Bromate oxidation of methyl orange. Part II. Effect of bromide and application to the determination of bromide

Abstract

The decolorisation of methyl orange by bromate proceeds by two paths. At low bromide concentrations, the direct reaction between methyl orange and bromate, although slow, produces bromide, which is oxidised by bromate to bromine to give rise to further decolorisation of methyl orange. This explains the induction period and autocatalysis that is observed in the system.

Bromide also acts as a retarding agent for the direct reaction of methyl orange with bromate. This effect is utilised for the determination of bromide at concentrations between 1 × 10^–7 and 5 × 10^–9 mol l^–1. At bromide concentrations approximately twice the methyl orange concentration the decolorisation of methyl orange occurs by the reaction with bromine. At intermediate bromide concentrations, the initial rapid decolorisation of methyl orange occurs by reaction with bromine, resulting in the depletion of bromide, with subsequent decolorisation occurring by a combination of the two processes, reaction with bromate and reaction with bromine.