Kosuke Izutsu

Electrochemistry in NonaqueousSolutions

Second, Revised and Enlarged Edition

57268File Attachmentcover.jpg

Kosuke Izutsu

Electrochemistry in Nonaqueous

Solutions

Related Titles

Endres, F., MacFarlane, D., Abbott, A. (eds.)

Electrodeposition from Ionic Liquids

2008

ISBN: 978-3-527-31565-9

Eftekhari, A. (ed.)

Nanostructured Materials in Electrochemistry2008

ISBN: 978-3-527-31876-6

Hamann, C. H., Hamnett, A., Vielstich, W.

Electrochemistry

Second, Completely Revised and Updated Edition

2007

ISBN: 978-3-527-31069-2

Kosuke Izutsu

Electrochemistry in NonaqueousSolutions

Second, Revised and Enlarged Edition

The Author

Prof. Dr. Kosuke Izutsu4-31-6-208 Kichijoji-honchoMusashino 180-0004Japan

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Contents

Preface to the First Edition XIPreface to the Second Edition XIII

Part One Fundamentals of Chemistry in Nonaqueous Solutions:Electrochemical Aspects 1

1 Properties of Solvents and Solvent Classification 31.1 Properties of Solvents 81.1.1 Physical Properties of Solvents 81.1.2 Chemical Properties of Solvents 131.1.3 Structural Aspects of Solvents 171.1.4 Toxicity and Hazardous Properties of Solvents 191.2 Classification of Solvents 201.3 Effects of Solvent Properties on Chemical Reactions:

An Outline 22References 24

2 Solvation and Complex Formation of Ions and Behaviorof Electrolytes 27

2.1 Influence of Ion Solvation on Electrolyte Dissolution 272.2 Some Fundamental Aspects of Ion Solvation 292.2.1 Ion–Solvent Interactions Affecting Ion Solvation 292.2.2 Structure of Solvated Ions 362.2.3 Ultrafast Ion Solvation Dynamics 392.3 Comparison of Ionic Solvation Energies in Different Solvents and

Solvent Effects on Ionic Reactions and Equilibria 402.3.1 Gibbs Energies of Transfer and Transfer Activity Coefficients

of Ions 402.3.2 Prediction of Solvent Effects by the Use of Transfer Activity

Coefficients 442.4 Solvent Effects on the Complexation of Metal Ions 46

V

2.5 Selective Solvation of Ions in Mixed Solvents 492.6 Ion Association and Solvent Permittivities 53

References 59

3 Acid–Base Reactions in Nonaqueous Solvents 633.1 Solvent Effects on Acid–base Reactions 633.1.1 Acid–Base Reactions in Amphiprotic Solvents of High

Permittivity 643.1.2 Acid–Base Reactions in Aprotic Solvents of High Permittivity 693.1.3 Acid–Base Reactions in Amphiprotic Solvents of Low Permittivity 793.1.4 Acid–Base Reactions in Aprotic Solvents of Low Permittivity 793.2 pH Scales in Nonaqueous Solutions 803.2.1 Definition of pH in Nonaqueous Solutions 803.2.2 pH Windows in Nonaqueous Solvents and pH Scales Common

to Multisolvents 82References 86

4 Redox Reactions in Nonaqueous Solvents 894.1 Solvent Effects on Various Types of Redox Reactions 894.1.1 Fundamentals of Redox Reactions 894.1.2 Solvent Effects on Redox Potentials and Redox Reaction

Mechanisms 934.1.3 Dynamical Solvent Effects on the Kinetics of Redox Reactions 1004.2 Redox Properties of Solvents and Potential Windows 1034.3 Redox Titrations in Nonaqueous Solutions 1064.3.1 Titrations with Oxidizing Agents 1064.3.2 Titrations with Reducing Agents 109

References 110

Part Two Electrochemical Techniques and Their Applicationsin Nonaqueous Solutions 111

5 Overview of Electrochemical Techniques 1135.1 Classification of Electrochemical Techniques 1135.2 Fundamentals of Electrode Reactions and Current–Potential

Relations 1145.2.1 Current–Potential Relation for Electron Transfer at the Electrode 1155.2.2 Current–Potential Relations and Mass Transport 1185.3 DC Polarography – Methods that Electrolyze Electroactive Species

Only Partially (1) 1215.4 New Types of Polarography – Methods that Electrolyze

Electroactive Species Only Partially (2) 1295.4.1 AC Polarography 1305.4.2 SW Polarography 1315.4.3 Pulse Polarography 131

VI Contents

5.5 Voltammetry and Related New Techniques – Methods thatElectrolyze Electroactive Species Only Partially (3) 133

5.5.1 Linear Sweep Voltammetry 1345.5.2 Cyclic Voltammetry 1365.5.3 Voltammetry at Rotating Disk and Rotating Ring-Disk Electrodes 1385.5.4 Ultramicroelectrodes 1405.5.5 Modified Electrodes 1415.5.6 Other Items Related to Voltammetry 1425.5.7 Combination of Voltammetry and Nonelectrochemical Methods 1445.6 Electrogravimetry and Coulometry – Methods that Completely

Electrolyze Electroactive Species 1475.6.1 Controlled-Potential Electrolysis and Controlled-Current

Electrolysis 1475.6.2 Electrogravimetry 1495.6.3 Coulometry and Coulometric Titrations 1505.7 Potentiometry – A Method that Does Not Electrolyze Electroactive

Species 1525.7.1 Potentiometric Indicator Electrodes and Reference

Electrodes 1535.7.2 Potentiometric Titrations 1575.8 Conductimetry – A Method that Is Not Based on Electrode

Reactions 1585.9 Electrochemical Instrumentation – Roles of Operational

Amplifiers and Microcomputers 1615.9.1 Application of Operational Amplifiers in Electrochemical

Instrumentation 1615.9.2 Application of Personal Computers in Electrochemical

Instrumentation 167References 167

6 Potentiometry in Nonaqueous Solutions 1716.1 Basic Techniques of Potentiometry in Nonaqueous Solutions 1716.1.1 Potentiometric Indicator Electrodes for Nonaqueous Solutions 1726.1.2 Reference Electrodes for Nonaqueous Solutions 1726.1.3 Method of Reporting Electrode Potentials in Nonaqueous

