Electrode reactions definitions

O Brien. 1235 Ohmic drop, 811, 1089, 1108 Ohmic resistance, 1175 Ohm s law, 1127. 1172 Open circuit cell, 1350 Open circuit decay method, 1412 Order of electrodic reaction, definition 1187. 1188 cathodic reaction, 1188 anodic reaction, 1188 Organic adsorption. 968. 978. 1339 additives, electrodeposition, 1339 aliphatic molecules, 978, 979 and the almost-null current test. 971 aromatic compounds, 979 charge transfer reaction, 969, 970 chemical potential, 975 as corrosion inhibitors, 968, 1192 electrode properties and, 979 electrolyte properties and, 979 forces involved in, 971, 972 977, 978 free energy, 971 functional groups in, 979 heterogeneity of the electrode, 983, 1195 hydrocarbon chains, 978, 979 hydrogen coadsorption and, 1340 hydrophilicity and, 982 importance, 968 and industrial processes, 968 irreversible. 969. 970 isotherms and, 982, 983... 

The most widely used reference electrode, due to its ease of preparation and constancy of potential, is the calomel electrode. A calomel half-cell is one in which mercury and calomel [mercury(I) chloride] are covered with potassium chloride solution of definite concentration this may be 0.1 M, 1M, or saturated. These electrodes are referred to as the decimolar, the molar and the saturated calomel electrode (S.C.E.) and have the potentials, relative to the standard hydrogen electrode at 25 °C, of 0.3358,0.2824 and 0.2444 volt. Of these electrodes the S.C.E. is most commonly used, largely because of the suppressive effect of saturated potassium chloride solution on liquid junction potentials. However, this electrode suffers from the drawback that its potential varies rapidly with alteration in temperature owing to changes in the solubility of potassium chloride, and restoration of a stable potential may be slow owing to the disturbance of the calomel-potassium chloride equilibrium. The potentials of the decimolar and molar electrodes are less affected by change in temperature and are to be preferred in cases where accurate values of electrode potentials are required. The electrode reaction is... 

The trends of behavior described above are found in solutions containing an excess of foreign electrolyte, which by definition is not involved in the electrode reaction. Without this excess of foreign electrolyte, additional effects arise that are most distinct in binary solutions. An appreciable diffusion potential q) arises in the diffusion layer because of the gradient of overall electrolyte concentration that is present there. Moreover, the conductivity of the solution will decrease and an additional ohmic potential drop will arise when an electrolyte ion is the reactant and the overall concentration decreases. Both of these potential differences are associated with the diffusion layer in the solution, and strictly speaking, are not a part of electrode polarization. But in polarization measurements, the potential of the electrode usually is defined relative to a point in the solution which, although not far from the electrode, is outside the diffusion layer. Hence, in addition to the true polarization AE, the overall potential drop across the diffusion layer, 9 = 9 + 9ohm is included in the measured value of polarization, AE. ... 

In electroanalysis, coulometry is an important method in which the analyte is specifically and completely converted via a direct or indirect electrolysis, and the amount of electricity (in coulombs) consumed thereby is measured. According to this definition there are two alternatives (1) the analyte participates in the electrode reaction (primary or direct electrolysis), or (2) the analyte reacts with the reagent, generated (secondary or indirect electrolysis) either internally or externally. 

The functional dependence of the activation energy of the anodic electrode reaction can be derived as follows. According to the definition of the rate of the electrode reaction, the partial current density... 

Several descriptions of electrode reaction rates discussed on the preceding pages and the difficulty to standardize electrode potential scales with respect to different temperatures imply several definitions of activation energies of electrode reactions. The easiest way to determine this quantity, for example, for an irreversible cathodic process, employs Eqs (5.2.9), (5.2.10) and (5.2.12) at a constant electrode potential,... 

Parsons, R., Electrode reaction orders, charge transfer coefficients and rate constants. Extension of definitions and recommendations for publication of parameters. Pure Appl. Chem., 52, 233 (1979). 

Assume that both the initial substances and the products of the electrode reaction are soluble either in the solution or in the electrode. The system will be restricted to two substances whose electrode reaction is described by Eq. (5.2.1). The solution will contain a sufficient concentration of indifferent electrolyte so that migration can be neglected. The surface of the electrode is identified with the reference plane, defined in Section 2.5.1. In this plane a definite amount of the oxidized component, corresponding to the material flux J0x and equivalent to the current density j, is formed or... 

When the rate of the electrode reaction is measurable, being characterized by a definite polarization resistance RP (Eq. 5.2.31), the electrode system can be characterized by the equivalent circuit shown in Fig. 5.22. 

Electrochemical reactions can be broken down into two groups outer-sphere electron-transfer reactions and inner-sphere electron transfer reactions. Outer-sphere reactions are reactions that only involve electron transfer. There is no adsorption and no breaking or forming of chemical bonds. Because of their simplicity, numerous studies have been performed, many entirely theoretical.18-25 By definition, though, electrode reactions are not outer-sphere reactions. However, if charge transfer is rate limiting for an electrode reaction, it typically takes a form similar to that of an outer-sphere reaction, which is described later in this section. 

