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PDF | This course presents the fundamentals of the Although these topics are already presented in several books this information is often. Fundamentals of Electrochemistry provides the basic outline of most topics of theoretical and applied electrochemistry for students not yet. text-books has become insistent. The variation in the teaching in different schools is so great The Prin DRAUGHTSMAN CIVIL - Text Books Online.

About this book Introduction An excellent way into the subject'- New Scientist Introduction to Electrochemistry is the first major new text in the field in recent years. The author takes the student from the basics through to a level suitable for beginning a post-graduate course. The chapters cover theory from electrolytes through electrodes to cells, both equilibrium and dynamic. Applications and methods are given great emphasis, and the second part of the text focuses on these aspects with coverage of electrosynthesis, electroanalytical chemistry, industrial electrochemistry, batteries and corrosion. Scattered throughout the text are panels of historical and anecdotal information illustrating unusual and often amusing aspects of electrochemistry not normally presented to the student. This, plus the highly readable style adopted by Brynn Hibbert, and his use of fully worked problems at the end of each chapter, make Introduction to Electrochemistry the ideal undergraduate textbook choice. Keywords electrochemical kinetics electrochemistry electrolyte electrosynthesis ion Authors and affiliations.

If the address matches an existing account you will receive an email with instructions to retrieve your username. Skip to Main Content. Fundamentals of Electrochemistry , Second Edition Author s: First published: Print ISBN: About this book Fundamentals of Electrochemistry provides the basic outline of most topics of theoretical and applied electrochemistry for students not yet familiar with this field, as well as an outline of recent and advanced developments in electrochemistry for people who are already dealing with electrochemical problems.

The content of this edition is arranged so that all basic information is contained in the first part of the book, which is now rewritten and simplified in order to make it more accessible and used as a textbook for undergraduate students.

More advanced topics, of interest for postgraduate levels, come in the subsequent parts. This updated second edition focuses on experimental techniques, including a comprehensive chapter on physical methods for the investigation of electrode surfaces. New chapters deal with recent trends in electrochemistry, including nano- and micro-electrochemistry, solid-state electrochemistry, and electrocatalysis.

Steps can be seen as a type of pre-equilibrium before the electron transfer. During the electron transfer itself all positions of the atoms are frozen, obeying the Franck-Condon principle adiabatic process. In the equations for energy changes a factor of 2 relative to electrode reactions appears, since whole reactions rather than half- reactions are being considered.

Theoretical and experimental com- parisons between electrode reactions and redox reactions in solution have 3 been made with satisfactory results. The reorientation and rearrangement causes the separation between the energy levels to be different in the activated complex than in the initial state. This level is the Fermi level, EF - electrons are always transferred to and from this level. The situation is shown schematically in Fig.

What is, then, the energy profile describing electron transfer? In a similar fashion to the description of the kinetics of homogeneous reactions, in the development of a model for electron transfer parabolic energy profiles have been used for reagents and products.

Nevertheless, the region where the profiles intersect is of paramount interest since this corresponds to the activated complex: Figure 4. The potential applied to the electrode alters the highest occupied electronic energy level, EF, facilitating a reduction or b oxidation. So for a reduction we can write 4. Values of aa and ac can vary between 0 and 1, but for metals are around 0. A value of 0. Substituting 4. On changing the potential applied to the electrode, we influence ka and kc in an exponential fashion.

The electrode is thus a powerful catalyst. When all the species that reach it are oxidized or reduced the current cannot increase further. If there are no effects from migration, diffusion limits the transport of electroactive species close to the electrode; the maximum current is known as the diffusion-limited current Section 5. Whatever the value of the standard rate constant, k0, if the applied potential is sufficiently positive oxidation or sufficiently negative reduction the maximum current will always be reached.

As indicated, for metals the activation barrier Fig. These situations occur with semiconductor electrodes, since the externally applied voltage appears as a potential difference almost totally across the semiconductor space charge layer. In many cases electrode processes involving the transfer of more than one electron take place in consecutive steps. The symmetry of the activation barrier referred to above relates to the rate-determining step. Thus extreme care must be Reaction Reaction Reaction coordinate coordinate coordinate a b c Fig.

Finally, since the anodic and cathodic reactions do not occur at the same potential, the mechanism for oxidation may not be the opposite of reduction. This occurs when there is multiple step electron transfer, possibly with intermediate chemical steps. Rewriting 4. Exactly the same result is obtained by following identical reasoning, using the anodic instead of the cathodic reaction in 4.

