Ruthenium(III)-catalyzed oxidation of alginate and pectate biopolymers by chromic acid in aqueous perchlorate solutions: a comparative kinetic study

Abstract

The kinetics of chromic acid oxidation of alginate and pectate carbohydrate biopolymers in aqueous perchlorate solutions was studied in the absence and presence of ruthenium(III) catalyst at a fixed ionic strength of 2.0 mol dm−3 and a temperature of 298 K. Both uncatalyzed and Ru(III)-catalyzed oxidation reactions were followed spectrophotometrically. Both the reactions exhibited first-order dependence on [Cr(VI)], less than unit order with respect to the biopolymer concentration and fractional second order with respect to [H+]. The catalyzed reactions were first order in [Ru(III)]. No significant effect of ionic strength or dielectric constant of the medium was observed. Addition of Mn(II) inhibited the oxidation. Probable mechanistic schemes for the oxidation reactions are proposed, and the rate laws associated with these mechanisms have been derived. In both cases, the final oxidation products were characterized by spectroscopic data and microanalysis as mono–keto derivatives of the biopolymers. Under comparable experimental conditions, the oxidation rates of alginate were found to be lower than those of pectate for both the uncatalyzed and Ru(III)-catalyzed paths. The activation parameters with respect to the slow step of the mechanisms have been computed.