Coupled homogeneous chemical reaction following

Commonly, in the description of chemical reactions coupled to electron transfer, the homogeneous chemical reaction is indicated by C and the heterogeneous electron transfer by E. The order of C with respect to E then follows the chronological order in which the two events occur. Furthermore, while Ox and Red indicate the electro active species, other non-electro active species which result from the coupled chemical complications are indicated by Y, Z, W, etc. 

Kong et al. [90] applied the electrochemical approach to the study of a two-phase azo coupling facilitated by reverse PTC. Cyclic voltammetry and chronoamperometry were employed to evaluate quantitatively the rate constants for the reaction. The process was interpreted in terms of an EC mechanism, i.e., diffusion-controlled electrochemical charge transfer followed by a homogeneous chemical reaction. The authors highlighted the usefulness of this approach based on the factors that enable the estimation of the contributions of the chemical reaction, mass transfer, partitioning, and the adsorption of reactants at the interface to the overall two-phase reaction. 

This section concerns heterogeneous electron transfer reactions coupled with homogeneous chemical reactions in which either the electroactive species A or the product of the electron transfer B participate as reactants. Perturbations of electrochemical responses of different techniques evoked by these reactions enable the elucidation of the mechaism and the evaluation of the kinetic parameters of the chemical steps. Chemical reactions that are indicated in the electrochemical way occur in the thin layer reaction layer) adjacent to the electrode surface only. This is illustrated in Fig. 1 where the concentration dependence of the product B on the distance from the electrode plane (with and without follow-up chemical reaction) is plotted. It must be stressed that the kinetics and the electrode mechanism are affected not only by the nature of the electroactive as well as electroinactive species including the type of the solvent, but also by the electrode material and substances adsorbed on the electrode surface. 

In view of the frequent occurrence in electrochemistry of certain reaction mechanisms involving coupled homogeneous reactions, a type of shorthand has been developed to describe them. In this shorthand the letter e refers to an electron transfer step, whilst the letter c refers to a homogeneous chemical reaction. Thus a ce reaction scheme implies a chemical step followed by an electron transfer. In all cases the electron transfer may be reversible, quasi-reversible, or irreversible, and the chemical steps may be reversible or irreversible and of first or higher order. The most commonly occurring reaction mechanisms are outlined below. 

Cyclic and square wave voltammograms showed that an ECjrrev mechanism appears as the most probable to describe the surface electrochemical reaction, where E represent a reversible electron transfer reaction, and Cinev an irreversible homogeneous chemical reaction (Laviron, 1972). The dependence between /pn and Epn on the logarithm of acid bulk concentration would indicate that a deprotonation reaction should be the fast follow-up chemical reaction coupled to the initial electron transfer reaction (Laviron, 1972 Mirceski Lovric, 2000). 

Let us first consider briefly how the use of mass transport as a variable can provide a guide to the reaction mechanism and give quantitative kinetic detail. As an illustration, we consider the behaviour of CE and EC processes (where E signifies electron transfer and C represents a chemical step) at a rotating disc electrode (RDE). This hydrodynamic system has already been discussed by Albery et al. and the reader is referred to Chap. 4 for details. CE and EC processes represent the simplest conceivable electrode reactions involving coupled homogeneous kinetics mechanistic examples of both types are shown in Table 1. In the discussion which follows, the electron-transfer reaction in the two mechanisms is considered to be a cathodic process the extension to the anodic case is trivial. 

Flow coulometry experiments were performed to study the reduction of U02 in nitric, perchloric, and sulfuric acid solutions [56]. The results of these studies show a single two-electron reduction wave attributed to the U02 /U + couple. The direct two-electron process is observed without evidence for the intermediate U02" " species because of the relatively long residence time of the uranium ion solution at the electrode surface in comparison to the residence time typically experienced at a dropping mercury working electrode. The implication here is that as the UO2 is produced at the electrode surface, it is immediately reduced to the ion. As the authors note a simplified equation for this process can be written, Eq. (7), but the process is more complicated. Once the U02" species is produced it experiences homogeneous reactions comprising Eqns (8) and (9) or (8) and (10) followed by chemical decomposition of UOOH+ or UO + to [49]. 

---