Kinetics and Mechanism of the Oxidation of 3,7-Bis(dimethylamino) Phenothionium Chloride (Methylene Blue) by Bromate Ion in Aqueous Hydrochloric Acid Medium

Abstract

The stoichiometry, kinetics and mechanism of the oxidation of 3,7–bis(dimethylamino) phenothionium chloride (methylene blue) by bromate ion, BrO₃^- was investigated in aqueous hydrochloric acid medium. The reaction follows the empirical rate law:

 -1/2 d[MB⁺] = (a + b[H⁺]²)[MB⁺][BrO₃^-]

   Where a = 210 dm³ mol^-1 s^-1, b = 1.6 dm⁹ mol^-3 s^-1 at 29 ± 1^oC. 

[H⁺] = 0.175 – 0.30 mol dm^-3, I = 0.50 mol dm^-3 (NaCl) and λ_max = 665 nm. 

The rate law indicates that the reaction occur via two pathways, involving acid - independent and second – order acid – dependent pathways. The inhibition of the reaction rate by the presence of added HCOO^- and SO₄^2-, the absence of both spectroscopic and kinetic evidences for the formation of a precursor binuclear intermediate prior to the electron transfer step are suggestive of the outer-sphere mechanism.