Kinetic and electrochemical aspects of two-electron reduction of ferrofluid by vitamin C in dilute aqueous solution

The two-electron reduction of ferrofluid (FF) by vitamin C (L-ascorbic acid, H₂A) and the reoxidation of the reduced product by molecular oxygen has been investigated in dilute aqueous acidic solutions (pH 2.5 - 4.0) as a function of the variants, temperature, ionic strength and concentrations of FF, H₂A, H⁺ and molecular oxygen using potentiometric, spectrophotometric and voltammetric techniques. The rate of reaction was found to be first order with respect to the concentrations of H₂A and FF, zero order with respect to molecular oxygen, inverse first order with respect to H⁺ and independent of ionic strength. The experimental results are interpreted in terms of a mechanism involving a rate determining outer sphere one-electron transfer. from vitamin C to FF followed by a subsequent and kinetically rapid transfer of the second electron of vitamin C to the same molecule of FF, which was already reduced by one electron in the rate determining step. Reduced product of FF gets reoxidized by molecular oxygen, making the overall two-electron oxidation of H₂A catalytic at appreciable lower thermodynamic quantities.