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 solutio ns containing the strong bases sodium and barium hydroxide was studied with gold microelectrodes in the absence and in the presence of different conce ntrations of supporting electrolyte. A well-defined oxidation wave attribut ed to the oxidation of hydroxide ions to oxygen was observed in all the sol utions investigated, regardless of both the nature of the base and the supp orting electrolyte employed. However, in solutions with excess electrolyte, the steady-state limiting current was found to depend on the actual concen tration 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, soluti ons with low or without supporting electrolyte yielded currents enhanced by migration contributions. Theoretical equations for the dependence of stead y-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. A t 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, t he pH of the basic solutions can be determined by amperometry in place of p otentiometry, with the use of an absolute equation.