Kinetics and mechanism of the oxidation of thiourea by bromate in acidic solution

published in Advance ACS Absrracrs, December I S , 1993. 0022-3654/94/2098-055 1504.50/0 Experimental Section Materials. The following reagent grade chemicals were used without further purification: sodium bromide, thiourea, perchloric acid (60%), sodium bromide, sodium perchlorate (Fisher), bromine, silver nitrate, sodium bromate, and sodium hydroxide (Aldrich). Neither of the major reactants-sodium bromate and thiourea-was standardized; they were assumed to be of high enough purity. Perchloric acid was standardized by standard sodium hydroxide with methyl orange as indicator. Stoichiometric determinations were performed by analyzing for sulfate, bromate, and bromine. Sulfate was analyzed gravimetrically as Bas04 only in excess thiourea, because Ba(BrO3)2 is not very soluble in water, and thus gravimetric analysis of solutions with excess bromate gave anomalously high values.21 Although barium bromite also forms a precipitate, it is sufficiently soluble that it does not precipitate in millimolar quantities. Generally, for quantitative determinations of sulfate, the reaction solutions were allowed to sit for at least 24 h before addition of barium chloride. Subsequently, the precipitate was allowed to settle for another 24 h before any analyses were performed. Excess bromate was acidified and mixed with excess iodide, and the liberated iodine was titrated against standard sodium thiosulate with starch as indicator. Bromine was evaluated from its absorbance at 390 nm (absorptivity coefficient 142 M-l cm-l). There was nodirect method of independently determining bromide concentrations. The bromide ion specific electrode was ineffective in the presence of sulfur compounds; almost instantly upon using it, its surface was coated with a black film of some sulfur compound, and the potential reading started to drift. Methods. The reaction was followed by monitoring changes in redox potential and optical absorbance of bromine. Redox potential measurements were carried out with platinum and Radiometer K601 HglHg2S041K2S04 electrodes. The use of mercury/mercuric sulfate as reference instead of the normal calomel electrode was to avoid possible interference by chloride ions from the electrode. Absorbance measurements were carried out on a Varian DMS 200 UV/visible spectrophotometer and on a Pye Unicam SP1750 spectrophotometer. The reaction was performed with high initial acid and bromateconcentrations such