Coupled homogeneous electrode reactions preceding reaction

Fig. 13. Diagnostic plots for electrode reactions with coupled homogeneous reactions, illustrated for the RDE. (a) CE mechanism. Curve A, no effect from chemical reaction (5k = 0) curve B, effect of preceding chemical reaction (5k >0). (b) Catalytic mechanism. Curve A, in the absence of parallel chemical reaction curve B, experimental dependence predicted from eqn. (175).

In the preceding three sections reaction mechanisms in which the homogeneous chemical reaction was coupled with the electrode process were discussed. This coupling enables exceptionally fast chemical reactions to be investigated and their rate constants determined. Nevertheless, voltammetric methods can also be exploited for kinetic studies on chemical reactions occurring independently of the electrode process in the bulk of the solution. For this purpose all voltammetric techniques can be used for which the dependence of voltammetric response on the concentration of one or more reactants is defined in a simple way. Various amperometric sensors are mostly applied, working at the potentials of limiting current. The response need not be a diffusion-controlled current. Kinetic currents within the diffusion-controlled zone can also be taken into account. 

Since the 1960s, cyclic voltanunetry has been the most widely used technique for studies of electrode processes with coupled chentical reactions. The theory was developed for numerous mechanisms involving different combinations of reversible, quasi-reversible, and irreversible heterogeneous ET and homogeneous steps. Because of space limitations, we will only consider two well-studied examples—(i.e., first-order reversible reaction preceding reversible ET) and E Ci (i.e., reversible ET followed by a first-order irreversible reaction)—to illnstrate general principles of the coupled kinetics measurement. A detailed discussion of other mechanisms can be found in Chapter 12 of reference (1) and references cited therein, including a seminal publication by Nicholson and Shain (19). 

---