Solutions (IUPAC Recommendation) 1756.1.4 Liquid Junction Potential Between Electrolyte Solutions in

the Same Solvent 1786.2 pH Measurements in Nonaqueous and Mixed Solvents 1806.2.1 IUPAC Method of pH Measurements in Aqueous Solutions 1806.2.2 Methods of pH Measurements in Nonaqueous and

Mixed Solvents 1826.2.3 Determination of Autoprotolysis Constants 1866.3 Applications of Potentiometry in Nonaqueous Solutions 1886.3.1 Acid–Base Reactions in Nonaqueous Solvents 188

Contents VII

6.3.2 Precipitation Reactions in Nonaqueous Solutions 1926.3.3 Complex Formation Reactions in Nonaqueous Solutions 1926.3.4 Redox Reactions in Nonaqueous Solutions 1946.3.5 Potentiometric Characterization of Solvents 1966.3.6 Potentiometric Study of Ion Solvation – Applications that Compare

Electrode Potentials in Different Solvents 1966.4 Liquid Junction Potentials between Different Solvents 201

References 206

7 Conductimetry in Nonaqueous Solutions 2097.1 Dissociation of Electrolytes and Electrolytic Conductivity 2097.1.1 Molar Conductivity of Dilute Solutions of Symmetrical Strong

Electrolytes 2097.1.2 Molar Conductivity and Association Constants of Symmetrical

Weak Electrolytes 2107.1.3 Molar Conductivity and the Formation of Triple Ions 2137.1.4 Conductivity of Solutions of Symmetrical Strong Electrolytes

at Moderate to High Concentrations 2147.1.5 Molar Conductivity and Ion Association of Asymmetric

Electrolytes 2167.2 Ionic Conductivities and Solvents 2177.2.1 Stokes Law and Waldens Rule – Role of Ultrafast

Solvent Dynamics 2177.2.2 Method for the Determination of Limiting Molar

Conductivities of Ions 2217.3 Applications of Conductimetry in Nonaqueous Solutions 2227.3.1 Study of the Behavior of Electrolytes (Ionophores) 2227.3.2 Conductimetric Studies of Acid–Base Equilibria 227

References 230

8 Polarography and Voltammetry in Nonaqueous Solutions 2338.1 Basic Experimental Techniques in Nonaqueous Solutions 2338.1.1 Experimental Apparatus for Nonaqueous Systems 2338.1.2 Solvents and Supporting Electrolytes 2368.2 Polarography and Voltammetry of Inorganic Species 2378.2.1 Polarographic Reductions of Metal Ions 2378.2.2 Polarography and Voltammetry of Metal Complexes 2478.2.3 Polarography and Voltammetry of Anions 2518.2.4 Electrode Reactions of Dissolved Oxygen, Dissolved Hydrogen,

Carbon Dioxide and Solvated Electrons 2538.3 Polarography and Voltammetry of Organic Compounds 2558.3.1 Reductions of Organic Compounds 2558.3.2 Oxidation of Organic Compounds 2688.4 Cyclic Voltammetry for Electrochemical Studies in Nonaqueous

Solutions 274

VIII Contents

8.4.1 Digital Simulation in Cyclic Voltammetry 2748.4.2 Ultramicroelectrodes in Cyclic Voltammetry 2758.4.3 Low-Temperature Electrochemistry and Cyclic Voltammetry 2778.5 Voltammetry of Isolated Nanoparticle Solutions

(Nanoelectrochemistry) 278References 281

9 Other Electrochemical Techniques in Nonaqueous Solutions 2879.1 Use of Electrolytic and Coulometric Techniques in Nonaqueous

Solutions 2879.2 Combination of Electrochemical and Nonelectrochemical

Techniques 2899.2.1 Spectroelectrochemistry 2899.2.2 Electrochemical-ESR Method 2969.2.3 Electrochemical Mass Spectroscopy 2999.2.4 Use of Electrochemical Quartz Crystal Microbalance 3019.2.5 Use of Scanning Electrochemical Microscopy 301

References 304

10 Purification of Solvents and Tests for Impurities 30710.1 Effects of Solvent Impurities on Electrochemical Measurements 30810.2 Procedures for the Purification of Solvents 30910.3 Tests for Purity of Solvents 31110.4 Purification Methods for Solvents in Common Use 314

References 320

11 Selection and Preparation of Supporting Electrolytes 32111.1 Selection of Supporting Electrolytes for Electrochemical

Measurements 32111.1.1 Solubility and Conductivity of Supporting Electrolytes 32111.1.2 Potential Windows and Supporting Electrolytes 32411.1.3 Influences of Supporting Electrolytes on Electrode Reactions

in Nonaqueous Solutions 32611.2 Methods for Preparing and Purifying Supporting Electrolytes 328

References 330

12 Use of Nonaqueous Solutions in ModernElectrochemical Technologies 333

12.1 Batteries Using Nonaqueous Solutions – Lithium Batteries 33312.2 Capacitors Using Nonaqueous Solutions 33712.2.1 Electrochemical Double-Layer Capacitors and Pseudocapacitors 33712.2.2 Aluminum Electrolytic Capacitors 33912.3 Conducting Polymers and Electrochemistry in Nonaqueous

Solutions 34012.4 Electrochemiluminescence (ECL) 343

Contents IX

12.5 Electrochemical Reduction of CO2 in Nonaqueous Solutions 34612.6 Use of Acetonitrile in Electrowinning and Electrorefining

of Copper 34912.7 Electrodeposition of Metals and Semiconductors from

Nonaqueous Solutions 351References 352

Part Three Electrochemistry in New Solvent Systems 355

13 Electrochemistry in Clean Solvents 35713.1 Introduction 35713.2 Supercritical Fluids 35813.2.1 General Aspects of Supercritical Fluids 35813.2.2 Electrochemical Aspects of Supercritical Fluids 36213.3 Ionic Liquids 36413.3.1 Physical, Chemical and Electrochemical Properties of ILs 36613.3.2 Voltammetry in ILs 37213.3.3 Applications of ILs in Electrochemical Technologies 37613.3.4 Risks of ILs and Their Decomposition 379

