Chronoamperometric behaviour of a CE process with fast chemical reactions at spherical electrodes and microelectrodes. Comparison with a catalytic reaction

A simple approximated time-dependent equation for the limit current corresponding to fast chemical reaction preceding the electrochemical step (CE process under kinetic steady state conditions) applicable to spherical electrodes and microelectrodes is deduced. From this equation, a simple method to elucidate the homogeneous kinetic is proposed. The particular cases of planar, microspherical and ultramicrospherical electrodes are obtained and the severe limitations for application of the time-independent equation for microelectrodes, usually used in the literature, are demonstrated. The equation deduced in this paper has been shown to be fundamental in the understanding of the reaction and diffusion layer thicknesses in steady state spherical diffusion, which have not been clearly defined until now. The different types of steady states that CE and catalytic mechanisms can reach are compared and explained. It is also shown that in the steady state, the CE mechanism becomes the catalytic one when reaction and diffusion layers thicknesses are equal.