Kinetics and mechanism of two-electron reduction of surfacial ferric ions of colloidal ferrofluid by L-ascorbic acid in aqueous acidic medium

The kinetics of electron transfer from L-ascorbic acid (H₂A) to ferrofluid (FF) in dilute aqueous acidic solutions (pH 2.5-4.0) was investigated spectrophotometrically in detail as a function of [H₂A], [FF], [H⁺], ionic strength, and temperature. FF was found to be reduced and the reduction was first-order dependent on both H₂A and FF concentrations. An inverse first order dependence in H⁺ ion concentration and no dependence on ionic strength were observed. Kinetic, electrochemical, spectrophotometric, and thermodynamic results are interpreted in terms of a mechanism involving a rate-determining outer-sphere one-electron transfer from H₂A to FF followed by a subsequent and kinetically rapid transfer of the second electron of H₂A to the same molecule of FF, which was already reduced by one electron in rate-determining step. This overall two-electron transfer process from H₂A to the same molecule of FF is discussed and compared with other closely related reactions.