References 381

14 Electrochemistry at the Liquid–Liquid Interfaces 38514.1 Interfaces Between Two Immiscible Electrolyte Solutions (ITIES) 38514.1.1 Fundamentals of Electrochemistry at the ITIES 38514.1.2 Practical Applications of Electrochemistry at the ITIES 39214.1.3 Three-Phase Electrochemistry 39414.2 Interfaces Between Ionic Liquids and Water 39514.2.1 Fundamentals 39514.2.2 Salt Bridges and Reference Electrodes 39714.2.3 Potential Windows and Voltammograms 397

References 399

Index 401

X Contents

Preface to the First Edition

A majority of chemical reactions are carried out in solution. The use of a solvent asreaction medium makes it easy to control reaction conditions such as temperature,pressure, pH, rate of mass transfer and concentration of reactant. Water is the mostpopular solvent. However, by using appropriate nonaqueous solvents, substancesthat are insoluble in water can be dissolved, substances that are unstable in waterremain stable and chemical reactions that are impossible in water become possible.The reaction environments are markedly wider in nonaqueous solvents than inwater.The widespread use of nonaqueous solvents, especially dipolar aprotic solvents,

began in the 1950s in various fields of pure and applied chemistry and has con-tributed greatly to advances in chemical sciences and technologies. From the verybeginning, electrochemistry in nonaqueous solutions has played an important role inexploring new chemical possibilities as well as in providing the methods to evaluatestatic solvent effects on various chemical processes. Moreover, many new electro-chemical technologies have been developed using nonaqueous solvents. Recently,electrochemistry in nonaqueous solutions has made enormous progress: thedynamic solvent effects on electrochemical processes have been greatly elucidatedand solvent effects are now understood much better than before. On the other hand,however, it is also true that some useful solvents have properties that are problematicto human health and the environment. Today, efforts are being made, under theframework of green chemistry, to find environmentally benign media for chemicalprocesses, including harmless nonaqueous solvents, immobilized solvents, ionicliquids, supercritical fluids, aqueous systems and even solventless reaction systems.For electrochemical purposes, replacing hazardous solvents with harmless solvents,ionic liquids and supercritical fluids appears to be promising.This book was written to provide readers with some knowledge of electrochem-

istry in nonaqueous solutions, from its fundamentals to the latest developments,including the current situation concerning hazardous solvents. The book is dividedinto two parts. Part I (Chapters 1–4) contains a discussion of solvent properties andthen deals with solvent effects on chemical processes such as ion solvation, ioncomplexation, electrolyte dissociation, acid–base reactions and redox reactions. Such

XI

solvent effects are of fundamental importance in understanding chemistry innonaqueous solutions; furthermore, their quantitative evaluations are often carriedout by means of electrochemical techniques. Part II (Chapters 5–12) mainly dealswith the use of electrochemical techniques in nonaqueous solutions. In Chapter 5,the fundamentals of various electrochemical techniques are outlined in preparationfor the following chapters. In Chapters 6–9, the applications of potentiometry,conductimetry, polarography, voltammetry and other new electrochemical techni-ques in nonaqueous solutions are discussed by focusing on the chemical informa-tion they provide. Chapters 10 and 11 examine methods of selecting and purifyingthe solvents and electrolytes of electrochemical importance. Finally, in Chapter 12,some practical applications of nonaqueous solvents in modern electrochemicaltechnologies are discussed. These include their use in batteries, capacitors anddisplay devices, and such processes as electrolytic refining, plating, synthesis andpolymerization. The applicability of ionic liquids and supercritical fluids as envir-onmentally benign media for electrochemical technology is also dealt with.Most chemists are familiar with chemistry in aqueous solutions. However, the

common sense in aqueous solutions is not always valid in nonaqueous solutions.This is also true for electrochemical measurements. Thus, in this book, specialemphasis is placed on showing which aspects of chemistry in nonaqueous solutionsare different from chemistry in aqueous solutions. Emphasis is also placed onshowing the differences between electrochemical measurements in nonaqueoussystems and those in aqueous systems. The importance of electrochemistry innonaqueous solutions is now widely recognized by nonelectrochemical scientists –for example, organic and inorganic chemists often use cyclic voltammetry in aproticsolvents to determine redox properties, electronic states and reactivities of electro-active species, including unstable intermediates. This book will therefore also be ofuse to such nonelectrochemical scientists.I obtained most of the information included in this book from the publications of

many scientists in this field. I would like to express my sincere thanks to all of them.I also would like to thank my coworkers for their cooperation, the editorial andproduction staff of Wiley-VCH for their help and support and my wife for herassistance and patience.

Matsumoto Kosuke IzutsuDecember 2001

XII Preface to the First Edition

Preface to the Second Edition

The second edition consists of three parts: Part I (Chapters 1–4) is for electro-chemical aspects of the fundamentals of chemistry in nonaqueous solutions, PartII (Chapters 5–12) deals with the electrochemical techniques and their applica-tions in nonaqueous solutions and Part III (Chapters 13 and 14) is concerned withthe electrochemistry in new solvent systems. Chapter 13 is on the electrochem-istry in clean solvents and special emphasis is placed on supercritical fluids and(room-temperature) ionic liquids. Chapter 14 is on the electrochemistry at theliquid–liquid interfaces and, in addition to charge transfers at the interfacebetween water and organic solvents, those at the interface between water andionic liquid are also discussed. Since the publication of the first edition, consider-able efforts have been made in pursuing green solvents that are benign to humanhealth and the environment. Especially, the movement in the field of ionic liquidshas been remarkable and the rapid progress is still continuing. Most of the ionicliquids are nonvolatile, nonflammable, less toxic, chemically and thermally stableand good solvents for both organic and inorganic materials. Due to their aproticproperties, many ionic liquids can replace aprotic organic solvents, particularly foruse in electrochemistry. Moreover, the immiscibility of ionic liquids with watermakes possible their applications to the water/ionic liquid interfaces in variousways, including electrochemical uses.Besides the addition of the two new chapters, each of the Chapters 1–12 has been

updated and revised. Especially, the revision in Chapter 12 is considerable becausethe use of nonaqueous solutions in modern electrochemical technologies is steadilyincreasing.Although some nonaqueous solvents that are hazardous to human health and the

environment cannot be used or should be used with great care, the needs for theelectrochemical use of less hazardous solvents, including supercritical fluids andionic liquids, are increasing. Thus, the knowledge of electrochemistry in nonaqu-eous solutions is also increasing its importance.Finally, I wish to thank the editorial and production staff at Wiley-VCH Verlag

GmbH, for their help and support in making this edition.

