Kinetics of the oxidation of L-ascorbic acid by diaquatetraamminecobalt(III) in acidic aqueous solution. Application of the Fuoss model and the Marcus-Sutin cross-relationship for electron-transfer processes

Abstract

The kinetics of the oxidation of L-ascorbic acid by diaquatetraamminecobalt(III) was studied as a function of pH, L-ascorbic acid concentration, ionic strength and temperature using a stopped-flow technique. The rate of the process was found to be first order with respect to both redox partners, whereas the [H +] concentration showed a retarding influence. The kinetic data are interpreted in terms of rate-determining oxidation of the deprotonated ascorbate anion, for which k=3.42±0.15)×10 −2 M −1 s −1 at 25 °C, ▵ H≠=81±6 kJ mol −1 and ▵ S≠=1±20 J K −1 mol −1. The ion-pair formation theory of Fuoss and the Marcus-Sutin cross-relationship for electron transfer were applied to this redox process to estimate the ion-pair formation constant, the rate constant for electron transfer and the self-exchange rate constant for the Co(NH 3) 4(H 2O) 2 2+/3+ couple.