Kinetics and mechanism of the oxidation of methylene violet by bromate at acidic pH and the dual role of bromide ion.
Methylene violet (MV(+)), a phenazine class of dye during oxidation by the bromate ion under acidic conditions, exhibited complex nonlinear behavior. The intricate kinetics of reaction of methylene violet [3-amino-7-(diethyleamino)-5-phenyl phenazium chloride, MV(+)] with acidic bromate was investigated using the stopped flow technique. Under excess acid and bromate concentration conditions, MV(+) exhibited a very slow reaction initially but a very rapid reaction after an induction time. The reaction had first-order dependence on both H(+) and BrO(3)(-) ions. The overall stoichiometric reaction is 2 MV(+) + 2 BrO(3)(-) + H(2)O --> 2 MP + 2 HON(CH(2)CH(3))(2) + N(2)O + 2 H(+) + 2 Br(-) where MP is the 3,7-dioxo-5-phenyl phenazine. The active roles of various bromo and oxybromo species in the mechanism are discussed. The rapid kinetics of the direct reaction of bromine with MV(+) is also reported. A 19-step mechanism, consistent with the experimental data and validated by simulations using Simkine-2, is proposed.
School of Chemistry, University of KwaZulu-Natal, Westville Campus, Chiltern Hills, P Bag X54001, Durban 4000, South Africa. jonnalagaddas@ukzn.ac.za
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