Musashino, Tokyo Kosuke IzutsuAugust 2009

XIII

Books, reviews and data compilations on nonaqueous solvents and the chemistry,especially electrochemistry, in nonaqueous solutions are shown below:

1 Lagowski, J.J. (ed.) (1966) The Chemistryof Non-Aqueous Solvents, Academic Press,New York, vol. 1; 1967, vol. 2; 1970, vol.3; 1976, vol. 4; 1978, vol. 5A; 1978, vol.5B. Includes many reviews.

2 Charlot, G. and Tremillon, B. (1969)Chemical Reactions in Solvents and Melts,Pergamon Press, Oxford.

3 Bard, A.J. (ed.) (1969) ElectroanalyticalChemistry, vol. 3, Marcel Dekker, NewYork, p. 57; 1975, vol. 8, p. 281, etc.

4 Coetzee, J.F. and Ritchie, C.D. (eds)(1969) Solute–Solvent Interactions, MarcelDekker, New York, vol. I; 1976, vol. II,includes reviews.

5 Mann, C.K. and Barnes, K.K. (1970)Electrochemical Reactions in NonaqueousSolvents, Marcel Dekker, New York.

6 Janz, G.J. and Tomkins, R.P.T. (eds)(1972) Nonaqueous Electrolytes Handbook,vol. 1, Academic Press, New York; 1973,vol. 2.

7 Covington, A.K. and Dickinson, T. (eds)(1973) Physical Chemistry in OrganicSolvent Systems, Plenum Press,New York, includes reviews and datacompilations.

8 Fritz, J.S. (1973) Acid–Base Titrations inNonaqueous Media, Allyn & Bacon,Needham Heights, MA.

9 Tremillon, B. (1974) Chemistry inNonaqueous Solvents, D. Reidel,Dordrecht, the Netherlands.

10 (a) Meites, L. and Zuman, P. (eds)(1977–1983) CRC Handbook Series inOrganic Electrochemistry, vols. I–VI, CRCPress, Boca Raton, FL; (b) (1981–1988)CRC Handbook Series in InorganicElectrochemistry, vols. I–VIII, CRC Press,Boca Raton, FL. Compilations ofpotential data.

11 Marcus, Y. (1977) Introduction to LiquidState Chemistry, John Wiley & Sons, Inc.,New York.

12 Burgess, J. (1978) Metal Ions in Solutions,Ellis Horwood, Chichester.

13 Gutmann, V. (1978) Donor–AcceptorApproach to Molecular Interactions,Plenum Press, New York.

14 Kolthoff, I.M. and Elving, P.J. (eds) (1979)Treatise on Analytical Chemistry, 2nd edn,Part I, vol. 2, John Wiley & Sons, Inc.,New York, Chapter 19. Excellent reviewson acid–base reactions in non-aqueoussystems.

15 Popovych, O. and Tomkins, R.P.T.(1981) Nonaqueous Solution Chemistry,John Wiley & Sons, Inc., New York.Treats electrochemical aspects in detail.

16 Coetzee, J.F. (ed.) (1982) RecommendedMethods for Purification of Solvents andTests for Impurities, Pergamon Press,Oxford. Reports from IUPAC.

17 Burger, K. (1983) Solvation, Ionic andComplex Formation Reactions inNonaqueous Solvents, Elsevier,Amsterdam.

18 Marcus, Y. (1985) Ion Solvation, JohnWiley & Sons, Inc., New York. Includeslarge amounts of data.

19 Riddick, A., Bunger, W.R. and Sakano,T.K. (1986) Organic Solvents, PhysicalProperties and Methods of Purification, 4thedn, John Wiley & Sons, Inc., New York.Includes detailed data on solventproperties and methods of solventpurification.

20 Safarik, L. and Stransky, Z. (1986)Titrimetric analysis in organic solvents,Comprehensive Analytical Chemistry,vol. 22, Elsevier, Amsterdam.

21 Izutsu, K. (1990) Acid–Base DissociationConstants in Dipolar Aprotic Solvents,IUPAC Chemical Data Series No. 35,Blackwell Science, Oxford. Datacompilation.

22 Krestov, G.A. (1991) Thermodynamics ofSolvation, Solution and Dissolution, Ionsand Solvents, Structure and Energetics,Ellis Horwood, New York.

23 Mamantov, G. and Popov, A.I. (eds)(1994) Chemistry of Nonaqueous Solutions,Current Progress, Wiley-VCH VerlagGmbH, Weinheim.

24 Galus, Z. (1994) in Advances inElectrochemical Science and Engineering,vol. 2 (eds H. Gerischer and C.W.Tobias), Wiley-VCH Verlag GmbH,

XIV Preface to the Second Edition

Weinheim, pp. 217–295.Thermodynamics and kinetics ofelectrode reactions in nonaqueous andmixed solvents.

25 Gutmann, V. and Resch, G. (1995)Lecture Notes on Solution Chemistry,World Science, Singapore.

26 Sawyer, D.T., Sobkowiak, A. andRoberts, J.L., Jr (1995) Electrochemistryfor Chemists, 2nd edn, John Wiley &Sons, Inc., New York. Useful referenceson electrochemical techniques innonaqueous solutions.

27 Kissinger, P.T. and Heineman, W.R.(eds) (1996), Laboratory Techniques inElectroanalytical Chemistry, MarcelDekker, New York. Includes manychapters on electrochemical techniquesin nonaqueous solutions.

