Why bother with approximate analytical models

The modelling of kinetics at modified electrodes has received much attention over the last 10 years [1-11], mainly due to the interest in the potential uses of chemically modified electrodes in analytical applications. The first treatment published by Andrieux et al. [5] was closely followed by a complimentary treatment by Albery and Hillman [1, 2]. Both deal with the simplest basic case, that is, the coupled effects of diffusion and reaction for a second-order reaction between a species freely diffusing in the bulk solution and a redox mediator species trapped within the film at the modified electrode surface. The results obtained by the two treatments are essentially identical, although the two approaches are slightly different. 

In their work, Andrieux and Saveant analyse the behaviour in terms of the characteristic current densities for each process. This has the advantage that, in general, the expressions for these characteristic current densities are fairly straightforward and obvious and, therefore, easily derived. In the case where the different processes involved are strictly first-order, the reciprocal of total current density can be obtained by combining the reciprocals of the current densities for the individual steps. For example, the limiting current density, //., for a reaction at the surface of a rotating disc coated by a permeable membrane is given by 

Analytical models of modified electrodes for NADH oxidation 

In this chapter, we present an overview of the detailed kinetic analysis of the oxidation of NADH at a poly(aniline) modified electrode, both to demonstrate the basic approach which can be used, and to illustate the type of information which can be extracted. In many ways the design of 

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