Kinetics of oxidation of thioacetic acid by methylene blue in methanol-water medium

Abstract

Thioacetic acid (TA) and methylene blue (MB) interact to form corresponding disulphide and the leucobase. The order in MB is two in the initial part (4050%) of e kinetic runs and in the latter part it changes to 1/2. The order in [TA] is unity. The second order rat constant increases linearly with increase in [H⁺] while the half order rate constant shows a fractional dependence on [H⁺] at low concentrations. At higher [H ⁺] (ca. ≥ 4.0 X 10^-2 mol dm^-3) however, again a linear relationship between <em>k</em>_1/2 and [H⁺] has been noticed. The rate decreases with increase in [MB]. The variation in ionic strength does not affect the rate. The psuedo-second order rate constant decreases an attains a limiting value while the half order rate constant continuously decreases on decreasing the dielectric constant of the medium. Addition of the products does not affect the rate but second order behaviour [MB] observed in the initial part disappears in the presence of leuco base. The order in TA remains unity while the pseudo-half order rate constant is not in influenced by varying [MB] under these conditions. The addition of methyl methacrylate does not affect the rate initially but appreciably retards the rate of the liar part of the reaction. Activation parameters have been evaluated and the participation of semireduced methylene blue is shown to cause the kinetic deviation.