Electrode reaction, reversibility

Pre-wave — In DC polarography, this is a part of the response of fast and reversible electrode reaction (- reversibility) complicated by the -+ adsorption of product on the - dropping mercury electrode surface Ox + ne (Red)ads Red, assuming that the reactant... 

Figure 6.16. Bi-amperometric titration curves, (a) Both electrode reactions reversible, e.g. Fe + against Ce4+. (b) Only titrant reaction is reversible, c.g. AsOJ against 12. (c) Only substance titrated is reversible, e.g. 12 against Na2S20j...

When a slowly changing potential is applied to an electrode that is immersed in an electrolyte solution containing a redox species, a current will be observed to flow as the applied potential reaches a certain value. The current arises from a heterogeneous electron transfer between the electrode and the redox species, resulting in either an oxidation or a reduction of the electroactive species. At sufficiently oxidizing or reducing potentials, the current can become a function of the mass transfer of the redox species to the electrode. Redox couples that give rise to such currents are frequently referred to as reversible couples, or the electrode reaction is referred to as a reversible electrode reaction. Reversibility, like beauty, lies in the eye of the beholder. If the rate of electron transfer between the redox species and the electrode is rapid when compared with the rate of mass transfer, then the electrode reaction is reversible. Under these circumstances the ratio of the concentrations of the oxidized and reduced forms of the couple at the electrode surface is described by the Nernst equation. 

The cyclic voltammetric response for this system (with the electrode reaction reversible, a CE, mechanism) is determined by X, and (k +k 2)/v. The limiting cases of the CE mechanism are (1) the chemical step is fast and at equilibrium, and (2) the forward chemical reaction is slow with respect to the time scale of the experiment. In case 1, the CV has an p determined by the E9 and Xgq. In case 2, the cyclic voltammogram will be reversible and will reflect only ° and the initial concentration of species Ox. 

Cyclic voltammetry provides a simple method for investigating the reversibility of an electrode reaction (table Bl.28.1). The reversibility of a reaction closely depends upon the rate of electron transfer being sufficiently high to maintain the surface concentrations close to those demanded by the electrode potential through the Nemst equation. Therefore, when the scan rate is increased, a reversible reaction may be transfomied to an irreversible one if the rate of electron transfer is slow. For a reversible reaction at a planar electrode, the peak current density, fp, is given by... 

Figure Bl.28.4. Cyclic voltaimnogram for a simple reversible electrode reaction in a solution containing only oxidized species.

Figure Bl.28.7. Schematic shape of steady-state voltaimnograms for reversible, quasi-reversible and irreversible electrode reactions.

Similarly to the response at hydrodynamic electrodes, linear and cyclic potential sweeps for simple electrode reactions will yield steady-state voltammograms with forward and reverse scans retracing one another, provided the scan rate is slow enough to maintain the steady state [28, 35, 36, 37 and 38]. The limiting current will be detemiined by the slowest step in the overall process, but if the kinetics are fast, then the current will be under diffusion control and hence obey the above equation for a disc. The slope of the wave in the absence of IR drop will, once again, depend on the degree of reversibility of the electrode process. 

The effects of ultrasound-enlianced mass transport have been investigated by several authors [73, 74, 75 and 76]. Empirically, it was found that, in the presence of ultrasound, the limiting current for a simple reversible electrode reaction exhibits quasi-steady-state characteristics with intensities considerably higher in magnitude compared to the peak current of the response obtained under silent conditions. The current density can be... 

The exchange current is directiy related to the reaction rate constant, to the activities of reactants and products, and to the potential drop across the double layer. The larger the more reversible the reaction and, hence, the lower the polarization for a given net current flow. Electrode reactions having high exchange currents are favored for use in battery apphcations. 

In the thermodynamic treatment of electrode potentials, the assumption was made that the reactions were reversible, which implies that the reactions occur infinitely slowly. This is never the case in practice. When a battery deUvers current, the electrode reactions depart from reversible behavior and the battery voltage decreases from its open circuit or equiUbrium voltage E. Thus the voltage during battery use or discharge E is lower than the voltage measured under open circuit or reversible conditions E by a quantity called the polari2ation Tj. 

It is not appropriate here to consider the kinetics of the various electrode reactions, which in the case of the oxygenated NaCl solution will depend upon the potentials of the electrodes, the pH of the solution, activity of chloride ions, etc. The significant points to note are that (a) an anode or cathode can support more than one electrode process and b) the sum of the rates of the partial cathodic reactions must equal the sum of the rates of the partial anodic reactions. Since there are four exchange processes (equations 1.39-1.42) there will be eight partial reactions, but if the reverse reactions are regarded as occurring at an insignificant rate then... 

A storage cell, unlike an ordinary dry cell, can be recharged repeatedly. This can be accomplished because the products of the reaction are deposited directly on the electrodes. By passing a current through a storage cell, it is possible to reverse the electrode reactions and restore the cell to its original condition. 

