Kinetics and mechanism of the oxidation of L -ascorbic acid by tris(oxalato)cobaltate( III ) and tris(1,10-phenanthroline)iron( III ) complexes in aqueous solution

Abstract

Kinetic studies of the oxidation of L-ascorbic acid by tris(oxalato)cobaltate(III), [CO(C2O4)3]3–, and tris-( 1,10-phenanthroline)iron(III), [Fe(phen)3]3+, have been made in an aqueous solution under varied conditions. The following mechanism of reaction is presented in accordance with the empirical results in solutions of pH 0.3–4.7 : (i) H2A [graphic omitted] HA–+ H+, (ii) H2A + Xn [graphic omitted] H2A˙++ Xn– 1, (iii) HA–+ Xn [graphic omitted] HA˙+ Xn– 1, (iv) Xn+ H2A˙+(and HA˙) [graphic omitted] Xn– 1+ A + 2 H+(and H+); where X = tris(oxalato)cobaltate(III) and tris(1,10-phenanthroline)iron(III) ions and n= 3– and 3+ respectively; H2A and HA– are the protonated species of the ascorbate ion. The rate constants k1 and k2, and the acid dissociation constant Ka were determined at 25 °C at ionic strength 0.1–1.0 mol dm–3, and at an ionic strength of 1.0 mol dm–3 at five temperatures between 10 and 30 °C. The results obtained are discussed in terms of Marcus theory for the outer-sphere electron-transfer reaction. The energy of the highest occupied molecular orbital of the species H2A and HA–, and the differences in total electronic energy between H2A+˙ and H2A, and between HA˙ and HA–, are calculated by using CNDO/2 MO, and they are presented for the comparison with the redox potentials estimated by applying the Marcus theory to the kinetic data.