Inner sphere oxidation of L-ascorbic acid by Ru(III) ion and its complexes in aqueous acidic medium

Abstract

The kinetics of electron transfer from L-ascorbic acid [H₂A] to oxidants, dichlorotetraaquoruthenium(III) [RuCl₂(H₂O)₄]⁺, iminodiacetatoruthenium(III) [Ru(III)---IMDA]⁺ and ethylenediaminetetraacetatoruthenate(III) [Ru(III)---EDTA]⁻ exhibit a first order dependence both on L-ascorbic acid and oxidants and inverse first order dependence on hydrogen ion concentration. Kinetic, spectroscopic and thermodynamic parameters are reported for the formation of intermediate Ru(III)--- (1:1) and Ru(III)---chelate---ascorbate (1:1:1) complexes during the oxidation of L-ascorbic acid. The results are interpreted in terms of a mechanism involving a rate-determining inner sphere one electron transfer from L-ascorbic acid to the oxidants used in the present investigation, followed by a subsequent and kinetically rapid transfer of the second electron of ascorbic acid to another molecule of the oxidant. A detailed discussion of the kinetic data, temperature and ionic strength dependence of the oxidation reactions is presented.