zima, Analytica Ch i mica Acta, 1 998, 362, 235. H. Shiraish i and R. Takahash i , Bioe/ectrochemistry and Bioenergetics, 1 99 3 , 3 1 , 2 0 3 .

204 H . T. L a m and M . Kopan ic a , Analytica Ch im ica A cta, 1 984, 1 6 1 , 3 1 5. 206 A . G. rogg, A . A . Barros a n d J.O. Cabral, Analyst, 1 986, I l l , 83 1 . 205

20 7 S . H u and Z. Zhao, Analytical Letters, 1 99 1 , 24, 827.

208 J. Rodriguez, C. Marin, A. S anchez and F. Vin a rge, f. Electroanalytical Chemistry, 1 98 8 , 25 6 , 77. 209 Metrohm Application N ot e No. V 56.

2 10

J, Wang, D - B. Luo and P.A.M. Farias, f. Electroanalytical Chemistry, 1 985, 1 85, 6 1 .

2 1 2 L . Hernandez, A . Zapardiel, J.A. Perez Lopez and E . B e rm ej o, A n alys t, 1 987, l l 2, 1 1 49. 2 1 3 L. He rn a nd ez , A. Zapardiel, J.A. Perez Lopez and E. Bermejo, ].Electroanalytical Chem istry, 1 988, 255, 85. 214 J.G. Osteryoung and K.J. Wikiel, Analytica Chimica Acta, 1 997, 35 1 , 6 5 . 2 1 5 V. Stara a n d M . Ko pa n i ca , Analytica Ch e m i ca Acta, 1 986, 1 86, 2 1 . 2 1 6 C . M . Fed urc o and I . Sestakova, Bioelectrochemistry and Bioenergetics, 1 996, 40, 2 2 3 . 2 1 7 B. Ogorevc, A . Krasna, V. Hudnik a n d S. G o m iscek, Mikroch im ica Acta, [ Wien ] , 1 99 1 , I , 1 3 1 . 2 1 8 A . M . M . Ali, N.A. El- Maali and M .A. Ghando ur, Electroanalysis, 1 993, 5 , 85. 2 1 9 V. S. Ferreira, M .V. B. Zanoni and A. G. Fogg, A nalytica Chim ica Acta, 1 998, 367, 2 5 5 . 220 S. Altinoz, D. Ozer, A. 'ietn i zer a n d N . Yu kscl, Analyst, 1 994, 1 1 9, 1 575. 2 2 1 J . Wang, M . L i n a n d V. Villa, Analyst, 1 987, I I 2 , 247. 222 E. Lo ren zo and L. Hernandez, Analytica Chimica Acta, 1 987, 20 1 , 275. 223 J. Wa ng , M. Bonakdar and M.M. Pa ck , Analytica Ch imica Acta, 1 98 7 , 1 92, 2 1 5 . 224 I. Gonza l ez M artin, C. G onzalez Perez and M.A. Bl anco Lopez, Analytica Chimica Acta, 1 998, 368, 1 75. 2 2 5 J. Wang, T. l'en g a nd M -S. Lin, Bioelectrochemistry and Bioenergetics, 1 9!16, 1 6 , 395. 226 A. Zapardiel, J.A. Pe rez Lopez, E. Bermejo, L. Herna n d ez and A. G . E s pa rtero , Analytical Letters, 1 99 1 , 24, 2 3 3 . 2 2 7 R. Lej eune, Analytica Chi mica Acta, 1 992, 2 5 6 , 59. 228 T. B. Jarbawi and W. R. Heineman, Analytica Ch imica Acta, 1 986, 1 86, 1 1 . 2 2 9 J. Wang and M . Ozscoz, Talattta, 1 990, 37, 783. 230 J . C . M oreira, S.E. Fo ster, J.A. Rodrigu es and A.G. Fogg, Analyst, 1 992, 1 1 7, 989. 2 3 1 I . Sestakova and M . Kopanica, Talanta, 1 98 11 , 35, 8 1 6 . 2 3 2 C.M.G. van den Berg, B.C. Houseman and J.P. Riley, ]. Electroanalytical Chemistry, 1 988, 239, 1 3 7 . 23 3 M et rohm Application Note No. V 58. 234 A.J. M . O rdieres, A. Costa Garcia, P. Tunon Blanco and W.F. Smyth, Analytica Chimica Acta, 1 987, 202, 1 4 1 . 2 3 5 ) . Wang, M . S . Lin and V. Villa, Analyst, 1 987, l l 2, 1 303. 236 1 . Wang, D. H . Grant, M . Ozsoz, X. Cai, B. Tian and J, Fernandes, Analytica Chim ica Acta, 1 997, 349, 77. 237 E. Palacek, P. Boublikova and F. Jelen, Analytica Ch imica Acta, 1 9!16, 1 87, 99. 238 R. Kalvoda, /. Electroanalytical Chemistry, 1 986, 2 1 4, 1 9 1 . 239 S.F. de Botono, J.M. Moreda, A . Arranz and ) . F. Arranz, Analytica Chim ica Acta, 1 996, 329, 2 5 . 2 4 0 A . Arra nz, S.F. d e Botono, ) . M . Moreda, A. C i d a n d J. F. Arranz, Analyst, 1 997, 1 22 , 849. 24 1 M. Chicharro, A. Zapardiel, E. Bermejo, J.A. Perez and L. Hern an dez , Analytical Letters, 1 994, 27, 1 809. 242 S. Hu, Q. He and Z. Zhao, Analyst, 1 992, 1 1 7, 1 8 1 . 243 M.J.F. Villamil, A.J. Miranda Ordieres, A . C os ta Garcia and P. Tunon nlanco, Analytica Ch im ica Acta, 1 993, 21 1

