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 CH 3 CN solutions containing 0.01 ≤ [NB] ≤ 9.1 M and 0.01 ≤ [BP] ≤ 4.0 M. Values of i lim exhibit a highly nonlinear dependence on redox concentration, obtaining maximum values in solutions containing either ∼3 M NB or ∼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/CH 3 CN and BP/CH 3 CN 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 NB and BP reduction.