Kinetics and Mechanism of the Oxidation of Thiourea by Bromate in Acidic Solution

Article abstract of DOI:10.1021/j100053a033

The reaction between bromate and thiourea has been studied in acidic medium.The stoichiometry of the reaction in excess thiourea is 4BrO3^- + 3SC(NH2)2 + 3H20 -> 3SO4^2- + 3OC(NH2)2 + 4Br^- + 6H⁺; in excess bromate the stoichiometry is 8BrO3^- + 5SC(NH2)2 + H2O -> 5SO4^2- + 5OC(NH2)2 + 4Br2 + 2H⁺.No bromine is formed in excess thiourea.In excess bromate the reaction displays an initial induction period.At the end of the induction period, the redox potential of the reaction mixture rises sharply, sulfate appears (signaled by precipitation of BaSO4 when BaCl2 is initially added), and a yellow coloration (due to bromine) is first noticeable.Consequently, bromine is not formed until all thiourea is consumed.A 14-step mechanism is proposed and used to simulate the observed kinetics.The rate-determining step for bromine appearance is formation of HOBr from the BrO3^--Br^- reaction.The oxidation of thiourea proceeds via oxygen additions on sulfur, successively forming HOSC(NH)NH2, HO2SC(NH)NH2, HO3SC(NH)NH2, and SO4^2-.The mechanism requires that cleavage of the X-C bond to form urea and SO4^2- occur at the sulfonic acid level and not before, in agreement with experimental observation.