X. Tan, J. Hu and Q . Li, Analyst, 1 997, 1 22, 99 1 .

273, 377.

244 245 246 247

R.M. Subietah, A . Z . Ab u Zuhri and A. G . Fogg, Fresenius' /. Analytical Ch em istry, 1 994, 348, 754. A. Zapardiel, E. Bermejo, J,A. Perez T .opez, L. He r n a n d ez and E. Gill, Microchemical ]., 1 995, 52, 4 1 . J.C Moreira and A . G . Fogg, Analyst, 1 990, 1 1 5 , 4 1 . 248 M.l. Paredes, M.C. Gonzalez. M .D. Vazquez, M.L. Ta sco n , P. S . B a n ta ne ro and G.J. Patriarch e, Eleclroanalysis, D - B . Luo, Analytica Chimica Acta, 1 986, 1 89, 277.

249 M.J. Honeych urch and M.J. Ridd, Electrocmalysis, 1 996, 8, 49. 250 J. Arcos, J. Vivre , A. F.l Jammal, G . j . Patri arche and G.D. Christian, 1alanta, 1 990, 37, 66 1 . 1 994, 6, 79 1 .

252 S. K i r, A . N . Onar and A. Temizer, Analytica Ch imica Acta, I 990, 229, 1 4 5 .

251

Metrnhm Application Note No. V 59.

j . Wan g , M . S . L i n and V. Villa, Analytical 1-etters, 1 986, 19, 2293.

254 j .C. Co rt in a Villar, A . Costa Garcia and 1'. Tunon Blanco, Analytica Chim ica Acta, 1 992, 256, 23 1 . 253

2 5 5 R . Kalvoda, J. Electroanalytical Chemistr;� 1 984, 1 80, 307.

256 M . Khodari, J.C. Vire, G.}. Patriarche and M.A. Ghandour, A nalytical Letters, 1 990, 23, 1 87 3 .

257 A . J. Barrio Diez- Caballero, L. Lo p ez de Ia Torre, J.F. Arranz Valetin a n d LA. A r r a nz Garcia, 'lalama, 1 989, 36, 50 1 .

246

I ntroduction to Voltammetric Analysis G. Stubauer and D. Obendorf, Analyst, 1 996, 1 2 1 , 35 1 .

