Uncatalyzed and V(V)-Catalyzed Reaction of Methylene Blue with Potassium Bromate in Aqueous Sulfuric Acid

Article abstract of DOI:10.1021/j100349a015

The uncatalyzed and V(V)-catalyzed kinetics of reaction between methylene blue (phenotiazinium, 3,7-bis(dimethylamino)-,chloride) and acidic bromate has been studied monitoring the absorbance of methylene blue (MB) at 665 nm.Both the reactions involved competitive and sequential steps, having slow reaction in the initial stages.A rapid reaction followed after an induction time.For the two reactions, the orders with respect to the reactants are the same: second order with H⁺, first order each with respect to bromate ion and MB.In addition, the catalyzed reaction had first order dependence on catalyst concentration.In both the reactions HOBr is found to be the reaction intermediate.HOBr competed with bromate ion in the depletion of MB to give an intermediate, possibly phenothiazin-5-ium, 3-methylamino-7-dimethylamino-, chloride.The intermediate is further oxidized possibly by HOBr in fast step to final product, phenothiazin-5-ium, 3-amino-7-dimethylamino, -chloride.The stoichiometric ratios of MB to bromate are 2:3.The dual role of bromide ion as an inhibitor at low concentration and as an autocatalyst at higher concentrations above a certain critical concentration in the reaction mechanism is discussed.