Any theory must be realistic and take into account the reorientation of the ionic atmosphere in mathematical terms. There have been many contributions in this area, especially by Marcus, Hush, Levich, Dog- nadze, and others5"9. The theories have been of a classical or quantum- mechanical nature, the latter being more difficult to develop but more correct.

It is fundamental that the theories permit quantitative com- parison between rates of electron transfer in electrodes and in homoge- neous solution. We illustrate the results obtained in the approximate model of Marcus, remembering that the activation barrier results predominantly from solvation changes.

The energy profile can be represented by a parabola. For the intersection of the two parabolas, assumed to be identical in form, one obtains, after a little algebraic manipulation, 4.

It is an example of a linear free energy relationship a kinetic parameter, In A: So, for very fast reactions, the theory predicts a variation of a with potential. There is some evidence that this occurs, but given the multistep nature of any electrode reaction no definitive conclusions can be taken, and mechanisms can be elaborated which have constant charge transfer coefficients.

Indeed the fact that the enthalpic and entropic parts of the coefficients have different temperature dependences leads to the question as to what is the real significance of the charge transfer coefficient, a topic currently under discussion9. Another aspect affecting electron transfer that has become more important with the increasing use of semiconductor electrodes 10 " 13 in, for example, solar energy conversion, but is also valid for metal electrodes, should be mentioned.

The Fermi energy is the electrochemical potential of the electrons in the electrode, see Chapter 3. The density of states is shown schematically in Fig. Overlap between EF and the distribution for Eo shows that oxidized species can be reduced. In order to relate Eredox, EF, and electrode potentials it is important to utilize the same reference state, namely vacuum In relation to vacuum the energy of the standard hydrogen electrode is —4.

It therefore seems logical, when describing the mechanism of an electrode reaction, to speak of an energy associated with the redox couple corresponding to that of the electrons in the solution species that are transferred, and equal to the Fermi energy in the actual electron transfer step after solvent reorganization, etc. X reflects the break in the structure of the solid and consequent variations in electronic distribution Fig. Energy corresponding to Volta potential Solution Fig.

A measurement of potential gives values of electrode potentials and never redox potentials. The crucial point is that the difference of potential available to effect electrode reactions and surmount activation barriers is not simply the difference between the Galvani potential i. On the side of the solid it is the Volta potential and on the side of the solution it is the potential at the inner Helmholtz plane, where species have to reach to in order for electron transfer to be possible.

Corrections to rate constants for the latter are commonly carried out using the Gouy-Chapman model of the electrolyte double layer and will be described in Section 6.

Marcus, J. Butler, Trans. Faraday Soc, , 19, and ; T. Erdey-Gruz and M. Volmer, Z. Marcus, Ann. Levich, Advances in electrochemistry and electrochemical engineering, ed. Dogonadze, Reactions of molecules at electrodes, ed. Kuznetsov, Faraday Disc. Soc, , 74, Bockris, pp. Holmes ed. Morrison, Electrochemistry at semiconductor and oxidised metal electrodes, Plenum, New York, Hamnett, Comprehensive chemical kinetics, Elsevier, Amsterdam, Vol.

Compton, Chapter 2. Khan, R. Soc, , , C. These, in turn, are affected not only by the electrode reaction itself but also by the transport of species to and from bulk solution.

This transport can occur by diffusion, convection, or migration Section 2. Normally, conditions are chosen in which migration effects can be neglected, this is the effects of the electrode's electric field are limited to very small distances from the electrode, as described in Chapter 3. In these conditions we need to consider only diffusion and convection. Forced convection considerably increases the transport of species, as will be demonstrated, and in many cases can be described mathematically.

Natural convection, due to thermal gradients, also exists, but conditions where this movement is negligible are generally used. In this chapter we consider systems under conditions in which the kinetics of the electrode reaction is sufficiently fast that the control of the electrode process is totally by mass transport. This situation can, in principle, always be achieved if the applied potential is sufficiently positive oxidation or negative reduction. First we consider the case of pure diffusion control, and secondly systems where there is a convection component.

Thus the species can be charged or neutral. The rate of diffusion depends on the concentration gradients. Fick's first law expresses this: Alternatively, the Nernst-Einstein or Stokes-Einstein relations discussed in Chapter 2 may be used to estimate values of D. The next question is: Consider an element of width djc Fig. Diffusion is in the direction opposing the concentration gradient.

