Voltammetry of the ferric/ferrous redox couple at platinum microelectrodes in aqueous hydrochloric acid

Abstract

In this paper, the Fe3+/Fe2+ redox system was investigated in 0.5 M HCl solutions, using steady-state voltammetry at a Pt microelectrode. Measurements were performed in solutions containing each ion alone or in mixtures containing both ions. The reduction of Fe3+ to Fe2+ provided a single sigmoidal wave, regardless of the ion concentration. For the oxidation process of Fe2+ to Fe3+, a wave split was observed at relatively high ion concentrations (i.e., ≥ about 10 mM). This feature, not observed or not considered in previous studies, was rationalized by considering that the Fe2+ oxidation process occurs over the potential region where passivating Pt oxides form on the electrode surface. Standard rate constant (k0′), and transfer coefficient (α) of the electrode process were evaluated using the method based on the quartile potentials ΔE1/4=E1/4-E1/2 and ΔE3/4=E1/2-E3/4, which can be obtained from the experimental steady-state voltammograms. k0′ and α values varied in the range 1.4 x10-3 cm s−1 – 3.1 x 10-2 cm s−1, and 0.44 – 0.64, respectively, depending on the specific status of the electrode surface, in turn depending on the care adopted in cleaning the electrode surface from the Pt oxides. From the quartile potentials approach, the formal potential of the redox system, equal to 0.495 (±0.005) V, vs. Ag/AgCl (KCl saturated) could also be evaluated. This value resulted in good agreement with 0.494 (±0.002) V, formal potential obtained from the cathodic-anodic voltammograms recorded in the mixtures of the two ions at equal concentration. Chronoamperometry was employed to determine the diffusion coefficients on the two ions by fitting the experimental current against time profiles and the theoretical equation that holds for microdisk electrodes. The values found where 4.67 x 10-6 cm2 s−1 and 5.61 x 10-6 cm2 s−1 for Fe3+ and Fe2+ ions, respectively.