Surface electrode reactions quasireversible

In the last two decades, significant attention has been paid to the study of surface electrode reactions with SWV and various methodologies have been developed for thermodynamic and kinetic characterization of these reactions. In the following chapter, several types of surface electrode processes are addressed, including simple quasireversible surface electrode reaction [76-84], surface reactions involving lateral interactions between immobilized species [85], surface reactions coupled with chemical reactions [86-89], as well as two-step surface reactions [90,91]. 

The physical meaning of the kinetic parameter m is identical as for surface electrode reaction (Chap. 2.5.1). The electrochemical reversibility is primarily controlled by 03 (Fig. 2.71). The reaction is totally irreversible for log(m) < —3 and electrochemically reversible for log(fo) > 1. Between these intervals, the reaction appears quasireversible, attributed with a quasireversible maximum. Though the absolute net peak current value depends on the adsorption parameter. Fig. 2.71 reveals that the quasireversible interval, together with the position of the maximum, is independent of the adsorption strength. Similar to the surface reactions, the position of the maximum varies with the electron transfer coefficient and the amphtude of the potential modrrlation [92]. 

Similar to the pure surface electrode reaction, the response of reaction (2.146) is characterized by splitting of the net peak under appropriate conditions. The splitting occurs for an electrochemically quasireversible reaction and vanishes for the pure reversible reaction. Typical regions where the splitting emerges are 3 < m < 10 and 0.1 < r < 10 for a = 0.5 and i sw = 50 mV. Contrary to the surface electrode reaction where the ratio of the split peak currents is solely sensitive to a, in the present system this ratio depends additionally on r. For instance, if a = 0.5 and r = 1 the ratio is = 1 for r = 10, > 1 and r = 0.1, < 1. Finally it is worth mentioning when experimentally possible, the electrode mechanism represented by (2.145) to (2.147) has to be simplified to a simple surface reaction (Sect. 2.5.1) in order to avoid the complexity arising from the effect of diffusion mass transport. 

For a given adsorption constant, the observed electrochemical reversibility depends on the kinetic parameter defined as (O = Xy, or (o =. This reveals that the inherent properties of reaction (2.208) are very close to surface electrode reactions elaborated in Sect. 2.5. The quasireversible maximum is strongly pronounced, being represented by a sharp parabolic dependence of vs. m. The important feature of the maximum is its sensitivity to the adsorption constant, defined by the following equation ... 

Beside the quasireversible maximum, the splitting of the net peak is the second intriguing feature of a surface electrode reaction [84]. The splitting emerges by... 

Due to the similarity with surface electrode processes, a quasireversible reaction in thin-film voltammetry exhibits a quasireversible maximum and splitting of the net peak. The reasons causing these voltammetric features are the same as for surface... 

Quasireversible maximum — is a feature of the square-wave voltammetric response (see - square-wave voltammetry) of a kinetically controlled - electrode reaction in which at least one component of the - redox couple is immobilized on the - electrode surface [i] and kinetically controlled electrode reaction occurring in a restricted diffusion space [ii] (see - thin-film electrode and... 

In contrast to classical cyclic voltammograms, AC-cyclic voltammograms have a clear baseline, which is advantageous for quantitative measurements. By extending AC linear sweep voltaimnetry by a reverse scan, AC cyclic voltammetry is obtained. If the surface concentrations of the electroactive species are the same at the same potential for forward and reverse scans, the peaks for forward and reverse scans are expected to be identical. If the DC process is not fully reversible, the surface concentrations of the electroactive species are different at a given DC potential for forward and reverse scans— that is, for quasi-reversible systems a displacement of the jjeaks for forward and reverse scan can be observed. This displacement can be used to derive kinetic parameters of the electrode reaction. For instance, the derivation (1, p. 393] (or Eq. 5-33) for sluggish one-step heterogeneous quasireversible and/or... 

Substituting the solutions for surface concentrations into the corresponding kinetic equations, one obtains integral equations for each cathodic stripping reaction. Numerical solution for the quasireversible electrode mechanism (2.204) is ... 

The shape of the polarographic wave is further influenced by the nature of the electrode process occurring at the drop surface. Polarographic waves may be reversible, irreversible, or quasireversible. The overall electrode process comprises the diffusion, electron transfer, and electrochemical reaction steps. 

In a chemically reversible redox system the rate of the following reaction(s) is insufficient to perturb the concentration of within the electrochemical reaction layer near the electrode surface. When the following reaction is rapid so that Y is depleted during the experiment, the couple is chemically irreversible. The term quasireversible should not be used in conjunction with chemical reversibility. That terminology is restricted to the electron-transfer step itself. This error is made frequently. Redox systems that are less than totally chemically reversible should be described as having limited chemical reversibility. 

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