Principles and applications of electrochemistry

295 Pages·1996·11.048 MB·English

by  Crow, David Richard

Principles and Applications of Electrochemistry

Principles and Applications of
Electrochemistry
Fourth edition
D. R. CROW
Professor of Electrochemistry and Dean of Research
University of Wolverhampton
Chapman & Hall/CRC
Taylor & Francis Group
Boca Raton London New York
Published in 1994 by
Chapman & Hall/CRC
Taylor & Francis Group
6000 Broken Sound Parkway NW, Suite 300
Boca Raton, FL 33487-2742
© 1974, 1979, 1988, 1994 by D. R. Crow
Chapman & Hall/CRC is an imprint of Taylor & Francis Group
First edition 1974, Second edition 1979, Third edition, 1988, This edition 1994
Originally published by Chapman & Hall
No claim to original U.S. Government works
International Standard Book Number-10: 0-7514-0168-4 (Softcover)
International Standard Book Number-13: 978-0-7514-0168-4 (Softcover)
Library of Congress catalog number: 94-70122
This book contains information obtained from authentic and highly regarded sources. Reprinted material is
quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts
have been made to publish reliable data and information, but the author and the publisher cannot assume
responsibility for the validity of all materials or for the consequences of their use.
No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic,
mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and
recording, or in any information storage or retrieval system, without written permission from the publishers.
Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only
for identification and explanation without intent to infringe.
Library of Congress Cataloging-in-Publication Data
Catalog record is available from the Library of Congress
Visit the Taylor & Francis Web site at
http://www.taylorandfrancis.com
Taylor & Francis Group and the CRC Press Web site at
is the Academic Division of Informa plc. http://www.crcpress.com
Preface
It has been my good fortune to have had the various editions of this book
in print for rather more than twenty years. Those who have used it have
been kind enough to comment on its appropriateness for them as either
teacher or student, and their identification of errors and obscurities has
always been communicated to me with diplomacy. There are now many
more books, both general and specific, which deal with the subject of
Electrochemistry: in some of them I flatter myself that there is evidence of
the influence of earlier editions of this text in regard to structure and
treatment. There is always something to learn from different approaches
and I acknowledge their help to me in formulating my own ideas for
presenting this update.
This fourth edition represents a considerable revision. Structurally it is
different from its predecessors in that principles and applications are
separated. Several derivations which at first (and maybe even subsequent)
readings of their context, may be omitted without in any way detracting
from the main theme have been placed in Appendices. Other areas have
been re-ordered, expanded or given greater prominence. An example is the
section on Electrochemical Sensors which now forms a chapter on its own;
another area is that of Electro-analytical Techniques which again has a
chapter devoted to it and which includes a number of modern methods not
included in earlier editions.
Maintenance of a proper balance between selectivity and detail is always
a problem in writing a text such as this one: however, in producing this new
edition, which is both re-structured and modestly expanded, the aim has
been to bring the essentials into focus and context, but to point the way to
further study, expanded treatment and greater intellectual depth. Above all,
it has been my concern that the shape of the subject of Electrochemistry
should be stated at the outset and not lost or obscured during the reading
of particular parts, that its coherence should be evident and that its wider
application should be appreciated.
There are a number of people whose help I would wish to acknowledge.
In terms of this edition I am indebted to my typist Ms Tracey Simcox whose
speed and accuracy of word-processing, combined with an ability to
decipher my drafts, contributed to my keeping to the schedule agreed with
the publishers. Which leads me to express my gratitude to the various
editors and staff of Chapman and Hall with whom I have worked over the
years: their courtesy, understanding and help have always been so suppor­
VI PREFACE
tive. My immediate colleagues and my own students are among those
already mentioned whose use of earlier editions and association with various
undergraduate courses have provided me with feedback on the effectiveness
of the book.
Finally, I am thankful for the inestimable benefit of a happy and settled
home where my wife Margaret, despite a busy academic life of her own, is
a constant source of support and encouragement.
D. R. C.
Contents
1 The development and structure of electrochemistry 1
1.1 The ubiquitous nature of electrochemistry 1
1.2 The historical dimension 1
1.3 The domains of electrochemistry 2
Part I Principles
2 Ionic interaction: the ways in which ions affect each other
in solution 9
2.1 The nature of electrolytes 9
2.1.1 Ion-ion and ion-solvent interactions. 9
2.1.2 Dissolution, solvation and heats of solution 10
2.2 Ion activity 11
2.2.1 Chemical and electrochemical potential 11
2.2.2 Mean ion activity 12
2.3 The Debye-Hiickel equation 13
2.3.1 A theoretical model for calculating activity coefficients 13
2.3.2 Limiting and extended forms of the Debye-Hiickel equation 16
