Mechanism of bleaching by peroxides. Part 2. - Kinetics of bleaching of alizarin and crocetin by hydrogen peroxide at high pH

Abstract

The kinetics of the oxidation of alizarin (1,2-dihydroxyanthraquinone) and crocetin (8,8-diapo-ψ,ψ-carotenedioic acid) by alkaline hydrogen peroxide have been studied in aqueous solution over a range of pH, ionic strength and temperature. In each case the rates of the reactions were found to be first order both in the organic colourant and in hydrogen peroxide. Use of free radical and singlet oxygen trapping agents have ruled out the involvement of these species in both reactions. Analysis of the kinetics of oxidation of alizarin has shown that over the pH range 10-12 the predominant reaction is that between the mono-ionised form of alizarin and the perhydroxyl anion, HOO-. The kinetic experiments with crocetin, coupled with spectrophotometric determination of its second ionisation constant, showed that the largest rate constant was that for reaction between HOO- and the doubly ionised form of crocetin. Experiments over the temperature range 21.0-45.0 °C led to activation energies of 68.8 and 69.9 kJ mol-1, and entropies of activation of -95 and -92 J K-1 mol-1 for the alizarin and crocetin reactions, respectively, at 298 K in hydrogencarbonate buffers. Mechanisms for both reactions are proposed.