259 A . Zapardiel, J.A. Perez Lopez, E . Bermejo, L . Hernandez and M . Chicharro, Analytica Chim ica Acta, 1 99 1 ,

258

E. Pinilla Gil, L. Calvo Blazquez, R.M. Garcia-Monco Carra and A . Sanchez Misiego, Fresenius Z Analytische Chemie, 1 988, 332, 82 1 . 26 1 M . Telting-Diaz, A.J. Miranda Ordieres, A . Costa Garcia, P. Tunon Blanco, D. Diamond and M .R. Smyth, 244, 49.

260

Analyst, 1 990, 1 1 5, 1 2 1 5 .

Metrohm Application Note No. V 5 5 . 263 M . J . Honeychurch a n d M . J. Ridd, Electroanalysis, 1 996, 8, 654. 264 D.V. Vukomanovic, D.E. Zoutman , G.S. Marks, J.F. Brien, G.W. van Loon and K.Nakatsu, ]. Pharmacological and Toxicological Methods, 1 996, 36, 97. 265 S. Altonoz, D. Ozer, A. Temizer and Y. Bayraktar, Analytical Letters, 1 992, 25, I I 1 . 266 J. Wang, D-B. Luo, P.A.M. Farias and J.S. Mahmoud, Analytical Chemistry, 1 9 8 5 , 57, 1 5 8. 2 6 7 A. Economou and P.R. Fielden, Elec t roanalysis, 1 995, 7 , 447. 268 S. Furlanetto, P. Gratteri, S. Pinzauti, R. Leard i, E. Dreassi and G. Santoni, ]. Pharmacological and Biomedical 262

Analysis, 1 995, 1 3, 43 1 .

J,J. Bergas, J . Rodriguez, J.M. Lemus and G . Castaneda, Electroanalysis, 1 997, 9, 474.

269 J . Wang and J.S. Mahmoud, Analytica Chi mica Acta, 1 986, 1 86, 3 1 .

J.A. Perez Lopez, E. Bermejo and L. Hernandez, Fresenius Z. A na lytis ch e Chemie, 1988, 330, 707. 2 7 2 j. Wang, P. A . M . Farias and J.S. Mahmoud, Analytica Chimica Acta, 1 9 8 8 , 1 7 1 , I 95. 273 J. Wang, T. Peng and M.S. Lin, Bioelectrochemistry and Bioenergetics, 1 986, 1 5 , 14 7. 274 R.M. Shubietah, A.Z. Abu Zuhri and A.G. Fogg, Analyst, 1 994, 1 1 9, 1 967. 275 J. Rarek, A. Berka, L. Dempirova and J. Zima, Collection of Czechoslovak Chemical Com m u n ications, 1 986, 5 1 , 2466. 276 Metrohm Application Note No. V 57. 277 Z.j. Suturovic and N.J. Marjanovic, Hlectroanalysis, 1 999, 1 1 , 207. 27!1 M. Fernandez, R.M. Alonso, R.M. Jiminez and M . J . l.egorburu, Analyst, 1 994, 1 1 9, 3 1 9. 279 J. Wang, M. Bonakder and C. Morgan, A nalytical Chemistry, 1 986, 58, 1 024. 280 J. Wang, V. Villa and T. Tapia, Bioelectrochemistry and Bioenergetics, 1 988, 1 9, 39. 281 H . Wa ng, H . Cui, A. Zhang and R. Liu, Analytical Com m u n ications, 1 996, 33, 2 7 5 . 28 2 J.P. Hart, S.A. Wri n g and I . C . M orga n , Analyst, 1 989, 1 1 4, 933. 283 J.C. Vire, N.A. El Maali, G.J. Patriarche and G . D. Christian, Talan ta, 1 988, 35, 997. 270 271

