Modification of adsorption components of the processes for reduction of selenium(IV) and oxidation of ferrocene at gold electrodes in frozen aqueous pe

Examination of the reduction of selenium(IV) and oxidation of ferrocene at gold electrodes in aqueous perchloric acid over the temperature range of 20 to -75°C demonstrates that the nature of adsorption at electrode surfaces may be modified in frozen electrolyte media. The four-electron irreversible reduction of selenium(IV) to elemental selenium occurs in aqueous perchloric acid by both adsorption (surface) and diffusion based mass transport processes at ambient temperatures. Oxidation of elemental selenium deposited on a gold electrode to selenium(IV) leads to both adsorbed and solution soluble products. At high selenium concentrations, gold-selenium intermetallic processes are observed. As the temperature is decreased below the freezing point of water (0°C) and then below the freezing point of aqueous perchloric acid electrolyte eutectic (-60°C), substantial discrimination against the adsorption and intermetallic based pathways occurs until almost only the diffusion based pathway is observed in totally frozen media. Results imply that an important step in the voltammetry at gold electrodes is the process {A figure is presented} with k₁ decreasing as the temperature is lowered. In the case of oxidation of ferrocene, Fecp₂, at ambient temperatures, the voltammetry at gold and other solid electrodes corresponds to the presence of relatively weak reactant adsorption, whereas, in the frozen electrolyte, strong product adsorption (or precipitation) of the ferrocenium cation, [Fecp₂]⁺, dominates the voltammetric response. The nature of the transformation from dominance of reactant to product adsorption is dependent on the perchloric acid concentration, electrode, and temperature. Results demonstrate that in aqueous media, the oxidation of Fecp₂ is better represented by the equation {A figure is presented} rather than as a single reversible one-electron diffusion controlled oxidation process.