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˙⁺+ X^{n– 1}, (iii) HA^–+ Xⁿ [graphic omitted] HA˙+ X^{n– 1}, (iv) Xⁿ+ H₂A˙⁺(and HA˙) [graphic omitted] X^{n– 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 K_a 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.