Assessment of Conditions under Which the Oxidation of Ferrocene Can Be Used as a Standard Voltammetric Reference Process in Aqueous Media

The one-electron oxidation process for ferrocene (Fc), Fc Fc⁺ + has been studied extensively by cyclic, normal pulse, and differential pulse voltammetry and chronocoulometry to determine the conditions under which this reaction can be used as a voltammetric standard In aqueous media. In water, the oxidation of ferrocene Is not a simple reversible one-electron process as Is the case In organic solvents. Rather, weak reactant adsorption Is exhibited, which Is electrode and electrolyte dependent. The oxidation of a saturated solution corresponds to considerably less than a monolayer of coverage. The relative order of adsorption with respect to the electrode material follows the trend Hg > glassy carbon > Au > Pt, while with electrolyte It Is NaCI0₄ > LI₂S0₄ > NaF. Despite the presence of weak reactant adsorption, essentially electrode, electrolyte, and technique Independent voltammetric data can be obtained at sufficiently slow scan rates or long pulse widths. The value calculated under these conditions is 0.400 ± 0.005 V vs NHE which agrees very well with the standard redox potential reported from potentlometrlc measurements under genuine equilibrium conditions. The data suggest that ferrocene can be used as a voltammetric standard under carefully chosen conditions where the Influence of adsorption Is minimal.