Examination of Conditions under Which the Reduction of the Cobaltocenium Cation Can Be Used as a Standard Voltammetric Reference Process in Organic and Aqueous Solvents

The two one-electron processes for reduction of the cobaltocenium cation as the hexafluorophosphate salt have been studied extensively In both aprotic and protic solvents at a hanging mercury drop electrode and at conventionally and microsized platinum, gold, and glassy carbon disk electrodes of various radii. Under carefully chosen conditions, both the Cc⁺/Cc° and Cc°/Cc^- couples (Cc = cobaltocene) can be used as reversible voltammetric reference systems. However, the Cc⁺/Cc° couple is the preferred reference process since the combination of a very negative potential and possible overlap problems with the solvent response sometimes prevents the Cc°/Cc^- couple from being used as a practical voltammetric reference system. Furthermore, the half-wave potential for the Cc⁺/Cc° system was calculated to be -1.35 ± 0.01 V vs the widely used Fc⁺/Fc reference couple (Fc = ferrocene) and is essentially Independent of electrode material, electrolyte, and solvent relative to the ferrocene reference potential. In contrast, the half-wave potential for the Cc°/Cc^- couple relative to the ferrocene reference potential is significantly dependent on the nature of the solvent.