Mechanism of bleaching by peroxides: Part 7. The pH dependence of the oxometalate catalysed bleaching of methyl orange☆

Abstract The kinetics have been studied over the pH range 8.95–11.0 of the bleaching of the azo dye methyl orange (4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt) by hydrogen peroxide catalysed by molybdate and tungstate at 25 °C. A plot of the overall rate constants against pH fell on a bell-shaped curve with molybdate catalyst, but with tungstate the rate constant rose to a plateau as the pH increased. The contributory rate constants for both the peroxometalate catalyst and the singlet oxygen pathways were determined by carrying out experiments both without and with the singlet oxygen trap 2,5-dimethylfuran. The singlet oxygen rate constants correlated well with 1 O 2 generation data in the literature. The results obtained with molybdate were consistent with MoO(O 2 ) 3 2− ion being the active metalate bleaching agent and also the precursor for 1 O 2 generation, but the tungstate results indicated that our knowledge of the peroxotungstate system is still incomplete.