KINETICS AND MECHANISM OF THE CHLORITE THIOUREA REACTION IN ACIDIC MEDIUM

The kinetics of the reaction between chlorite and thiourea in acidic medium have been studied. The stoichiometry of the reaction for pH > 3 is 2ClO2- + SC(NH2)2 + H2O --> 2Cl- + OC(NH2)2 + SO42- + 2H+; for pH < 3 OC(NH2)2 is hydrolyzed to NH4+ and CO2. The reaction is characterized by an induction period after which chlorine dioxide is produced. For [ClO2-]/[SC(NH2)2] < 2 the formation of chlorine dioxide is only transient. Thiourea is progressively oxidized to sulfonic acid before cleavage of the sulfur-carbon bond and formation of sulfate. A 13-step mechanism is proposed with rate-determining step ClO2- + SC(NH2)2 + H+ --> HOCl + HOSC(NH)NH2. Computer simulation using this mechanism gives good agreement with experiment.