Substituting, we reach Fick's second law: Definition of the coordinates used in Table 5. The solution of Fick's second law gives the variation of flux, and thence diffusion-limited current, with time, it being important to specify the conditions necessary to define the behaviour of the system boundary conditions. Since the second law is a partial differential equation it has to be transformed into a total differential equation, solved, and the transform inverted1.

The Laplace transform permits this Appendix 1. In the next two sections we use the Laplace transform to solve Fick's second law for two important cases under conditions of pure diffusion control: Note that in the first case the potential is controlled and the current response and its variation with time is registered, chronoamperometry, and in the second case the value of the current is controlled and the variation of potential with time is registered, chronopotentiometry. Potential step to obtain a diffusion-limited current of the electroactive species.

This gives rise to a diffusion- limited current whose value varies with time. If it were a reduction a minus sign would be introduced into 5. From 5. Variation of concentration with distance at a planar electrode for various values of t after the application of a potential step, following 5. This critical time can vary between some seconds and several minutes depending on the system's experimental arrangement. It should also not be forgotten that, from a practical point of view, for small values of t there is a capacitive contribution to the current, due to double layer charging, that has to be subtracted.

This contribution arises Fig. Variation of current with time according to the Cottrell equation.

Thus the method of solution is the same. Two extreme cases can be considered: The second term in 5. Diffusion at a sphere can be treated as linear diffusion. This is very important for the dropping mercury electrode Section 8.

Diffusion currents for planar and spherical electrodes: The spherical term dominates, which represents a steady- state current. However, due to the effects of natural convection this steady state is never reached at conventionally-sized electrodes. The smaller the electrode radius, the faster the steady state is achieved. It is possible to achieve a steady state at microelectrodes.

These are described further in Section 5. The equations for the diffusion-limited current at planar and spherical electrodes are shown in Table 5. Fick's second law is solved using the Laplace transform as in the previous section; the first two boundary conditions are the same, but the third is different: The boundary condition 5. Figure 5. It should be noted that the equation for the transition time at a spherical electrode is equal to that for a plane electrode.

This result, perhaps unexpected, shows that it is only the current density that determines the transition time and not the curvature of the electrode surface.

Common geometries include spherical, hemispherical, disc, ring, and line. Their applications are many, and will be referred to throughout the book. They exhibit high current densities, but low total currents, so that the percentage electroly- sis is small, and permit the attainment of steady states in situations, such as in the absence of added electrolyte, not possible with larger electrodes. The diffusion-limited current at spherical and hemispherical microelec- trodes follows directly from 5.

We consider a hemispherical elec- trode as shown in Fig. Additionally, and in general, as a result of the high rate of diffusion the current density is sufficiently high that interference from natural, and even forced, convection is negligible. Finally, we consider the case of a plane disc microelectrode. In these cases the current density is not uniform. However, it is easier to make disc microelectrodes of solid materials than hemispheres.

In fact, the similarity of 5.

However, for reasons of comparison it is useful to speak of a diffusion layer defined in the following way: The diffusion layer results, therefore, from the extrapolation of the concentration gradient at the electrode surface until the bulk concentration value is attained.

This approximation was introduced by Nernst6. Applying 5. The mass transfer coefficient is compare with 5. Both these conditions can be satisfied by microelectrodes. They can also be obtained by imposition of forced convection at larger electrodes—hydrodynamic systems.

The solution to 5. The velocity profile obtained depends on whether the flow corresponds to a laminar, transition, or turbulent regime. In general, electrochemical investigations are done under laminar flow conditions. Studies of mass and heat transfer have been conducted in fluid dynamics for many types of system, including surface heterogeneous reactions. The results can often be applied directly to hydrodynamic electrochemical systems using the so-called 'similarity principle'.

An important exception to laminar flow is the calculation of velocity profiles in industrial electrochemical reactors. These often function in the turbulent regime in order to maximize the yield of the process, given that the mass transfer coefficients are higher. In Section 5. This assumption should be verified for any hydrodynamic system under study. The development of convective-diffusion theories is due principally to Prandtl9 and Schlichting10, and their application in electrochemistry to Levich Levich was the first to solve the equations for the rotating disc electrode.

In practice one uses values that differ by 5 per cent from their values at infinite distance from the electrode surface, given that the components tend asymptotically to their values in bulk solution.

It is therefore reasonable to suppose that there is no convection within the diffusion layer. Diagram showing the relative thicknesses of the hydrodynamic and diffusion layers in a hydrodynamic system in aqueous solution. Below a certain critical value of Re, Recriu the flow is laminar; above it is turbulent with a transition regime around ReCTli.