Equilibrium Potential The minimum potential, which is necessary to perform a (reversible) reaction, is the equilibrium potential E, defined for zero cell current. It is typical for a given reaction. By definition, it is related to the NHE, which represents the potential zero. If the electrode reaction is coupled with the reaction 2 + 2e H2 at the NHE, theoreti-... 

For the electron transfer of hydrated redox particles (the outer-sphere electron transfer), the electrode acts merely as a source or sink of electrons transferring across the compact double layer so that the nature of the electrode hardly affects the reaction kinetics this lack of influence by the electrode has been observed for the ferric-ferrous redox reaction. On the other hand, the electron transfer of adsorbed redox particles (the inner-sphere electron transfer) is affected by the state of adsorption so that the nature of the electrode exerts a definite influence on the reaction kinetics, as has been observed with the hydrogen electrode reaction where the reaction rate depends on the property of electrode. 

The overall effect of the preceding chemical reaction on the voltammetric response of a reversible electrode reaction is determined by the thermodynamic parameter K and the dimensionless kinetic parameter . The equilibrium constant K controls mainly the amonnt of the electroactive reactant R produced prior to the voltammetric experiment. K also controls the prodnction of R during the experiment when the preceding chemical reaction is sufficiently fast to permit the chemical equilibrium to be achieved on a time scale of the potential pulses. The dimensionless kinetic parameter is a measure for the production of R in the course of the voltammetric experiment. The dimensionless chemical kinetic parameter can be also understood as a quantitative measure for the rate of reestablishing the chemical equilibrium (2.29) that is misbalanced by proceeding of the electrode reaction. From the definition of follows that the kinetic affect of the preceding chemical reaction depends on the rate of the chemical reaction and duration of the potential pulses. 

A quasireversible electrode reaction is controlled by the film thickness parameter A, and additionally by the electrode kinetic parameter k. The definition and physical meaning of the latter parameter is the same as for quasireversible reaction under semi-infinite diffusion conditions (Sect. 2.1.2). Like for a reversible reaction, the dimensionless net peak current depends sigmoidally on the logarithm of the thickness parameter. The typical region of restricted diffusion depends slightly on K. For instance, for log( If) = -0.6, the reaction is under restricted diffusion condition within the interval log(A) < 0.2, whereas for log(if) = 0.6, the corresponding interval is log(A) <0.4. 

One definite problem in establishing a volcano curve is the actual values of the quantity D(M-B), which is seldom known for conditions resembling those relevant to an electrode reaction. As illustrated later, this issue is approached case by case, depending on the nature of the electrode reaction. 

In words, the overpotential is a deviation of the Galvani potential of the electrode from the value it has when the rate of charge flow across its interface with the solution is equal in each direction for the reaction concerned (i.e., Mz+. + ze M). The definition given in Eq. (7.14) is general, although it is necessary to define the electrode reaction concerned, the solution composition, etc., because the value of the thermodynamic A e is different for each electrode reaction, concentration of reactant, and temperature. 

Under equilibrium conditions the currents and i , and also icp and i, are equal to each other by the absolute value, in accordance with the principle of detailed balancing (see, for example, Landau and Lifshitz, 1977). These equilibrium values (i ) = (i )° = i° and (i )° = (i )° = i represent, by definition, exchange currents of an electrode reaction passing through the valence band (i°) and through the conduction band (i ). 

N.B. In the literature, some authors refer the EC scheme to the trivial case where the RP, RE scheme given before applies, but the electrode reaction is studied by considering a net oxidation. Here, we disregard this definition. 

In this section, a non-reversible electrode reaction will be addressed. An exact definition of a slow charge transfer process is not possible because the charge transfer reaction can be reversible, quasi-reversible, or irreversible depending on the duration of the experiment and the mass transport rate. So, an electrode reaction can be slow or non-reversible when the mass transport rate has a value such that the measured current is lower than that corresponding to a reversible process because the rate of depletion of the surface species at the electrode surface is less than the diffusion rate at which it reaches the surface. Under these conditions, the potential values that reduce the O species and oxidize the R species become more negative and more positive, respectively, than those predicted by Nemst equation. 

These laws (determined by Michael Faraday over a half century before the discovery of the electron) can now be shown to be simple consequences of the electrical nature of matter. In any electrolysis, an oxidation must occur at the anode to supply the electrons that leave this electrode. Also, a reduction must occur at the cathode removing electrons coming into the system from an outside source (battery or other DC source). By the principle of continuity of current, electrons must be discharged at the cathode at exactly the same rate at which they are supplied to the anode. By definition of the equivalent mass for oxidation-reduction reactions, the number of equivalents of electrode reaction must be proportional to the amount of charge transported into or out of the electrolytic cell. Further, the number of equivalents is equal to the number of moles of electrons transported in the circuit. The Faraday constant (F) is equal to the charge of one mole of electrons, as shown in this equation ... 

