SYSTEMATIC DESIGN OF CHEMICAL OSCILLATORS .88. KINETICS AND MECHANISMOF THE OXIDATION OF THIOUREA BY BROMATE IN ACIDIC SOLUTION

The reaction between bromate and thiourea has been studied in acidic m edium. The stoichiometry of the reaction in excess thiourea is 4BrO(3) (-) + 3SC(NH2)2 + 3H(2)O-->3SO(4)(2-) + 30C(NH2)2 + 4Br(-) + 6H(+); in excess bromate the stoichiometry is 8BrO(3)(-) + 5SC(NH2)2 + H2O --> 5SO(4)(2-) + 4Br(2) + 2H(+). No bromine is formed in excess thiourea. In excess bromate the reaction displays an initial induction period. A t 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 unti l all thiourea is consumed. A 14-step mechanism is proposed and used t o simulate the observed kinetics. The rate-determining step for bromin e appearance is formation of HOBr from the BrO3--Br- reaction. the oxi dation of thiourea proceeds via oxygen additions on sulfur, successive ly forming HOSC(NH)NH2, HO2SC(NH)NH2, HO3SC(NH)NH2), and SO42-. The me chanism requires that cleavage of the X-C bond to form urea and SO(4)( 2-)occur at the sulfonic acid level and not before, in agreement with experimental observation.