A. Zapardiel,

I n dex

A.c. polarography, 46 basic principles of, 46-48 base ( charging) current depression in, 5 1 charging current i n , 49 definition of, 4 6 effects of adsorption in, 5 1 fundamental harmonic current, 47 irreversible electrode processes in, 5 1 peak width, 49 phase angle, 49 phase selective, 49 phase selective second harmonic, 50 quasi-reversible e lectrode processes in , 50 reversible electrode p ro cess es in, 48 second h a rm onic, 50 theory, 48 time domain of, 6 with digitise d instrumentation, 46, 48 A.c. voltamm etry, 46 Adsorption, 1 4 current, 1 4 o f analytes on electrodes , 1 4 o f i mp ur ities o n electrodes, 5 1 waves in d.c. polarography, 1 4 Adsorptive stripping v oltammetry 72 catalytic re a ctions in, 77, 1 64 effect of accumulation time on the current peak in, 74 effect of stripping w aveform in riboflavin analysis by, 79 limits of detection in, 77 linearity of current peak height with concentration in, 74 of metals as complexes, 72, 76 of organic compo u n d s 206 of pesticides, 208 using mercury elect ro des 71 using modified electrodes, 1 1 4 ,

,

,

Aluminium determination, 1 68 A m p ero m etric, biosensors, 2 1 2 detection in, ion chromatography, 1 38 high performance capillary electrophoresis, 1 3 8 detection of organic compounds i n flow­ through detectors, 2 1 4 detectors, 33, 1 28, 1 7 1 factors affecting the sensitivity of, 1 30 gas sensors, 1 7 1 titrations, 3 1 Amperometry, 3 1 , 1 70 Analyte, 1 Anodic current, 2 Anodic stripping voltammetry, 60 ap p li ca t i o ns, 1 53 An tim o ny, determin ation, 1 59 Arsenic d eterminatio n by cathodic str ipp i n g voltammetry, 1 6 1 As hi ng, cold plasma, 96 high pressure, 95 Automated voltammetric analysis, 1 40 Auxiliary electrode, 4 Biamperometry, 33 Bismuth determination by anodic stripping voltam metry, 1 5 9 Butyl-tin c omp o unds , determination of, 2 1 1 Cadmium determination by anodic stripping voltamm etry, 1 53, 1 5 7 using the RAMTM electrode, 1 59 Calibration cu rves 1 67, 209 Capillary characteristics, 2 1 Capacitive current (see charging current) ,

247

248

I ndex

Catalytic currents, 4, 1 0 in the determination of trace metals, 1 1 , 1 49

Cathodic current, 2 Cathodic stripping voltammetry, 60, 70 insoluble m e rcury compound formation

in,

70

in, 72 of orga n ic sulfur c o m p o unds 2 1 0 C EC mechanism , 1 2 metal co- deposition

Cells, 1

flow-through, 1 26- 1 30

micro, 99 thin layer, 1 26, 1 29 tubular, 1 26 voltammcrtric, 98 wall-jet, 1 26 CE mechanism, 1 1 Charge transfer coefficient, 7 effect on polarographic wave shape, 27 Ch a rge transfer r e a c t io n , 4 rate of, 6 Charging current, 1 2, 29, 3 8 , 5 1 Chemically modified electrodes, 1 1 3-1 1 7 Ch r o m ium , determination , 1 49, 1 66 Chronopotentiometric stripping analysis, 80 comparison with anodic stripping voltammetry, 8 1 derivative, 8 2 differential, 82 peak width in 84 effect of surfactants on, 84 li mit of detection, 82 th eo ry , 80 Chronopotentiometry, 52 basic pri n c i pl e s of, 53 d e riva t i ve , 55 irreversible processes i n , 54 rev ers i b l e processes in, 54 with time-dependent current, 55 sensitivi ty of, 56

Clarke cell, 33, 1 7 1 Cobalt determination, 1 4 1 Complexing agents, 26 use in improving selectivity, 69, 1 48 u s e in trace metal adsorptive stripping analyses, 72, 76, 77 Concentration gradient at electrode surface, 7, 8