28 Marcus, Y. (1997) Ion Properties, MarcelDekker, New York.

29 Tremillon, B. (1997) Reactions inSolution: An Applied Analytical Approach,John Wiley & Sons, Inc., New York.

30 Barthel, J.M.G., Krienke, H. and Kunz,W. (1998) Physical Chemistry of ElectrolyteSolutions: Modern Aspects, Topics inPhysical Chemistry, vol. 5, Springer,Berlin.

31 Marcus, Y. (1998) The Properties ofSolvents, John Wiley & Sons, Inc.,New York.

32 Chipperfield, J.R. (1999) Non-AqueousSolvents, Oxford University Press, Oxford.

33 Aurbach, D. (ed.) (1999) NonaqueousElectrochemistry, Marcel Dekker,New York. Mainly concerned withlithium batteries.

34 Burgess, J. (1999) Ions in Solution, BasicPrinciples of Chemical Interactions,Horwood Publishing, Chichester.

35 Wypych, G. (ed.) (2001) Handbook ofSolvents, ChemTec Publishing, Toronto.

36 (a) Lund, H. and Hammerich, O. (eds)(2001) Organic Electrochemistry, 4th edn,Marcel Dekker, New York; (b)Lund, H.and Baizer, M.M. (eds) (1991) OrganicElectrochemistry, 3rd edn, Marcel Dekker,New York. Detailed treatment ofelectrochemical techniques and electrodeprocesses of organic substances.

37 Marcus, Y. (2002) Solvent Mixtures:Properties and Selective Solvation, MarcelDekker, New York.

38 Buncel, E. (2003) The Role of the Solventin Chemical Reactions (Oxford ChemistryMaster 6), Oxford University Press,Oxford.

39 Fawcett, W.R. (2004) Liquids, Solutions,and Interfaces: From Classical MacroscopicDescriptions to Modern MicroscopicDetails, Oxford University Press, Oxford.

40 Reichardt, C. (2005) Solvents and SolventEffects in Organic Chemistry, 3rd edn,Wiley-VCH Verlag GmbH, Weinheim.

41 Wasserscheid, P. and Welton, T. (eds)(2008) Ionic Liquids in Synthesis, 2ndedn, vols. 1–2, Wiley-VCH VerlagGmbH, Weinheim.

42 Endres, F., Abbott, A.P. and MacFarlane,D.R. (eds) (2008) Electrodeposition fromIonic Liquids, Wiley-VCH Verlag GmbH,Weinheim.

Examples of books dealing with thefundamentals of electrochemistry:

1 Rossiter, B.W. and Hamilton, J.F. (eds)(1986) Electrochemical Methods, PhysicalMethods of Chemistry, vol. II, 2nd edn,John Wiley & Sons, Inc., New York.

2 (a) Brett, C.M.A. and Brett, A.M.O.(1993) Electrochemistry: Principles,Methods and Applications, OxfordUniversity Press, Oxford; (b) (1998)Electroanalysis, Oxford University Press,Oxford.

3 Koryta, J., Dvorak, J. and Kavan, L.(1993) Principles of Electrochemistry,2nd edn, John Wiley & Sons, Inc.,New York.

4 Oldham, H.B. and Myland, J.C. (1994)Fundamentals of Electrochemical Science,Academic Press, New York.

5 Galus, Z. (1994) Fundamentals ofElectrochemical Analysis, 2nd edn, JohnWiley & Sons, Inc., New York.

6 Rubinstein, I. (ed.) (1995) PhysicalElectrochemistry: Principles, Methods,and Applications, Marcel Dekker,New York.

7 Fisher, A.C. (1996) Electrode Dynamics,Oxford University Press, Oxford.

8 Bockris, J.OM. and Reddy, A.N. (1998)Modern Electrochemistry, 2nd edn, vol. 1,Ionics; Plenum Press, New York; (2000)Vol. 2A, Fundamentals of Electrodics;(2000) vol. 2B, Electrodics in Chemistry,

Preface to the Second Edition XV

Engineering, Biology and EnvironmentalScience.

9 Bard, A.J. and Faulkner, L.R. (2001)Electrochemical Methods: Fundamentalsand Applications, 2nd edn, John Wiley &Sons, Inc., New York.

10 Bond, A.M. (2002) Broadening Electroche-mical Horizons: Principles and Illustrationof Voltammetric and Related Techniques,Oxford University Press, Oxford.

11 Bard, A.J. and Stratmann, M., et al.(2002�2007) Encyclopedia ofElectrochemistry, vols. 1–11, Wiley-VCHVerlag GmbH, Weinheim.

12 Bagotsky, V.S. (2005) Fundamentalsof Electrochemistry (The ECS Series of

Text and Monographs), 2nd edn,Wiley-Interscience, New York.

13 Savéant, J.-M. (2006) Elements ofMolecular and BiomolecularElectrochemistry: An ElectrochemicalApproach to Electron Transfer Chemistry,John Wiley & Sons, Inc., NJ.

14 Wang, J. (2006) AnalyticalElectrochemistry, 3rd edn, Wiley-VCHVerlag GmbH, New York.

15 Compton, R.G. and Banks, C.E. (2007)Understanding Voltammetry, WorldScientific, London.

16 Hamann, C.H., Hamnett, A. and Vielstich,W. (2007) Electrochemistry, 2nd edn,Wiley-VCHVerlag GmbH,Weinheim.

XVI Preface to the Second Edition

Part OneFundamentals of Chemistry in Nonaqueous Solutions:Electrochemical Aspects

1Properties of Solvents and Solvent Classification

Three types of liquid substances, i.e. molecular liquids, ionic liquids and atomicliquids, can serve as solvents. They dissolve solutes that are solid, liquid or gaseousand form solutions. Molecular liquid solvents are the most common and include,besides water, many organic solvents and some inorganic solvents, such as hydrogenfluoride, liquid ammonia and sulfur dioxide. Ionic liquid solvents are mostly moltensalts and usually used at relatively high temperatures. Nowadays, however, variousroom-temperature ionic liquids (or simply called ionic liquids) are being designedand used as a kind of green solvents.1) There are only a few atomic liquid solvents atroom temperatures, metallic mercury being a typical example. Besides these liquidsolvents, supercriticalfluids are sometimes used asmedia for chemical reactions andseparations (see footnote 1).Apart from Chapter 13, which deals with electrochemistry in new solvents

(supercritical fluids, ionic liquids, etc.), this book mainly considers molecular liquidsolvents. Thus, the term solvents means molecular liquid solvents. Water existsabundantly in nature and has many excellent solvent properties. If water is appro-priate for a given purpose, it should be used without hesitation. If water is notappropriate, however, some other solvent (or ionic liquids) must be employed.Molecular solvents other than water are generally called nonaqueous solvents.Nonaqueous solvents are oftenmixedwithwater or someother nonaqueous solvents,to obtain desirable solvent properties. These mixtures of solvents are called mixedsolvents.