2.4 Ion association 18
2.4.1 Ionization, dissociation and association 18
2.4.2 The Bjerrum equation 19
Problems 21
3 Ionic equilibria: the behaviour of acids and bases 22
3.1 Classical theory. The Arrhenius dissociation model 22
3.2 The Bronsted-Lowry concept of acids and bases 23
3.2.1 The importance of solvent in generating acid-base properties 23
3.2.2 Relative strengths of conjugate pairs 24
3.2.3 Types of solvent and general acid-base theory 24
3.3 Strengths of acids and bases in aqueous solution 25
3.3.1 Dissociation constants of acids and the self-ionization constant
of water 26
3.3.2 Dissociation constants of bases 26
3.3.3 Zwitterions 28
3.3.4 The values of dissociation constants 29
3.4 Extent of acidity and the pH scale 29
3.4.1 Calculation of pH for solutions of strong acids and bases 29
3.4.2 Calculation of pH for solutions of weak acids and bases 30
3.5 Hydrolysis. Salt solutions showing acid-base properties 31
3.6 Calculation of the pH of salt solutions 32
3.6.1 Salts derived from weak acids and strong bases 32
3.6.2 Salts derived from weak bases and strong acids 33
3.6.3 Salts derived from weak acids and weak bases 33
viii CONTENTS
3.7 Buffer systems 34
3.7.1 The Henderson-Hasselbalch equation 34
3.7.2 Efficiency of buffer systems: buffer capacity 36
3.8 Operation and choice of visual indicators 39
3.8.1 Functioning of indicators 40
3.8.2 Titrimetric practice 41
Problems 41
4 The conducting properties of electrolytes 43
4.1 The significance of conductivity data 43
4.1.1 Measurement of conductivity 43
4.1.2 Molar conductivity 45
4.1.3 Empirical variation of molar conductivity of electrolyte solutions with
concentration 46
4.1.4 The independent migration of ions 47
4.2 Conductivity and the transport properties of ions 50
4.2.1 Diffusion and conductivity: the Nemst-Einstein equation 52
4.2.2 Ion speeds and conductivity: the Einstein and Stokes-Einstein
equations 53
4.3 Rationalization of relationships between molar conductivity and electrolyte
concentration 58
4.3.1 Strong, completely dissociated electrolytes 58
4.3.2 Weak, incompletely dissociated electrolytes 61
4.3.3 Electrolyte systems showing ion pairing 62
4.4 Conductivity at high field strengths and high frequency of alternation of
the field 63
4.5 Electrical migration and transport numbers 65
Problems 67
5 Interfacial phenomena: double layers 68
5.1 The interface between conducting phases 68
5.2 The electrode double layer 68
5.3 Polarized and non-polarized electrodes 71
5.4 Electrocapillarity: the Lippmann equation 71
5.4.1 Variation of charge with applied potential at a mercury/solution
interface 72
5.4.2 Specific adsorption 75
5.5 Models for the double layer 76
5.5.1 Distribution of charge according to Helmholtz, Gouy and Chapman,
and Stem 76
5.5.2 The diffuse double layer 77
5.5.3 The zeta potential 77
5.6 Electrokinetic phenomena 78
5.6.1 Electro-osmosis 79
5.6.2 Streaming potential 81
5.6.3 Electrophoresis 83
5.7 Behaviour of colloidal systems 85
5.7.1 Stability of colloidal dispersions 85
5.7.2 Colloidal electrolytes 86
5.7.3 Polyelectrolytes 86
Problems 87
CONTENTS ix
6 Electrode potentials and electrochemical cells 88
6.1 Comparison of chemical and electrochemical reactions 88
6.2 Electrode potentials: their origin and significance 89
6.2.1 Types of potential operating at the electrode/solution interface 90
6.2.2 Measurable and non-measurable quantities 93
6.3 Electrode potentials and activity: the Nemst equation 93
6.4 Disturbance of the electrode equilibrium 96
6.4.1 Why electrons transfer 96
6.4.2 The distinction between fast and slow systems 96
6.5 The hydrogen scale and the IUPAC convention 102
6.5.1 The standard hydrogen electrode 103
6.5.2 Electrode potential and cell emf sign conventions 105
6.5.3 Calculation of cell emf values from tabulated data 108
6.6 Other reference electrodes 108
6.7 Concentration cells and emf measurements 111
6.8 Concentration cells without liquid junctions 112
6.8.1 Cells with amalgam electrodes 112
6.8.2 Cells with gas electrodes operating at different pressures 113
6.8.3 Concentration cells without transference 114
6.9 Concentration cells with liquid junctions 116
6.9.1 Cells with a liquid junction potential 116
6.9.2 Cells with eliminated liquid junction potentials 118
6.9.3 Calculation of liquid junction potentials 119
6.10 Membrane equilibria 120
6.10.1 Membrane potentials 120
6.10.2 Dialysis 124
6.10.3 Ion-exchange resins 125
Problems 126
7 Electrode processes 129
7.1 Equilibrium and non-equilibrium electrode potentials 129
7.1.1 Current-potential relationships for fast and slow systems 129
7.1.2 Mass transfer and electron-exchange processes 130
7.1.3 Types of mass transfer 132
7.2 The kinetics of electrode processes: the Butler-Volmer equation 133
7.3 The relationship between current density and overvoltage: the Tafel equation 138
7.4 The modern approach to the interpretation of electrode reactions 140
7.5 Electrolysis and overvoltage 143
7.5.1 Activation overvoltage (77^) 144
7.5.2 Resistance overvoltage (%) 144
7.5.3 Concentration overvoltage (7jc) 144
7.5.4 Summary of overvoltage phenomena and their distinguishing features 147
7.6 Hydrogen and oxygen overvoltage 148
7.6.1 Decomposition potentials and overvoltage 148
7.6.2 Individual electrode overvoltages 149
7.7 Theories of hydrogen overvoltage 151
Problems 152
Part II Applications
8 Determination and investigation of physical parameters 157
8.1 Applications of the Debye-Hiickel equation 157
8.1.1 Determination of thermodynamic equilibrium constants 157
8.1.2 Dependence of reaction rates on ionic strength 157