Any surface (typically a piece of metal) on which an electrochemical reaction takes place will produce an electrochemical potential when in contact with an electrolyte (typically water containing dissolved ions). The unit of the electrochemical potential is volt (TV = 1JC1 s 1 in SI units).The metal, or strictly speaking the metal-electrolyte interface, is called an electrode and the electrochemical reaction taking place is called the electrode reaction. The electrochemical potential of a metal in a solution, or the electrode potential, cannot be determined absolutely. It is referred to as a potential relative to a fixed and known electrode potential set up by a reference electrode in the same electrolyte. In other words, an electrode potential is the potential of an electrode measured against a reference electrode. The standard hydrogen electrode (SHE) is universally adopted as the primary standard reference electrode with which all other electrodes are compared. By definition, the SHE potential is OV, i.e. the zero-point on the electrochemical potential scale. Electrode potentials may be more positive or more negative than the SHE. 

As mentioned before, ESR spectroscopy has been used extensively for the study of electrochemically generated radicals and radical ions 40 A word of caution is necessary with regard to the interpretation of such results the detection of a particular radical species is no definite proof that the radical is an intermediate in the formation of products. This can only be established by supporting the ESR studies by kinetic investigations. Also the failure to detect radicals from an electrode process does not mean that radicals are not intermediates, only that they may be too short-lived to be detectable. Generally, one can estimate the lower limit for detection of radicals from electrode reactions at a half-life of about 0.1 sec for external generation and 0.01 sec for internal generation. 

The second and third terms on the right hands side of Eq. 9.8 remain constant for a given electrode-electrolyte system, and hence the electrode potential is a linear function of the interfacial potential A MIS of the electrode. This definition of the electrode potential holds valid for all electronic and ionic electrodes, whether the electrode reaction is in equilibrium or non-equilibrium. The potential defined by Eq. 9.8 is called the absolute electrode potential. 

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. 

Irreversible electrode phenomena polarization and over-potential. Most of the electrode reactions mentioned in the preceding paragraph are nearly reversible that is, the electrode when dipped into the electrolyte immediately assumes a definite potential difference from the solution, which is but slightly affected by small currents passing across the electrode. Should the potential of the electrode be raised slightly above the equilibrium reversible value, the current flows from the electrode to the solution if the potential falls slightly, the current flows in the opposite direction. For a perfectly reversible electrode, an infinitesimal departure of the potential from the equilibrium value should cause a considerable current to flow in one or the other direction. 

Refs. [i] (1972) Definition, terminology and symbols in colloid and surface chemistry, Part I. Pure Appl Chem 51 77 [ii] Horanyi G (2002) Specific adsorption. State of art Present knowledge and understanding. In Bard AJ, Stratmann M, Gileadi M, Urbakh M (eds) Thermodynamics and electrified interfaces. Encyclopedia of electrochemistry, vol. 1. Wiley-VCH, Weinheim, pp 349-382 [Hi] Calvo EJ (1986) Fundamentals. The basics of electrode reactions. In Bamford CH, Compton RG (eds) Comprehensive chemical kinetics, vol. 26. Elsevier, Amsterdam, pp 1-78 [iv] Baltruschat H (1999) Differential electrochemical mass spectrometry as a tool for interfacial studies. In Wieckowski A (ed) Interfacial electrochemistry, theory, experiment, and applications. Marcel Dekker, New York, pp 577-597... 

The reactions of DPAt and radical cations of other aromatic hydrocarbons with pyridine and substituted pyridines are among the most intensively studied electrode reactions of positive ions. The first definitive study of the mechanism of the reaction employed the rotating disk electrode (Manning et al 1969). Data were found to fit ECE working curves (Fig. 21) for the reactions of DPA7 with 4-cyanopyridine, 4-acetoxypyridine, pyridine and 4-methylpyridine. Pseudo first order rate constants of about 3, 10, 30, 300... 

Although Eqs. 4D and 5D look similar, the transition from one to the other is by no means trivial and is the subject of detailed discussion later. Here we shall limit ourselves to a brief discussion of two points. First, the charge on the particle z, which appears in Eq. 4D has been dropped from Eq. 5D, since it is tacitly assumed that electrode reactions occur by the transfer of one electron at a time. Tlius, for any rate-determining step, the value of z is always taken as unity. Second, the parameter P, called the symmetry factor, has been introduced. By definition it can take values from zero to unity,... 

The reproducibility of the effective mobility is governed by temperature and pH effects. An acceptable run-to-run pH reproducibility can usually be assured, at least in equipment where the effects of possible electrode reactions can be avoided. Again, temperature plays a leading role The effective mobility has a temperature coefficient of 2-3% per degree. Most buffer solutions, incidentally, have a temperature-dependent pH value, so that changing the temperature in the capillary will, for weak ions, even lead to changing degrees of dissociation and, hence, effective mobility. These effects are, by definition, different for different buffers and different sample components. 

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