For aqueous solution, 5c - 1 0 3. Sh is proportional to the mass transfer coefficient kd. In practice the electrode bodies usually have the form of a cylinder, with the sheath around the disc significantly larger than it is, so as to approximate a surface of infinite dimension Fig.

The first step consists in deducing the velocity profile. This fluid dynamics problem was solved by von Karman 14 and Cochran15 and gives the velocity components: The coordinates are shown in Fig.

As a result of rotation, solution is sucked towards the disc and spread out sideways. Streamlines for a rotating disc. Some other hydrodynamic electrodes are not uniformly accessible such as, for example, tubular and impinging jet electrodes.

However, the method for calculating the diffusion-limited current is always the same, see Section 8. The various hydrodynamic electrodes and their use in investigating electrode processes are described in Chapter 8. Cottrell, Z. Sand, Philos. Fleischmann, S. Pons, D. Rollison, and P. Schmidt, Ultramicroelectrodes, Datatech Systems Inc. Montenegro, M. Queiros, and J. Daschbach eds. Nernst, Z. Physik, Chem. Newman, Advances in electrochemistry and electrochemical engineering, ed.

Prandtl, Proc. Schlichting, Boundary layer theory, Pergamon Press, London, Riddiford, Advances in electrochemistry and electrochemical engineering, ed. Tobias, Vol. Brett and A.

Oliveira Brett, Comprehensive chemical kinetics, ed. Bamford and R. Cochran, Proc. In this chapter these two parts of the electrode process are combined and we see how the relative rates of the kinetics and transport cause the behaviour of electrochemical systems to vary In hydrodynamic systems, forced convection increases the flux of species that reach a point corresponding to the thickness of the diffusion layer from the electrode.

The mass transfer coefficient kd describes the rate of diffusion within the diffusion layer and kc and k. Simplified scheme for an oxidation-reduction reaction on an electrode surface. In general these coefficients differ because the diffusion coefficients differ. From Chapter 4 we have the Butler-Volmer expressions for the kinetic rate constants: The steady state also means that the applied potential has a fixed value.

Only R present in solution: From 6. We now consider the factors that affect the variation of kc or ka and kd. The kinetic rate constants depend on the applied potential and on the value of the standard rate constant, k0. As was seen in Chapter 5, kd is influenced by the thickness of the diffusion layer, which we can control through the type of experiment and experimental conditions, such as varying the forced convection.

By altering kc or k. At the moment we note that there are two extremes of comparison between k0 and kd: The calculation of the current requires the relation between the flux and current: The current is determined only by the electronic energy differences between the electrode and the donor or acceptor species in solution and their rate of supply.

Figure 6. The characteristic sigmoidal profile results from the logarithmic term in 6. Equation 6. In fact, the same expression is reached for non-uniformly accessible electrodes, but the reasoning is a little more complex. In all cases 6. From the expressions obtained above, we can write a diagnostic of reversibility: E for a reversible reaction; inverse ' slope is 0. E—equation 6. Kinetics has an important role, especially for potentials close to Eeq.

It is necessary to apply a higher potential than for a reversible reaction in order to overcome the activation barrier and allow reaction to occur—this extra potential is called the overpotential, rj. Because of the overpotential only reduction or only oxidation occurs and the voltammogram, or voltammetric curve, is divided into two parts. At the same time it should be stressed that the retarding effect of the kinetics causes a lower slope in the voltammograms than for the reversible case.

The expressions 6. The transport term has to appear, since only at the beginning of an irreversible voltammogram can the effects of transport be neglected. This is because kc or ka increases on increasing the potential negatively or positively so that we finally reach the limiting current plateaux in Fig.

Schematic voltammogram for an irreversible electrode reaction. Ex- ample: The half-wave potential for reduction or oxidation varies with kd, since there is not equilibrium on the electrode surface.

For cathodic and anodic processes respectively we may write 6. Once more, similarly to the reversible case in Fig. Plots of the type shown in Fig. For example, for the rotating disc electrode one constructs a plot of 7"1 vs. Thus 6. Whatever the relative values of k0 and kd, we should obtain the limiting current. The effect of the value of k0 on the current density close to Eeq; a k0 large; b k0 smaller.

In these conditions we can approximate the exponentials by the first term in the Maclaurin expan- sion Appendix 1.

We obtain. The proportionality constant is highly dependent on the value of ac and thence also on ara , Fig. This is the Tafel region. They are an example of a linear free energy relationship linear relation between a kinetic and a thermodynamic parameter the parameters in this case being the flux or the current and the potential.

