Reaction Kinetics And Mechanism Analysis On The Oxidation Of Thiourea By Potassium Persulfate

Kinetics and mechanism of the oxidation of thiurea by persulfate were studied by the means of High Performance Liquid Cheomatography monitoring sulfur containing species such as thiurea, formamidine disulfide, thiourea monoxide, dioxide and trioxide simultaneously. Based on several sets of kinetics curves recorded by changing initial concentrations of thiourea and persulfate, the rate law in the form of v=kp H[TU][S2O82-] at different pH values was determined by the initial rate method, which means that the reaction is first order with respect to both thiourea and persulfate. The calculated second-order rate constants and the pH dependence research indicated that acid catalysis would appear in weak-acid solution while kpH was constant at neutral conditions. Alcohol was added in the reaction system has no effect on reaction dynamic behavior. It indicated that thiourea radicals were generated during the reaction, due to thiourea radical was more reactive than that of ethanol. The early stage of the reaction mainly generated formamidine disulfide, and further was oxidized in the presence of excess oxidizing agents. Besides, a simple 12-step radical mechanism was proposed to explain and simulate the kinetic traces.TU₂ was an important intermediate species for the reaction of K₂S₂O₈ and TU, so the stability and oxidation of TU₂ could explain the detail mechanism. The kinetic results showed that the hydrolysis of TU₂ was the first-order reaction, and the hydrolysis rate constant was 7.97× 10-6 s-1 at experimental conditions. Fixed TU₂ concentration, we studied on the effects of K₂S₂O₈ concentration in wide scope. It found that the TU₂ consumption rate was smaller than that of hydrolysis（[K₂S₂O₈]0 < 20 mM）. The initial rate method was adopted for K₂S₂O₈- TU₂ reaction, and thus the order of persulfate and thiourea was 1 and 0.6, respectively. Non-simple order showed that the reaction process included both direct oxidation and hydrolysis process of the TU₂. The model of 12 steps was proposed based on the reaction kinetics for reasonable interpretation of the above experimental phenomena.