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

The oxidation reaction of Aniline Blue (AB(+)) with bromate at low pH exhib its interesting non-linear phenomena. The depletion of AB(+) in the presenc e 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 rea ction is 3AB(+) + 2BrO(3)(-) + 3H(+) + 3P(1) --> 3P(2)(2+) + 2Br(-), where P-1 and P-2 are sulfonate derivatives of 4-anilinobenzoic acid and 4,4'-dip henoquinone-4,4'-(bis) phenylimine, respectively. Under excess bromate cond itions, bromide is oxidised to bromine and the stoichiometry changes to 5AB (+) +/- 4BrO(3)(-) --> 5P(1) + 5P(2) + 2Br(2) + H+ + 2H(2)O. A 12-step reac tion mechanism, consistent with the reaction dynamics, is described. The ro le of different bromo- and oxybromo-species is discussed. The proposed mech anism is supported by simulations.