When the titration curve is symmetrical about the equivalence point the end point, defined by the maximum value of AE/AV, is identical with the true stoichiometrical equivalence point. A symmetrical titration curve is obtained when the indicator electrode is reversible and when in the titration reaction one mole or ion of the titrant reagent reacts with one mole or ion of the substance titrated. Asymmetrical titration curves result when the number of molecules or ions of the reagent and the substance titrated are unequal in the titration reaction, e.g. in the reaction... 

In such reactions, even though the indicator electrode functions reversibly, the maximum value of AE/AV will not occur exactly at the stoichiometric equivalence point. The resulting titration error (difference between end point and equivalence point) can be calculated or can be determined by experiment and a correction applied. The titration error is small when the potential change at the equivalence point is large. With most of the reactions used in potentiometric analysis, the titration error is usually small enough to be neglected. It is assumed that sufficient time is allowed for the electrodes to reach equilibrium before a reading is recorded. 

Electrical units 503, 519 Electrification due to wiping 77 Electro-analysis see Electrolysis and Electrogravimetry Electrochemical series 63 Electro-deposition completeness of, 507 Electrode potentials 60 change of during titration, 360 Nernst equation of, 60 reversible, 63 standard 60, (T) 62 Electrode reactions 505 Electrodeless discharge lamps 790 Electrodes antimony, 555 auxiliary, 538, 545 bimetallic, 575... 

Hydrogen-absorbing alloy can reversible absorb and desorb a large amount of hydrogen. Hydrogen gas is rapidly absorbed in the gas phase, then desorbed on the alloy (gas-solid reaction). In the electrode reaction, the alloy electro-chemically absorbs and desorbs hydrogen in an alkaline solution (electrochemical reaction) ... 

In redox flow batteries such as Zn/Cl2 and Zn/Br2, carbon plays a major role in the positive electrode where reactions involving Cl2 and Br2 occur. In these types of batteries, graphite is used as the bipolar separator, and a thin layer of high-surface-area carbon serves as an electrocatalyst. Two potential problems with carbon in redox flow batteries are (i) slow oxidation of carbon and (ii) intercalation of halogen molecules, particularly Br2 in graphite electrodes. The reversible redox potentials for the Cl2 and Br2 reactions [Eq. (8) and... 

The detection of the AC component allows one to separate the contributions of the faradaic and charging currents. The former is phase shifted 45° relative to the applied sinusoidal potential, while the background component is 90° out of phase. The charging current is thus rejected using a phase-sensitive lock-in amplifier (able to separate the in-phase and out-of-phase current components). As a result, reversible electrode reactions yield a detection limit around 5 x 10 7m. 

As in chemical systems, however, the requirement that the reaction is thermodynamically favourable is not sufficient to ensure that it occurs at an appreciable rate. In consequence, since the electrode reactions of most organic compounds are irreversible, i.e. slow at the reversible potential, it is necessary to supply an overpotential, >] = E — E, in order to make the reaction proceed at a conveniently high rate. Thus, secondly, the potential of the working electrode determines the kinetics of the electron transfer process. 

The formation of an ion will have a negative A F and hence electrode reactions which produce an anion or a cation from a neutral substrate wiU be favoured by an increase in pressure. That is, the reversible potential for the reaction... 

Observance of a mixed potential of about 1.0 V (instead of the equilibrium thermodynamic reversible potential Ec= 1.23 V vs. SHE) due to the formation of surface oxides at the platinum electrode, according to different electrode reactions ... 

The values of exchange current density observed for different electrodes (or reactions) vary within wide limits. The higher they are (or the more readily charges cross the interface), the more readily will the equilibrium Galvani potential be established and the higher will be the stability of this potential against external effects. Electrode reactions (electrodes) for which equilibrium is readily established are called thermodynamically reversible reactions (electrodes). But low values of the exchange current indicate that the electrode reaction is slow (kinetically limited). 

It is sometimes said that this electrode is reversible with respect to the anion. This claim must be examined in more detail. An electrode potential that depends on anion activity still constitutes no evidence that the anions are direct reactants. Two reaction mechanisms are possible at this electrode, a direct transfer of chloride ions across the interface in accordance with Eq. (3.34) or the combination of the electrode reaction... 

Different electrode reactions will occur independently, and their kinetic coefficients are unrelated. But for the forward and reverse process of a given reaction, such a correlation should exist, since at the equilibrium potential the corresponding partial current densities assume equal values. 

The standard rate constant kP characterizes the rates of both the forward and reverse processes. Its value is independent of the reference electrode selected, in contrast to what holds true for the values of k and and it is also independent of the component concentrations, in contrast to what holds true for the exchange CD. Therefore, this constant is an unambiguous characteristic of the kinetic properties exhibited by a given electrode reaction. 

The silver-zinc cell is a storage battery After discharge, it can be recharged by forcing through it an electric cnrrent in the reverse direction. In this process the two electrode reactions (19.3) and (19.4) as well as the overall reaction (19.2) go from right to left electrons flowing in the sense of arrow r in Fig. 19.1. 

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