KINETICS AND MECHANISM OF THE OXIDATION OF THIOUREA BY METHYLENE BLUE IN ACIDIC MEDIUM

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Abstract
The title reaction has been investigated kinetically in methanol-water medium (3% v/v) and in presence of hydrochloric acid. The reaction follows a half order kinetics in methylene blue (MB) and a first order kinetics in thiourea (TU). At higher [TU] or at lower [MB] (ca. 1.5 × 10−5 M), the order in the oxidant changes from 1/2 to zero. The rate increases linerarly on increasing [H+]. Addition of EDTA and the variation in ionic strength has no influence on the rate except the addition of potassium chloride which may be attributed to the environmental effect. The increase in dielectric constant increases the rate. The external addition of the reaction products does not influence the rate but higher concentrations of leucomethylene blue tend to exert a slightly retarding influence. Activation parameters have been evaluated and a reaction scheme has been proposed postulating a sterically controlled nucleophilic attack of thiourea molecule on half-reduced methylene blue radical as the rate limiting step.