Mechanism of bleaching by peroxides. Part 3.—Kinetics of the bleaching of phenolphthalein by transition-metal salts in high pH peroxide solutions

Abstract

The kinetics of the oxidation of phenolphthalein by hydrogen peroxide catalysed by transition-metal complexes (1–50 µmol dm^–3) have been studied in aqueous solution at pH 10 and over a range of temperatures and ionic strengths. The rate laws for the reactions were first order in each of phenolphthalein, hydrogen peroxide and the metal catalyst. Complexes of the early transition metals in their highest oxidation states catalysed the reaction; the order of reactivity was Mo^VI Ti^IV > W^VI > V^V≈ Cr^VI. Suggestions as to the identities of the active catalytic species are made on the basis of the ionic strength dependence of the reactions. In the Mo^VI-catalysed reaction the active species is thought to be [Mo(O₂)₄]^2–. The greatest catalytic activity was exhibited by [Co(NH₃)₅Cl]²⁺, which is ascribed to an electrostatically favourable reaction between a positively charged peroxocobalt(III) ion with anionic phenolphthalein species. The temperature dependence of the reactions has been investigated over the range 10–30 °C and mechanisms for the reactions are proposed. Use of trapping agents showed that neither singlet oxygen nor radical species were involved in the oxidations.

At higher (0.1–4 mmol dm^–3) concentrations of Mo^VI and W^VI the rate laws of the reactions included a term in the square of the hydrogen peroxide concentration. Experiments using a singlet oxygen trapping agent showed that here ¹O₂ was produced in significant quantities and made a major contribution to the overall rates of the oxidations.