1) Green chemistry is the utilization of a set ofprinciples that reduces or eliminates the use orgeneration of hazardous substances in thedesign, manufacture and application ofchemical products (Anastas, P.T., Warner, J.C.(1998) Green Chemistry, Theory and Practice,Oxford University Press, New York, p. 11).Under the framework of green chemistry,efforts are being made to find environmentallybenign media (green solvents) for chemicalprocesses; among such media are harmless

nonaqueous solvents, immobilized solvents,ionic liquids, supercritical fluids, aqueous re-action systems and solvent-free reaction sys-tems. For the recent situation, see, for example,Abraham, M.A., Moens, L. (eds) (2002) CleanSolvents: Alternative Media for Chemical Reac-tions and Processing, Oxford University Press,New York; Nelson, W.M. (2003) Green Solventsfor Chemistry: Perspectives and Practice (GreenChemistry Series), Oxford University Press,New York.

j3

Table 1.1 Physical properties of organic solvents and some inorganic solvents of electrochemical importance.

Solvent Abbreviatedsymbol

Bp (�C) Fp (

�C) Vapor

pressurea

(mmHg)

Densitya

(g cm�3)Viscositya

(cP)Conductivitya

(S cm�1)Relativepermittivitya

Dipolemomenta

(D)

Toxicityb

(1) Water 100 0 23.8 0.9970 0.890 6· 10�8 78.39 1.77

Acids(2) Hydrogen fluoride 19.6 �83.3 — 0.9529 0.256 1· 10�4 84.0 1.82 0.5(3) Formic acid 100.6 8.27 43.1 1.2141 1.966 6· 10�5 58.516 1.8230 5(4) Acetic acid HOAc 117.9 16.7 15.6 1.0439 1.130 6· 10�9 6.19 1.6830 10(5) Acetic anhydride 140.0 �73.1 5.1 1.0749 0.78330 5· 10�9 20.719 2.82 5

Alcohols(6) Methanol MeOH 64.5 �97.7 127.0 0.7864 0.551 1.5· 10�9 32.7 1.71 200,T(7) Ethanol EtOH 78.3 �114.5 59.0 0.7849 1.083 1.4· 10�9 24.6 1.74 1000,C(A4)(8) 1-Propanol 1-PrOH 97.2 �126.2 21.0 0.7996 1.943 9· 10�918 20.5 1.71 200,C(A3)(9) 2-Propanol 2-PrOH 82.2 �88.0 43.3 0.7813 2.044 6· 10�8 19.9 1.6630 200,C(A4)(10) Methyl cellosolve (i) 124.6 �85.1 9.7 0.9602 1.60 1.1· 10�6 16.9 2.04 5(11) Cellosolve (ii) 135.6

Ketones(17) Acetone Ac 56.1 �94.7 231 0.7844 0.303 5 · 10�9 20.6 2.720 500,C(A4)(18) 4-Methyl-2-pentanone MIBK 117.4 �84 18.8 0.7963 0.546

Table 1.1 (Continued)

Solvent Abbreviatedsymbol

Bp (�C) Fp (

�C) Vapor

pressurea

(mmHg)

Densitya

(g cm�3)Viscositya

(cP)Conductivitya

(S cm�1)Relativepermittivitya

Dipolemomenta

(D)

Toxicityb

Sulfur compounds(37) Sulfur dioxide �10.01 �75.46 1.46�10 0.4290 15.60 1.62 2, C(A4)(38) Dimethyl sulfoxide (xiii) DMSO 189.0 18.5 0.60 1.095 1.99 2· 10�9 46.5 4.06 NE(39) Sulfolane (xiv) TMS 287.3 28.5 5.0118 1.26030 10.330

A3 and A4 show confirmed human carcinogen, confirmed animal carcinogen with unknown relevance to humans and not classifiable as a human carcinogen,respectively. Themark Tshows the solvent has been listed in Title III of the Clean Air Act Amendments of 1990 as a hazardous air pollutant. NE stands for not established.

(i) CH3OCH2CH2OH

(ii) C2H5OCH2CH2OH

(iii) O-CH2CH2CH2CH2

CH2CH2

CH2CH2O (iv) O

(v) CH3OCH2CH2OCH3

(vi) (CH3OCH2CH2)2O

(viii) HCON(CH3)2

N(CH3)2

N(CH3)2

N-P=O

(x) CH3CON(CH3)2

(ix) CH3CONH(CH3)

(vii) HCONH(CH3)

(xii) H2C CH2

N

CH3

(xiii) (CH3)2S=O

(xiv) H2C

(xv) H2C

H2C

H2C

H2C

CH2 CH2

CH2

CH2

C=O

C=O

O

O

O

S

(xvi) CH3CH

O O C

O

(xi) (CH3)2

1Properties

ofSolventsand

SolventClassificationj

7

There are a great many kinds of neat nonaqueous solvents. Substances that aresolid or gaseous at ambient temperatures also work as solvents, if they are liquefiedat higher or lower temperatures. For mixed solvents, it is possible to vary the mixingratio and thus the solvent properties continuously. Therefore, if bothnonaqueous andmixed solvents are included, the number of solvents really is infinite.When a nonaqueous solvent is to be used for a given purpose, a suitable one must

be selected from the infinite number available. This is not easy, however, unless thereare suitable guidelines available on how to select solvents. In order to make solventselection easier, it is useful to classify solvents according to their properties. Theproperties of solvents and solvent classification have been dealt with in detail in theliterature [1, 2]. In this chapter, these problems are briefly discussed in Sections 1.1and 1.2, and then the influences of solvent properties on reactions of electrochemicalimportance are outlined in Section 1.3.Organic solvents and some inorganic solvents for use in electrochemical mea-

surements are listed in Table 1.1, with their physical properties.

