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

The kinetics of the oxidation of L-ascorbic acid by diaquatetramminecobalt(II1) 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) x lo-’ M-’ s-r at 25 “C, AEZf =81+6 kJ mol-’ and AS+ =1+20 J K-r mol-‘. 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(NH3)4(H20)~+n+ couple.The authors gratefully acknowledge financial support from CICYT (Madrid) and the Deutsche Forschungsgemeinschaft.Peer reviewe