Rotating disc electrode electron transfer kinetics

In this section, we present a kinetic model for the oxidation of NADH within a polymer film at a rotating disc electrode. In this model, we assume that the rate of electron transfer between the mediating species in the film and the electrode is sufficiently fast so that it is not rate limiting [43]. We also assume that the substrate diffuses into the film where it is adsorbed at the reaction sites within the film and where it undergoes oxidation to the product. The possibility that the product is able to compete for these reaction sites with the film in a reversible manner is also considered. 

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. 

The theoretical description of electrocatalysis that takes into account electron and ion transfer and the transport process, the permeations of the substrates, and their combined involvement in the control over the overall kinetics has been elaborated by Albeiy and Hillman [312,313,373] and by Andrieux and Saveant [315], and a good summary can be found in [314]. Practically all of the possible cases have been considered, including Michaelis-Menten kinetics for enzyme catalysis. Inhibition, saturation, complex mediation, etc., have also been treated. The different situations have also been represented in diagrams. Based on the theoretical models, the respective forms of the Koutecky-Levich eqrration have been obtained, which make analyzing the resirlts of voltarrrmetry on stationary artd rotating disc electrodes a straightforward task. 

The hydrodynamically modulated rotating disc electrode The idea of this technique [15-17] is to allow the separation of kinetic and mass transport controlled components of a measured current (either in the region of mixed electron transfer/mass transport control or in situations where there are two competing electrode reactions, one mass transport controlled and the other kinetically controlled, (e.g. solvent decomposition) by using a sinusoidal modulation of the rotation rate and employing a phase sensitive detection method to measure the perturbation of the current. Only the mass transport limited component of the current will respond to the modulation. 

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