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

BLACKIE ACADEMIC & PROFESSIONAL An Imprint of Chapman & Hall

London • Glasgow • Weinheim • New York • Tokyo • Melbourne • Madras

Contents

1 The development and structure of electrochemistry

1.1 The ubiquitous nature of electrochemistry 1.2 The historical dimension 1.3 The domains of electrochemistry

Part I Principles

2 lonic interaction: the ways in which ions affect each other in Solution

2.1 The nature of electrolytes 2.1.1 Ion-ion and ion-solvent interactions 2.1.2 Dissolution, solvation and heats of Solution

2.2 Ion activity 2.2.1 Chemical and electrochemical potential 2.2.2 Mean ion activity

2.3 The Debye-Hückel equation 2.3.1 A theoretical model for calculating activity coefficients 2.3.2 Limiting and extended forms of the Debye-Hückel equation

2.4 Ion association 2.4.1 Ionization, dissociation and association 2.4.2 The Bjerrum equation

Problems

3 lonic equilibria: the behaviour of acids and bases

3.1 Classical theory. The Arrhenius dissociation model 3.2 The Brensted-Lowry concept of acids and bases

3.2.1 The importance of solvent in generating acid-base properties 3.2.2 Relative strengths of conjugate pairs 3.2.3 Types of solvent and general acid-base theory

3.3 Strengths of acids and bases in aqueous Solution 3.3.1 Dissociation constants of acids and the self-ionization constant

of water 3.3.2 Dissociation constants of bases 3.3.3 Zwitterions 3.3.4 The values of dissociation constants

3.4 Extent of acidity and the pH scale 3.4.1 Calculation of pH for Solutions of strong acids and bases 3.4.2 Calculation of pH for Solutions of weak acids and bases

3.5 Hydrolysis. Salt Solutions showing acid-base properties 3.6 Calculation of the pH of salt Solutions

3.6.1 Salts derived from weak acids and strong bases 3.6.2 Salts derived from weak bases and strong acids 3.6.3 Salts derived from weak acids and weak bases

Vlll 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. L 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 Nernst-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 ahernation 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 Stern 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 Nernst 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 (rjA) 144 7.5.2 Resistance overvoltage (%) 144 7.5.3 Concentration overvoltage (r/c) 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-Hückel equation 157 8.1.1 Determination of thermodynamic equilibrium constants 157 8.1.2 Dependence of reaction rates on ionic strength 157

CONTENTS

8.2 Determination of equilibrium constants by conductivity measurements 159 8.2.1 Solubilities of sparingly soluble salts 159 8.2.2 The ionic product of self-ionizing solvents 160 8.2.3 Dissociation constants of weak electrolytes, e.g. weak acids 160

8.3 Thermodynamics of cell reactions 161 8.4 Determination of Standard potentials and mean ion activity coefficients 162 8.5 The determination of transport numbers 164

8.5.1 Determination by the Hittorf method 165 8.5.2 Determination by moving boundary methods 170 8.5.3 Determination using cell emf 173 8.5.4 Interpretation and application of transport numbers 173

8.6 Determination of equilibrium constants by measurements of potential 174 8.6.1 Dissociation constants of weak acids 174 8.6.2 The ionization constant of water 179 8.6.3 Solubility products 180 8.6.4 Equilibrium constants of redox reactions 181 8.6.5 Formation (stability) constants of metal complexes 182

8.7 The experimental determination of pH 183 8.7.1 The hydrogen electrode 183 8.7.2 The glass electrode 185

Problems 186

Electroanalytical techniques 189

9.1 What constitues electroanalysis? 189 9.2 Conductimetric titrations 189 9.3 Potentiometrie titrations 190

9.3.1 Zero current potentiometry 190 9.3.2 Constant current potentiometry 193 9.3.3 Potentiometry with two indicator electrodes 194

9.4 Classical voltammetric techniques 196 9.4.1 Polarography 197 9.4.2 Characteristics of diffusion-controlled Polarographie waves 200 9.4.3 Amperometric titrations 204 9.4.4 Wave characteristics and the mechanism of electrochemical processes 205

9.5 Modern Polarographie methods 209 9.5.1 Variation of current during the life of mercury drops 209 9.5.2 Pulse polarography 211 9.5.3 Differential pulse polarography 211 9.5.4 Stripping voltammetry 213

9.6 Voltammetry based on forced controlled convection 214 9.6.1 Rotating disc voltammetry 214 9.6.2 The ring-disc electrode 214

9.7 Cyclic voltammetry 215 9.8 Ultramicroelectrodes 216 9.9 Electrogravimetry 217

9.10 Coulometric methods 217 Problems 219

Electrochemical sensors 221

10.1 Ion-selective electrodes 221 10.1.1 Glass membrane electrodes 221 10.1.2 Solid-state electrodes 222 10.1.3 Liquid membrane electrodes 223

10.2 Problems with ion-selective electrodes 223 10.3 Chemically modified electrodes 225

CONTENTS XI

10.4 Gas-sensing electrodes 226 10.5 Enzyme electrodes 227 10.6 Sensors based on modified metal oxide field effect transistors (MOSFETs) 229 10.7 The wall-jet ring-disc electrode (WJRDE) 230

11 The exploitation of electrode processes 233

11.1 Mixed Potentials and double electrodes 233 11.1.1 Pourbaix diagrams 233 11.1.2 Corrosion prevention 236

11.2 Electrochemical processes as sources of energy 239 11.2.1 Primarycells 239 11.2.2 Secondary cells 241 11.2.3 Fuel cells 246

11.3 Electrocatalysis and electrosynthesis 249 11.3.1 Anodically initiated process 251 11.3.2 Cathodically initiated process 252

11.4 Electrochemistry on an industrial scale 252 Problems 254

Further Reading 256

Solutions to problems 259

Appendix I 271

Appendix II 274

Appendix III 275

Appendix IV 277

Index 279