Cu(II) catalysed oxidation of o-mercapto-benzoic acid by methylene blue in acidic medium: A kinetic study

Abstract

Kinetics of oxidation of <i>o</i>-mercaptobenzoie acid by methylene blue catalyzed by Cu(II) in presence of sulphuric acid and in aqueous ethanol medium (4% <i>v/v</i>)<i> </i>have been studied. Two moles of the substrate are oxidized by one mole of methylene blue forming the disulphide and the leuco dye as the end products. The order in methylene blue is 3/2<i> </i>which increases to two at lower concentrations of Cu(II). The older in <i>o</i>-mercaptobenzoic acid is nearly zero. The rate is independent of variation in [H⁺] but at higher concentrations, the rate decreases to give a limiting value. The rate is not influenced on varying [Cu(II)] at higher levels but at lower concentrations (<i>ca</i>. < 6.0×10^-6 <i>M</i>),<i> </i>there is a linear relationship between the rate and [Cu (II)]. The addition of reaction products does not influence the rate. Activation parameters have been evaluated and a plausible reaction scheme, presuming the reaction between metal - TSA complex and MB dimer as the rate determining step, has been proposed.