Kinetics of proton depended oxidation of diethylthiourea with methylene green in aqueous medium

Diethylthiourea (DETU) reacts with methylene green (MG) and reduced it into leuco dye where it oxidized into smaller fragments like urea, nitrate and sulphate. The reaction was favorable in acidic condition. A direct relation in between oxidation of DETU and MG to the concentration of H+ ions was observed through typical kinetics in two acids like HCl and H2SO4. The second order with respect to HCl and the first order with respect to H2SO4 indicated the protonation of DETU with the reduction of dye. In alkaline medium, the rate of reaction decline may be due to the suppressing effects of alkali on oxidation/degradation of DETU into smaller fragments or may be attributed with the de-protonation of DETU. Influence of addition of dye into reaction mixture retarded the bleaching process and follows zero order kinetics while rate of reaction enhanced by the addition of DETU in the reaction mixture and shows ¾th order with respect to DETU. Negative effect of added different electrolytes reflects that change of ionic strength of reaction mixture have some bearing on redox reaction of MG and DETU. Reaction was monitored at elevated temperature and activation parameters were computed. The mechanisms of oxidation of DETU with reduction of dye MG were discussed. Reduction of MG into leuco dye with the fragmentation of DETU was observed in acidic condition at elevated temperature.   Key words:  Methylene green (MG), diethylthiourea (DETU), proton dependence, smaller fragments, redox reaction, ions.