A kinetic study on Ru(III)–catalysed oxidation of mercaptosuccinic acid by methylene blue in acidic medium

Abstract

Mercaptosuccinic acid and methylene blue interact in a molar ratio of 2:1 in presence of hydrochloric acid and Ru(III) as a catalyst in aqueous acetone medium (40% <i style="">v/v</i>) forming dithiodimalic acid and dihydromethylene blue. The reaction follows a half order kinetics in methylene blue. The order in mercaptosuccinic acid is unity. The average value of half order rate constant remains practically unaffected on varying [H⁺] up to 2.0 × 10^-2 <i>M</i>. The order in methylene blue changes from 1/2 to 1 at lower [H⁺] (1.0 × 10^-2 <i>M</i>)as well as at lower concentrations of catalyst (1.0 × 10^-5 <i>M</i>), while at higher [Ru (III)] (≥ 2.0 × 10^-5 <i>M</i>) the order in the oxidant becomes zero. This indicates that the relationship between the rate constant and [Ru(III)] is not simple. The rate constant decreases on increasing the time of equilibration of Ru(III) with other ingredients of the reaction system. SEM images suggest the formation of square shaped crystals with size ranging between 50-200 μm. The addition of reaction products does not influence the rate. Activation parameters have been evaluated and a plausible reaction scheme, presuming the reaction between the transient complex (Ru(III)SR’)and MBH⁺ as the rate determining step, has been proposed.