1.1Properties of Solvents

Physical and chemical properties that are important in characterizing solvents asreaction media are listed in Table 1.2 and are briefly discussed in Sections 1.1.1and 1.1.2. The data of solvent properties have been compiled in Refs [2–4] for anumber of solvents. In addition to these properties, structural aspects of solvents areoutlined in Section 1.1.3 and the effects of toxicity and the hazardous properties ofsolvents are considered in Section 1.1.4.

1.1.1Physical Properties of Solvents

Each of the physical properties in Table 1.2 has its own significance.2) The boilingpoint, Tb, and the melting (or freezing) point determine the liquid range of solvents.The vapor pressure is a fundamental vaporization property, and it is also importantwhen considering the problem of toxicity and other hazards of solvents(Section 1.1.4). The heat of vaporization, DvH, determines the cohesive energydensity, c, defined by c¼ (DvH�RT )/Vm, and the solubility parameter, d, is definedby d¼ c1/2¼ [(DvH�RT)/Vm]1/2, where Vm is the molar volume. The cohesiveenergy density is a measure of the stickiness of a solvent and is related to thework necessary to create cavities to accommodate solute particles in the solvent.Conversely, the solubility parameter proposed by Hildebrand is useful in predictingthe solubilities of nonelectrolyte solutes in low-polarity solvents. In many cases, twoliquid substances with similar d values are miscible, while those with dissimilar

2) See Refs [1–3] or advanced textbooks of physicalchemistry.

8j 1 Properties of Solvents and Solvent Classification

d values are immiscible.3) The heat of vaporization at the boiling point, DvH(Tb) inkJmol�1, determines Troutons constant, (DvS(Tb)/R), which is equal to DvH(Tb)/Tb.Solvents with DvS(Tb)/R� 11.6 are usually nonstructured (e.g. DvS(Tb)/R¼ 7.2 foracetic acid, 10.2 for hexane, 10.5 for benzene and 10.9 for acetone), while thosewith DvS(Tb)/R� 12 are structured (e.g. DvS(Tb)/R¼ 12.5 for methanol and 13.1 forwater). The viscosity (h) influences the rate of mass transfer in the solvent andtherefore the conductivity of electrolyte solutions.The relative permittivity, er, influences the electrostatic interactions between

electric charges. If two charges, q1 and q2, are placed in a vacuum at a distance rfrom each other, the electrostatic force Fvac between them is expressed by Eq. (1.1):

Fvac ¼ q1q24pe0r2 ð1:1Þ

where e0 is the permittivity of a vacuum and e0¼ 8.854� 10�12 Fm�1. Fvac is arepulsive force if q1 and q2 are of the same sign, while Fvac is an attractive force if they

Table 1.2 Physical and chemical properties of solvents.

Physical properties Bulk properties: boiling point, melting (or freezing) point, molar mass,density, viscosity, vapor pressure, heat capacity, heat of vaporization,refractive index, relative permittivity, electric conductivity; molecularproperties: dipole moment, polarizability

Chemical properties Acidity (including the abilities as proton donor, hydrogen bond donor,electron pair acceptor and electron acceptor)a; basicity (including theabilities as proton acceptor, hydrogen bond acceptor, electron pairdonor and electron donor)a

aThe terms acidity and basicity are used in somewhat wider ways than usual (see text).

3) The primary role of solvents is to dissolvesubstances. There is an old principle – likedissolves like. In general, polar solvents candissolve polar substances, while nonpolar sol-vents can dissolve nonpolar substances. Thefollowing table shows the relationship betweenthe polarities of solvents and solutes and theirmutual solubilities:

The necessary condition for dissolution of asubstance is that energetic stabilization is

obtained by dissolution. The energetic stabili-zation depends on the energies of three inter-actions, i.e. solute–solvent, solute–solute andsolvent–solvent interactions. When both thesolvent and the solute are nonpolar, all of thethree interactions are weak. In that case, theenergy gained by the entropy of mixing of thesolvent and the solute plays an important role

in the high mutual solubility. For the dissolu-tion of electrolytes, see Section 2.1.

Solvent A Solute B Interaction Mutual solubility

A � � � A B � � � B A � � � B

Nonpolar Nonpolar Weak Weak Weak HighNonpolar Polar Weak Strong Weak LowPolar Nonpolar Strong Weak Weak LowPolar Polar Strong Strong Strong High

1.1 Properties of Solvents j9

are of opposite sign. If the two charges are placed in a solvent of relative permittivity erand at a distance r, the electrostatic force Fsolv between them is expressed by Eq. (1.2):

Fsolv ¼ q1q24pe0err2 ¼Fvacer

ð1:2Þ

Because er is larger than �1.8 for most solvents (1.84 for n-pentane and 1.88 forn-hexane are examples of lowest er values), the electrostatic interaction betweencharges is always weakened by solvents. As discussed in Chapter 2, the relativepermittivity of a solvent has a decisive influence on the electrostatic solute–solute andsolute–solvent interactions as well as on the dissolution and dissociation of electro-lytes. Thus, it is used in classifying solvent polarity or solvating capability. Solvents ofhigh permittivities (er � 15 or 20) are called polar solvents, while those of lowpermittivities are called apolar or nonpolar solvents (Section 1.2). Many of the solventslisted inTable1.1arepolar solvents, because thesolvents for electrochemicalusemustdissolve anddissociate electrolytes. The relative permittivities ofN-methylformamide(NMF) and N-methylacetamide (NMA) are exceptionally high, at 182 and 191,respectively. This is because the molecules of these solvents mutually interact byhydrogenbondingandarelinearlyarranged,causinghighpermittivities(Section1.1.3).However, somenonpolar solvents, e.g. hexane andbenzene (er� 2), arenowalsousedin electrochemical measurements, as will be discussed in Section 8.4.If a solvent is placed in a low-frequency electric field (

ion solvation, homogeneous and heterogeneous electron-transfer reactions and ionicmigrations, as discussed in Sections 2.2.2, 4.13, 7.2.1, 8.2.2 and 8.3.1.The refractive index, nD, defined as the ratio of light speed at the sodium D-line

in a vacuum to that in the medium, is used in obtaining the polarizability, a, of

18161412108

UV/visFIR + IRMicrowavees

e∞eop1

Pe

rmitt

ivity

log (n /Hz)

Figure 1.1 Dielectric dispersion spectra for a polar solvent with asingle Debye relaxation process in the microwave region and tworesonant transmissions in the IR and UV ranges [5b].

