MECHANISM OF BLEACHING BY PEROXIDES .3. KINETICS OF THE BLEACHING OF PHENOLPHTHALEIN BY TRANSITION-METAL SALTS IN HIGH PH PEROXIDE SOLUTIONS

The kinetics of the oxidation of phenolphthalein by hydrogen peroxide catalysed by transition-metal complexes (1-50 mumol dm-3) have been st udied in aqueous solution at pH 10 and over a range of temperatures an d ionic strengths. The rate laws for the reactions were first order in each of phenolphthalein, hydrogen peroxide and the metal catalyst. Co mplexes of the early transition metals in their highest oxidation stat es catalysed the reaction; the order of reactivity was Mo(VI) much gre ater than Ti(IV) > W(VI) > V(V) almost-equal-to Cr(VI). Suggestions as to the identities of the active catalytic species are made on the bas is of the ionic strength dependence of the reactions. In the Mo(VI)-ca talysed reaction the active species is thought to be [Mo(O2)4]2-. The greatest catalytic activity was exhibited by [Co(NH3)Cl5]2+, which is ascribed to an electrostatically favourable reaction between a positiv ely charged peroxocobalt(III) ion with anionic phenolphthalein species . The temperature dependence of the reactions has been investigated ov er the range 10-30-degrees-C and mechanisms for the reactions are prop osed. Use of trapping agents showed that neither singlet oxygen nor ra dical species were involved in the oxidations. At higher (0.1-4 mmol d m-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 O- 1(2) was produced in significant quantities and made a major contribut ion to the overall rates of the oxidations.