Constructing plots of In y vs. E we obtain Fig. Plot of In y vs. E showing how to measure y0 and a from the slopes of the lines. For a reduction, from 6. As this is the Nernst equation, the value of E can be identified with the equilibrium potential Eeq. In this way we have the advantage of obtaining the current as a function of the difference in applied potential and equilibrium potential, i. Additionally, the concentration of electroactive species will be, in general, less at distance JCH from the electrode than outside the double layer in bulk solution.

These assump- tions can be treated quantitatively. The practical consequence is variation of k0 with potential. As is perhaps to be expected, the double layer can also affect the values of the measured, i. This situation corresponds to potentials far from Ez. As can be seen, the apparent value of occ can be positive or negative. This example shows the extreme importance in correcting values of ac and aa for double layer effects.

We can ask how effects of the double layer on electrode kinetics can be minimized and if the necessity of correcting values of a and of rate constants can be avoided? This can be achieved by addition of a large quantity of inert electrolyte —1. As stated elsewhere, other ad- vantages of inert electrolyte addition are reduction of solution resistance and minimization of migration effects given that the inert electrolyte conducts almost all the current.

In the case of microelectrodes Section 5. In this section we consider metal ions given that, at least apparently, there are no other species involved, except for molecules of solvation etc. If the reactions are irreversible we can investigate their kinetics.

For a two-electron reduction Fig. The kinetics of these two steps is conditioned by the medium where they occur and this will determine the type of voltammetric wave that is observed. Second step occurs at a more negative potential than the first: We observe a one-electron reduction until the applied potential is sufficiently negative for reduction of the second electron. In other words we observe two separated voltammetric waves Fig. First step rate-determining: Second step rate-determining: This means that ocji — 1.

In actual fact, arc —0. Supposing the reduction follows path 2: We cannot ever assume a priori that the reverse of a multistep reaction with known mechanism will be the inverse. It is also fairly evident that for certain combinations of rate constants we can change mechanisms by changing the applied potential.

The application of these concepts to the electroreduction of oxygen, important for fuel cells, with hydrodynamic electrodes is described in Chapter 8. Finally in this section, we remember that multiple electron transfer has to follow the reaction coordinate and has consecutive steps, even if the first step is rate determining.

The possibility of multiple electron transfer reactions without intermediate chemical steps has been questioned, with experimental evidence from, for example, the supposedly relatively 6 simple reduction of Cd II and similar ions at mercury electrodes. This is because solvation and interaction with the environment, adsorption, etc. A selection of possible schemes is shown in Table 6. Note the presence of many organic compounds: Electrode reactions with coupled homogeneous reactions, adapted from Ref.

Such mechanisms are com- monly summarized in the scheme of squares, shown below for two electron transfers and two protonation steps. We consider three simple schemes, shown in Fig. T h e equations to calculate the rate constants from experimental measure- ments for the various types of electrode can be found in the specialized literature.

In most studies the electrochemical step has b e e n considered reversible—thus, in the following, the rate constant for the electrode reaction is not indicated. The position of the voltammetric curve on the potential axis is not affected by the homogeneous step Fig. The effect of coupled homogeneous reactions on electrode reactions illustrated for an oxidation.

Absence of homogeneous reaction ; presence. This shift can be directly related to the kinetics of the homogeneous reaction. The concentration of Ax will be larger than expected and the current bigger than in the absence of the homogeneous reaction Fig. Altering kd through the diffusion layer thickness permits the determination of the kinetics of the homoge- neous reaction.

In all these schemes for coupled homogeneous reactions, it is useful to consider in the deduction of the equations the concept of a reaction layer associated with the homogeneous reaction; all the homogeneous reaction occurs within a distance equal to the thickness of the reaction layer from the electrode. Other more complex mechanistic schemes are studied by a variety of techniques.

Double hydrodynamic electrodes are particularly useful for investigating schemes involving two electron transfer steps, such as ECE and DISP schemes. Some of the applications of the different electroche- mical techniques in the elucidation of these reactions are described in the following chapters.

Thirsk and J. Bongenaar, A. Remijnse, M. Sluyters-Rehbach, and J. Sluyters, J. Electroanal Chem. Brett, Port. Acta, , 3, Electrode materials specially designed for potentiometric measurements, which rely on the material selectivity, are discussed in Chapter We shall not give many practical details, but only those of greatest interest in the planning and design of electrochemical experiments.

