Synergetic influence of mercaptoacetic acid on the reaction of thiourea with methylene blue in acidic medium

Abstract

Thiourea and methylene blue interact in a mole ratio of 2:1 in the presence of HCl forming the corresponding disulphide and leucomethylene blue. The order in thiourea is unity while that in methylene blue is zero at higher concentrations (<i style="">ca</i>.>2.5×10^-5<i style="">M</i>). At lower concentrations, the order in methylene blue is -1/2; this kinetic deviation depends on the nature of the thiol acid exhibiting synergetic effect. Mercaptoacetic acid does not react with methylene blue under the prevailing conditions; however it exerts a synergetic effect on the reaction. The rate increases linearly on increasing [H⁺] with the reaction having a large negative entropy of activation. Electrospray induced mass spectrometry, ¹H NMR and FTIR data indicate the participation of a persulphide-like species formed due to thiourea–mercaptoacetic acid interaction, which is perhaps responsible for the synergetic effect of mercaptoacetic acid.