ROTATING MICRODISK VOLTAMMETRY

The influence of the supporting electrolyte concentration on the steady-state voltammetric behavior of a 12.5-mu m-radius rotating Pt disk electrode (angular velocity, omega = 0-378 rad/s) has been investigated for several electrochemical reactions in acetonitrile solutions, The results demonstrate that the voltammetric response is a strong function of the ratio of the supporting electrolyte and redox concentrations (C-elec/C-redox) as well as the charge of the reactant (z), For the oxidation or reduction of monovalent and divalent species (e,g,, [(trimethylammonio)methyl]-ferrocene (z = +1) and methylviologen (z = +2)), the observed voltammetric limiting currents are found to increase linearly with omega(1/2), independent of C-elec/C-redox. Voltammetric currents corresponding to the oxidation or reduction of neutral reactants (e,g,, ferrocene and nitrobenzene (z = 0)) show a more complex dependence on C-elec/C-redox In solutions containing an appreciable quantity of supporting electrolyte, C-elec/C-redox > 0.1, mass transport limited currents are found to increase linearly with omega(1/2). However, for C-elec/C-redox < 0.1, the voltammetric currents decrease with increasing omega, The unusual behavior observed for neutral species in low ionic strength solution is interpreted in terms of the rate of migration of charge-balancing electrolyte ions to the electrode surface, relative to the rate of removal of the same ions by forced convection; In low ionic strength solutions, electrolyte ions are removed by forced convection more rapidly than they are replenished by migration, resulting in a decrease in the electrical driving force for electron transfer.