Convection , 5

Copper determ ination by a no di c stripping vo lt am m e t ry, 1 53 using the RAM"'"M electrode, I 59 Cottrell e qu at i o n , 9 Coulometric detectors, 1 30 Counter el e ct rode , 2 Current, diffusion, 4 faradaic, 4 faradaic to c h a rgi n g cu r r en t ra t i o, 1 4 peaks, measurement of, 1 2 0 vol tammetric, 7 waves, measurement of, 1 2 0 Current-sampled d.c. polarography, 3 0 C ycl ic voltammetery, 36 ap p l i c a ti on of, 37 influence of chemi cal reaction of the electrode product, 37 irreversible electrode processes in, 37 reve r s i ble electrode processes in, 36 theory of, 36 Cyanide, determination of, 1 5 1 Cysteine and cystine, determinati on of, 202 Data processing, graphical 1 I 9 digitally, 1 20 D.c. polarography, I 8-3 0 adsorption currents in, 1 4 catalytic currents i n , 1 1 charging current in, 29 detection limits in, 3 1 diffusion current in, 1 9-2 1 effect of capillary characteristics on, 2 1 effec t of concentration on, 20 effect of mercury column height on, 2 1 effect of temperature on, 2 I effect of drop time on, 20 potential dependence of, 20 invention of, 3 irreversible electrode processes i n , 27 kin e t i c a lly controlled currents in, 1 0 l imiting current in, 9 , 20 maxima in, 2 7 migration current in, 5 residual current in, 2 8 reversible electrode processes in, 22 selectivity of, 28 theory of, I 9 D.c ( linear scan ) stripping voltam metry, 66

Index

Derivitisation 1 38, 204 Desorp ti o n , 5 1 Detection limits, de t ermining factor of, 1 4 Diazep am, determination of, 1 94 Dibucaine, determination of, 1 98 Differential p ulse polarography, 43-46 current measurement in, 44 peak half-widths in, 45 peak resolution in, 45 sensitivity of, 46 theory of, 44 waveform used in, 43 with d igitised eq u i p ment , 43 D i fferenti al p u lse stripping voltam met ry, 66 Differential p ul s e volt am metry, 43 Diffusion, 5 coefficient s, 8 controlled c u r rents , 4, 1 9 current, average, 2 1 c u rrent constant, 2 1 Fick's laws of, 8 , 9 layer, 5, 1 27, 1 29 linear, 7, 1 9, 107 spherical, 1 9, 108 up to a dropp i n g m er cury electro d e , 1 9 Di gest ion, 93 microwave, 93, 96 U.V., 93 wet, 94 Digitised instrumentation, 1 1 7 Do u ble layer, 1 2 capacitance, 1 3 differential capacitance of, 1 4 D ro p ping mercury electrode, 1 , 1 9, 1 03 controlled drop time, 20 dro p time, 20, 3 1 potential dependence of gravity controlled, 20 EC mechanism, 1 1 Electroactive organic fu n ct i onal groups, 1 76 Electrocapillary maximum, 14, 29 Electrode potential, effect on electrode reaction rate, 6 Electrode processes, 4 catalytic, 1 49, 1 0 , 1 64 and coupled chemical r e a ct i o n s , 6, l 0, 3 7 d iffus i o n controlled, 5 , 7 irreversible, 6 ki net ically controlle d , I 0

249

quasi-reversible, 7, 50 and rate determining ste p s, 6 reversible, 6 Electrodes, 1 auxiliary, 4 calomel, 1 00 carbon paste, 1 06, 1 3 1 charge on, 1 2 , 1 4 c h e m i ca lly modified, 1 1 3 counter, 2 disc, mic r o , 1 09 dro p p i ng mercury, 1 , 1 9, 1 03 glassy carbon, 1 06 g old , 1 06, 1 3 1 , 1 6 1 gra p hi te, 1 06 gr ap h i te u ltra - t r ace, 1 1 2 hanging me rcu ry drop, 6 1 , 1 03, 1 40 mercury pool, 4 mercury t h in film, 6 1 , 1 0 5 micro, 1 0 7 construction, 1 1 0 rapid response time of in flow-through analysis , 1 36 response time, 1 09 sensitivity, 1 09 mic ro-array, 109, 1 1 2 multi-modeTM, 1 05 platinum, 1 06, 1 3 1 "" random assembly micro d isc array � M , 1 1 2 reference, 2, 4, 99, 1 02 ro ta t ing disc, 64 silver, 70 silver - silver chloride, 1 00 static mercury drop, 3 1 , 1 03 thick film gra ph ite, 1 1 2 thick film m odified grap h ite , 1 1 4 tubular , 1 2 7 wall-j et, 1 3 3 working, 2, 1 0 1 Electroinactive, 1 1 Errors, sources of, 90 Ethylenediaminetetraacetic acid d ete r m i n atio n , 1 86 Extraction p olaro g raph y, 1 5 1 Faradaic currents, 4 Fast sweep p ol a r ogr a p hy ( see single sweep polarogra p hy ) Fenchlorazol-ethyl, determination of by a dsorpt i v e stri p ping voltammetry, 207