Table 1.3 Solvent dielectric and related properties at 25b

C.a

Solvent es (¼ er) eop e1 e� 1op � e� 1s sD (ps) sL (ps)

Debye solventsAN 37.5 1.80 2 0.528 3.3 �0.2Ac 21 1.84 2 0.495 3.3 0.3DMSO 46.7 2.18 5.7 0.438 19.5 2.4HMPA 29.6 2.12 3.3 0.438 80 8.9NB 35.7 2.40 4.1 0.389 45.6 5.2Py 13.3 2.27 2.3 0.365 6.9 1.2THF 7.58 1.97 2.3 0.376 3.3 1.0

Non-Debye solventsEtOH 24.5 1.85 4.2 0.499 130 22FA 110 2.09 7.0 0.469 37 2.35MeOH 32.7 1.76 5.6 0.628 48 8.2NMF 182 2.04 5.4 0.485 123 3.71-PrOH 20.4 1.92 2.2 0.472 390 42

Debye or non-Debye (?) solvents (different viewpoints exist)DMF 36.7 2.04 4.5 0.472 11.0 1.3PC 65 2.02 4.1 0.480 43 2.7

aFrom McManis, G.E., Golovin, M.N. and Weaver, M.J. (1986) J. Phys. Chem., 90, 6563; Barthel, J.,Bachhuber, K., Buchner, R., Gill, J.B. and Kleebauer, M. (1990) Chem. Phys. Lett., 167, 62; Galus, Z.(1995)Advances in Electrochemical Science and Engineering, vol. 4 (edsH. Gerischer and C.W. Tobias),Wiley-VCH Verlag GmbH, Weinheim, p. 222.es: static permittivity; eop: optical permittivity; e¥: infinite frequency permittivity; (e� 1op � e� 1s ):solvent Pekar factor; tD: Debye relaxation time and tL: longitudinal relaxation time.

1.1 Properties of Solvents j11

solvent molecules. The relationship between a and nD is given by a ¼ ð3Vm=4pNAÞðn2D � 1Þ=ðn2D þ 2Þ, whereNA is the Avogadro constant andVm is themolar volume.4)Solvent molecules with high a values tend to interact easily with one another or withother poralizable solute particles by dispersion forces.5)

Most solvents consist of molecules that are intrinsic dipoles and have permanentdipole moments (m). If such molecules are placed between the two plates of acapacitor as a vapor (or as a dilute solution in a nonpolar liquid), they are oriented bythe electric field. Then, the orientational polarization and the induced polarizationoccur simultaneously, as described above. If er is the relative permittivity of the vapor,there is a relationship

er � 1er þ 2 ¼

4pNA3Vm

aþ m2

3kBT

� �ð1:3Þ

where kB is the Boltzmann constant. By plotting the relation between Vm(er� 1)/(er þ 2) and 1/T, the valueofm is obtained simultaneouslywith the valueofa, althougha more accurate value of m is obtainable from the Stark splitting of microwave lines.Thedipolemomentisalsousedtoassessthesolventpolarity:solventswithhighdipole

moments (e.g. m� 2.5D, 1 D¼ 3.33564� 10�30Cm) are called dipolar solvents, whilethosewith lowdipolemomentsare calledapolarornonpolar solvents.Manysolventswithhigh er values also have high m values (see Table 1.1). However, the m value of water(1.77D) is lower than expected from its high solvating abilities. The dipole moment

Table 1.4 Dielectric relaxation parameters of water and loweralcohols determined by femtosecond terahertz pulsespectroscopy at 25

b

C.a

Solvent es s1 (ps) e2 s2 (ps) e3 s3 (ps) e1

Water 78.36 8.24 4.93 0.18 3.48MeOH 32.63 48 5.35 1.25 3.37 0.16 2.10EtOH 24.35 161 4.15 3.3 2.72 0.22 1.931-PrOH 20.44 316 3.43 2.9 2.37 0.20 1.97

aFrom Kindt, J.T. and Schmuttenmaer, C.A. (1996) J. Phys. Chem., 100, 10373.

4) Examples of nD values: methanol 1.326, water1.332, AN 1.341, hexane 1.372, PC 1.419, DMF1.428, DMSO 1.477, benzene 1.498, Py 1.507,NB 1.550 and DMTF 1.576 (Table 3.5 in Ref.[2a]). For all solvents, the value of nD is between1.2 and 1.8. There is a relationship eop � n2D.

5) Dispersion forces (instantaneous dipole–induced dipole interactions): Even in atoms andmolecules with no permanent dipole moment,the continuous movement of electrons results,at any instant, in small dipole moments, whichfluctuatingly polarize the electronic system ofthe neighboring atoms or molecules. Thiscoupling causes the electronicmovements to besynchronized in such a way that a mutual

attraction results (Ref. [1a], p. 13). The disper-sion forces, which are universal for all atomsandmolecules, are proportional to the productsof the polarizabilities (a) of the two interactingspecies but are short range in action.Among theintermolecular forces, the dispersion forces areoften stronger than the dipole–dipole anddipole–induced dipole forces, though they areweaker than the hydrogen bonding. Due to thedispersion forces, benzene exists as a liquid atnormal temperatures, and hydrogen and argonare condensed to liquids at low temperatures.See, for example, Israelachvili, J.N. (1992)Intermolecular and Surface Forces, 2nd edn,Academic Press, London, Chapter 6.

12j 1 Properties of Solvents and Solvent Classification