Mechanism of bleaching by peroxides ☆: Part 6. Kinetics of the peroxide bleaching of methyl orange catalysed by MoO42− and WO42− at pH 10

Abstract The kinetics of bleaching of the azo dye methyl orange (4-[[(4-dimethylamino)phenyl]-azo]benzenesulfonic acid sodium salt) by hydrogen peroxide both alone and catalysed by [MoO 4 ] 2− and [WO 4 ] 2− have been studied at 25 °C at pH 10. The effects of adding the sequestrant CIX (diethylenetriamine pentakis- methylenephosphonic acid), free radical and singlet oxygen traps were investigated. It was established that two parallel catalytic pathways were involved: bleaching by peroxomolybdate or peroxotungstate species and oxidation by singlet oxygen, the latter being generated by peroxo species.