However, it is hoped that the discussion in this chapter will prove an aid to consulting more detailed expositions in this area, for example Refs. The usable potential range is limited by one or more of the following factors: Additionally, solid electrodes can be adversely affected by poisoning through contact with solutions containing contaminants.

We now con- sider some frequently used materials and look at their properties as electrodes in more detail. Metals Much has been written about solid metal electrodes, which have now largely displaced liquid mercury. Those most often used as redox 'inert' electrodes for studying electron transfer kinetics and mechanism, and determining thermodynamic parameters are platinum, gold, and silver. However, it should be remembered that their inertness is relative: Platinum also exhibits catalytic properties.

A general advantage of metal electrodes is that their high conductivity results in low usually negligible background currents. It is usually fairly easy to increase sensitivity and reproducibility at solid electrodes by forced convection. Their surfaces can be modified by electrodeposition or chemical modification, although the latter is more common with carbon electrodes see below.

Another advantage of the use of metal electrodes is the ease of construction of the electrode assembly, and ease of polishing. Electrodes of many metals can undergo corrosion or passivation— formation of a salt film on the surface—and other reactions, depending on the medium and experimental conditions.

Electrochemical techniques can be used to investigate the mechanisms of these processes. Carbon Carbon7 exists in various conducting forms. Electrochemical reactions are normally slower at carbon than at metallic electrodes, electron transfer kinetics being dependent on structure and surface preparation8. Carbon has a high surface activity, which explains its susceptibility to poisoning by organic compounds.

Bonds with hydrogen, hydroxyl and carboxyl groups, and sometimes quinones, can be formed at the carbon surface. The presence of these groups signifies that the behaviour of these electrodes can be very pH-sensitive. Properties of various carbon materials from Ref.

Various types of carbon are used as electrodes. These include glassy carbon, carbon fibres, carbon black, various forms of graphite, and carbon paste, which consists of graphite particles in contact, incorporated in an inert matrix. They are all sp2 carbons, and can be compared structurally by considering the length of microcrystallites, La, in the graphite lattice plane a-axis , and the thickness of the microcrystallites perpendicular to the graphite planes c-axis , Lc.

These values, together with apparent density and resistivity are shown in Table 7. Probably the most widely used of these is glassy carbon, which is isotropic. However, due to its hardness and fragility, electrode fabrica- tion is difficult, which essentially limits its use to the dimensions and forms that can be acquired commercially. Since glassy carbon has some amorphous characteristics, as can be seen from Fig.

Carbon fibres have a diameter similar to that of a hair jum , and exhibit a stiffness greater than steel in the fibre direction. Fabrication is, in general, either from polyacrylonitrile PAN , which gives circular concentric graphitic ribbon rings, or from pitch, which tends to give a radial structure of graphite lamellae Apart from use as microelec- trodes, they are used as bundles in porous electrodes where a high electrolysis efficiency is required Chapter Representation of the structure of glassy carbon, showing La and Lc from Ref.

If this is pressure annealed at high temperature it turns into highly ordered pyrrolytic graphite HOPG , which is highly anisotropic as shown in Table 7. In graphite Fig. Pores arising in graphite are sometimes impregnated with ceresin or paraffin under vacuum in order to impede the entry of solution into the electrode. The structure of graphite. Comparative studies between the various types of carbon paste electrode have been carried out 1 1.

Other solid materials Other solid electrode materials used are semiconductors, for example metal oxides 1 2 1 3 , and conducting organic salts These last are of much interest at present for the immobilization of organic compounds such as enzymes, given their compatibility with these macromolecules Chapter For spectroelectrochemical and photoelectrochemical studies, optically semi-transparent electrodes have been fabricated by vapour deposition techniques on glass or quartz substrates Chapter Tin and indium oxides, platinum, and gold have been used.

Mercury For many years, mercury was the most used electrode material in the laboratory in the dropping mercury and hanging mercury drop elec- trodes, and more recently in the static mercury drop electrode. The relationship between the concentration of ions and electrode potential is given by Nernst equation. For a electrochemical cell, Concentration of pure solids and liquids is taken as unity.

Nernst equation and Kc Here,? Relationship between free energy change and equilibrium constant? Conductance G It is the ease of flow of electric current through the conductor. It is reciprocal of resistance R.

Unit of cell constant is cm-1 or m Specific conductivity decreases on dilution. This is because concentration of ions per cc decreases upon dilution. Molar Conductivity? It is related to specific conductance as? Equivalent conductivity?