Rotating Microdisk Voltammetry

Abstract

The influence of the supporting electrolyte concentration on the steady-state voltammetric behavior of a 12.5-μm-radius rotating Pt disk electrode (angular velocity, ω = 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 ω 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 ω 1/2 . However, for C elec /C redox < 0.1, the voltammetric currents decrease with increasing ω. 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