Chemistry of a non-linear reaction between Aniline Blue and acidic bromate

Abstract

The oxidation reaction of Aniline Blue (AB⁺) with bromate at low pH exhibits interesting non-linear phenomena. The depletion of AB⁺ in the presence of excess of bromate and acid occurs at a distinctly slow rate, followed by a very rapid reaction. Detailed reaction kinetics were studied by using spectrophotometric and potentiometric methods. The stoichiometry of the reaction is 3AB⁺+2BrO₃^-+3H⁺→3P₁+3P₂²⁺ +2Br^-, where P₁ and P₂ are sulfonate derivatives of 4-anilinobenzoic acid and 4,4′-diphenoquinone-4,4′-(bis)phenylimine, respectively. Under excess bromate conditions, bromide is oxidised to bromine and the stoichiometry changes to 5AB⁺+4BrO₃^-→5P₁+5P₂+2Br₂+H⁺+2H₂O. A 12-step reaction mechanism, consistent with the reaction dynamics, is described. The role of different bromo- and oxybromo-species is discussed. The proposed mechanism is supported by simulations.