Steady-state voltammetry for hydroxide ion oxidation in aqueous solutions in the absence of and with varying concentrations of supporting electrolyte

The steady-state voltammetric behavior for the oxidation of aqueous solutions containing the strong bases sodium and barium hydroxide was studied with gold microelectrodes in the absence and in the presence of different concentrations of supporting electrolyte. A well-defined oxidation wave attributed to the oxidation of hydroxide ions to oxygen was observed in all the solutions investigated, regardless of both the nature of the base and the supporting electrolyte employed. However, in solutions with excess electrolyte, the steady-state limiting current was found to depend on the actual concentration of the supporting electrolyte, as the diffusion coefficient of the electroactive species varies with both the ionic strength and viscosity of the medium. Since the hydroxide ion is a negatively charged species, solutions with low or without supporting electrolyte yielded currents enhanced by migration contributions. Theoretical equations for the dependence of steady-state limiting currents with ionic strength were derived; theoretical and experimental data compared satisfactorily. The usefulness of the oxidation wave of hydroxide ions for analytical applications was shown by examining the dependence of steady-state limiting currents on concentration and pH. At a given ionic strength, the steady-state limiting current is proportional to the concentration of hydroxide ion over the range 0.5-5 mM. Moreover, the pH of the basic solutions can be determined by amperometry in place of potentiometry, with the use of an absolute equation.