Mechanism of bleaching by peroxides: Part 5. Kinetics of the peroxide bleaching of crocetin catalysed by [Co3IIW{CoIIW9O34}2(H2O)2]12−

Abstract The kinetics of the bleaching of crocetin (8,8-diapocarotenedioic acid) by hydrogen peroxide catalysed by the triple sandwich polyoxometalate Na 12 [Co 3 II W{Co II W 9 O 34 } 2 (H 2 O) 2 ]·47H 2 O have been studied at pH 10 and 25 °C. The effect of the concentration of the added sequestrant CIX (diethylenetriamine pentakis methylenephosphonic acid) was particularly investigated. The mechanism which fitted the experimental results involved the formation of an intermediate crocetin–catalyst complex which was then oxidised by the hydrogen peroxide. In the absence of CIX the formation of the intermediate was rate-determining while, in the presence of excess CIX, the oxidation of the intermediate became rate-determining with a Michaelis–Menten-type rate equation.