Direct determination of diffusion coefficients of substrate and product by chronoamperometric techniques at microelectrodes for any level of ionic support.

A new method for the determination of the diffusion coefficients of both the substrate, DS, and the product, DP, of an electrode process has been developed. The method proposed is based on the analysis of the transient currents and can be applied to some reactions of the type SzS = PzP + ne and, in contrast to the concept based on the steady-state current, to any ratio of the concentrations of supporting electrolyte and substrate. The diffusion coefficients can be evaluated sequentially from the two parts of the double-potential step chronoamperogram, since the magnitude of the normalized chronoamperometric current of the first step depends on the DS value, while that of the second step is controlled by both DS and DP values. The corresponding, easy-to-use equations and procedures are given in the paper. The equations were derived on the basis of the numerical simulation data. The proposed methods of determination of diffusion coefficients for the substrates and products have been examined experimentally with the charged and uncharged ferrocene derivatives under diffusional and mixed diffusion-migration conditions. Only the chronoamperometric DS values obtained for the substrates could be compared to those determined from the steady-state diffusional current. It was found that for the systems investigated the agreement between these two methods was good. In this limited comparison, the standard deviations for the transient techniques were slightly larger than those obtained for the excess supporting electrolyte steady-state voltammetry.