Diffusion currents in concentrated redox solutions

Diffusion-limited currents, i(lim), corresponding to the 1 - e(-) reductions of nitrobenzene (NB) and benzophenone (BP) at Pt microdisk electrodes are reported for CH3CN solutions containing 0.01 less than or equal to [NB] less than or equal to 9.1 M and 0.01 less than or equal to [BP] less than or equal to 4.0 M. Values of i(lim) exhibit a highly nonlinear dependence on redox concentration, obtaining maximum values in solutions containing either similar to 3 M NE or similar to 2 M BP (i.e., above these intermediate concentrations, i(lim) decreases with increasing redox concentration). The data are analyzed within the framework of a redox species/solvent interdiffusion model, employing the Cullinan-Vignes equation to estimate concentration-dependent binary mutual diffusion coefficients for the NB/CH3CN and BP/CH3CN systems, An analytical equation is derived that allows i(lim) values for a microdisk to be computed using the viscosities and hard-sphere molecular radii of the solvent and redox species in their pure state. Predicted values of i(lim) are found to be in reasonable agreement with voltammetric currents measured for NE and BP reduction.