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.