Mechanism of bleaching by peroxides. Part 1.—Kinetics of bleaching of phenolphthalein by hydrogen peroxide at high pH

Abstract

The kinetics of the oxidation of phenolphthalein by alkaline hydrogen peroxide have been studied in aqueous solution over a range of pH, ionic strength and temperature. The rate of reaction was found to be first order in both phenolphthalein and hydrogen peroxide. Use of free radical and singlet oxygen trapping agents have ruled out the involvement of these species in the reaction. The variation of the rate constant with pH and ionic strength has shown that at pH 11 the main reaction is that between the pink bisphenolate form of phenolphthalein and the perhydroxyl anion, HO₂^–. At pH 9, epoxidation by the hydrogen peroxide molecule contributed most to the overall rate constant. Borate-buffered solutions gave a faster rate of reaction than those in hydrogencarbonate or ammonia buffers. Experiments over the temperature range 21.0–45.0 °C lead to activation energies of 43.3 and 48.4 kJ mol^–1 and entropies of activation of –149 and –131 J K^–1 mol^–1 at 298 K, in carbonate and borate buffers, respectively. A mechanism for the reaction is proposed.