250

Index

Flow-through amperometric detectors in the analysis of, phenols, 1 33 polyaromatic hydrocarbons, 1 34-1 37 Flow- through cells, 1 2 6- 1 30 Flow-through electrodes, 1 27 Flow-through stripping voltammetry, 1 3 9 electrolyte exchange in, 1 4 1 , 142 Folic acid determination, 201 Formaldehyde determination, 1 80 Galvanostat, 52, Gold electrode, 1 6 1 Half-wave potential, 22 definition of, 9 dependence on concentration of analyte, 23, 24 dependence on concentration of supporting electrolyte, 23 dependence on solution composition, 26 effect of metal -ion complexation on, 26, 1 48 influence of pH on, 26 irreversible processes and, 27 of metal ions in various supporting electrolytes, 1 46 ranges of classes of organic compounds, 178 relationship t o E0, 2 3 reversible processes and, 2 3 Heterogeneous rate constants, 6 Heyrovsky-Ilkovic equation, 23 High performance liquid chromatograpy, with amperometric detection, 1 29, 1 32 with voltammetric detection, 1 36 High pressure ashing, 9 5 Hydrodynamic voltammetry, 1 34 Hydrogen sulfide sensor, 1 7 1 Hydrogen waves, catalytic, 1 2 Ilkovi c equation, 20 Insoluble product formation, 24, 70 Instrumentation, 1 1 7 automation of, 1 1 8 , 1 40 computerised, 1 1 7 multi functional, 1 1 7 iR drop, 3 Iron ( III) determination , 1 64 Irreversible electrode processes, 2 7

Karl Fisher titration, 33 Kinetic current, 4, 1 0 Lead determination by anodic stripping voltammetry, 1 53, 1 5 7 using the RAMrM electrode, 1 69 Linear sweep voltammetry, 33 charging current in, 35 influence of scan rate in, 3 5 limits of detection i n, 35 irreversible electrode processes in, 35 peak potential in, 34 resolution in, 35 reversible electrode processes in, 34 shape of curves in, 36 theory of, 33 Mass transport, 5 Medium exchange, 69 Mercury, determination of, 1 6 1 oxidation i n the presence o f organ ic compounds, 70, 209 Migration current, 5 Molybdenum determination, 1 1 , 1 68 Nernst, diffusion layer, 7 equation, 22 Nickel determina tion, 1 4 1 Nicotine determination, 1 97 Nitrilotriacetic acid determination, 1 86 Non-faradaic current, 1 4 Normal pulse polarography, 43 charging current in, 38 selectivity of, 43 sensitivity of, 43 waveform used in,44 Normal pulse voltammetry, 43 Organic solvents, 98 Osteryoung square-wave voltammetry, 39 Oxygen, interference, 28, 6 1 reduction at a mercury electrode, 98 removal of from solution, 28, 98 Peak current, 1 0, 6 1 Peak potential, 1 0, 62 Platinum determination by adsorptive

