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Cooperativity effect of mercaptosuccinic acid on the oxidation of thiourea by methylene blue in acidic medium

The principal substrate, thiourea (TU) interacts with methylene blue (MB) in a molar ratio of 2: 1 and the rate of oxidation is considerably enhanced in presence of mercaptosuccinic acid (TMA). The order of reaction is zero in MB and unity with respect to TU. The rate initially increases on increasing [TMA] but it is not affected at higher concentrations of TMA (ca ≥ 2.0 × 10−4 M). The order in MB changes from zero to –½ at lower concentrations of MB (ca ≤ 1.0 × 10−5 M). The reaction is characterised by a high negative entropy of activation. The kinetic features have been explained in terms of a competitive reaction between the half-reduced MB radical (HM·) and the TU molecule. This postulate has been verified by making the runs in presence of EDTA. The possibility of the participation of metal ion(s) as an impurity was ruled out on the grounds that the reactions of TU and TMA with MB individually in acidic medium do not show a negative order.