I ndex

stri p pin g voltammetry, 1 64 Polarographic speciation, 1 45 Polarographic spectrum, 1 4 5 P olaro graphy, definition of, 1 hi sto rical aspects of, 3 of anions, 1 50 of 1 ,4 b en zodiaz epi n es, 1 9 3 of c arbo n yl co m pounds, 1 80 of carb ox yl ic a cids , 1 8 3 of hydrocarb ons, 1 78 of multivalent i ons, 1 48 of n itrates , 1 92 of n i tro compounds 1 90 of nitroso c omp o unds, 1 92 of N-heterocycles, 1 96 of N -o xide s, 1 92 of organ ic halides, 1 79 of organic peroxides, 1 87 of o rgan i c s u l fur c o mpou n ds, 202 of o rg a no - met a ll i c com p o unds , 205 of qui n ones , 1 8 9 po te n t i a l ran ge i n , 2 8 Potassium chloride, d.c. charg i ng current in, 14 Potential o f zero charge, 1 4, 29 Potentiom etric st r ip pi n g analysis, 85 diffe r ent i al, 86 Potentiostat, 3 Q uarter-transition time, 54 Randles-Sevci k e quatio n , 33 Rapid scan square - wav e v o l tam metry in flow-th ro u gh analyses, 1 37 Rate c onsta n ts , 6 Reference elect rodes, 99 p os i t ioni n g of, 98 Reversible electrode processes, 22 Reversibi lity, 6 Riboflavin determination, 2 0 1 Sample pre-treatment, 92 Sand equati o n , 53 Selen ium det e rmi n a tio n , 1 62 S i n gle sweep po l aro gra p hy, 33 Solid phase extraction, 96 Sphericit y effect, 20, Square-wave polarography, 39 charging current in, 38

251

d ig ita ll y con t r o l led, 41 sen s itivi ty a n d se l ec t i vit y of, 39 theory of, 39 waveform used in, 39 Square-wave voltammetry, 39 Osteryoung, 39 Staircase vo l t age, 3 1 Staircase v o lt a m metry, 39, 4 1 , 45, 48, 66 dis c rimi n a t ion against charging c u rren t in, 66 waveform used in, 4 1 Staircase s tripping volt am metry , 66 Standard addition, 1 24, 1 52, 1 6 1 , 1 88, 207 Standard redox pote nt i al, 23 Stationary electrodes in flowing solutions, 1 27 Stripp in g volt a m metry, 60 acc umulation step i n , 58 accumulation po te n tial , 60, 62 accumulation time, 60 comparison of thin film and mercury drop e l e ctrodes i n , 62,64 electrodes used in, 62 in envi ronmen ta l studies, 77 in flow-th rou gh analysis, 1 39- 1 42 i n fl uen ce of sur facta nt s in, 77 inter-metallic co m po un d formation in, 65 limits of detection in, 66, 67 m e tal compl exa t i o n in, 68 peak width i n , 62 resolution in, 66 rest period in, 58 stripping step summary, 87 theory of, 62-64 tra ce met al analysis using, 1 53-1 70 trace metal spec i a t io n u si n g, 1 55 waveform used in, 66 Styrene, determination following derivitisation, 204 Sulfide dete rm i natio n , 1 50 Suppo r t i ng el ec trolytes, 30, 97 T ast

p o l ar o graphy, 30 Tellurium, dete rmina t io n of, 1 63 Tensammetric p e a ks , 5 1 Te nsammet ry, 5 1 applications, 5 2 Tomes equation, 2 4 Thiourea determination, 209 Tributyltin d eterm i n a tio n , 2 1 1

252

I ndex

Titanium determination, 1 52 Transfer c o efficient 6, 7 Trace metal speciation 1 5 5 Transition time, 5 3 , 80

Voltage , applie d, 2 Voltammetric determination of trace metals in polluted waters, 1 56 Voltammetry definition, 1

Uranium determination by adsorptive stripping voltammetry, 77, 1 66 U.V. i rradiation, 93 effect on trace metal determinations, \ 53

Water analysis, 1 55 Wo rki n g e lectrodes 2, 1 0 1

,

,

Vitamins, polarographic determination of, 1 84

,

,

Zinc determination by anodic stripping voltammetry, 1 53 half-wave potential effect of supporting electro lyte concentration on, 23 ,