KINETICS OF THIOUREA DIOXIDE DECOMPOSITION IN WATER-ETHANOL-AMMONIA SOLUTION
Abstract
A stoichiometric mechanism for thiourea dioxide decomposition in water- ethanol-ammonia solution is proposed based on dependences of concentrations of thiourea dioxide over time and literature data. The concentration of thiourea dioxide was measured with iodometry, while the intermediates were qualitatively detected using the polarography. The set of the obtained data allows to consider that at low concentration of alcohol (approximately up to 0.1 molar ratio) the thiourea dioxide decomposition proceeds on the heterolytic mechanism with formation of sulfoxylic acid, and at high concentrations of alcohol – on homolytic mechanism with the formation of anion radicals. Rate constants for individual stages of heterolytic mechanism are obtained by mathematical modeling, presented a system of differential equations. Absolute errors of rate constants, correlation coefficients, and F-factors were also calculated. The linear correlation was found between the equilibrium constant logarithm of the decay stage of thiourea dioxide and the inverse value of the dielectric constant of water-alcohol solutions. The kinetics of reduction of nickel ions with thiourea dioxide in an aqueous ammonia solution with ethanol additives was also studied. Сoncentration of nickel ions was determined using complexometric titration. Obtained kinetic data showed that at alcohol concentrations of 0.13 molar ratio and more, reaction kinetics is described by a first-order equation. At lower ethanol concentrations, a fractional order is observed on the concentration of nickel ions. A stoichiometric mechanism of this reaction is proposed due to sulfoxylic acid molecules with alcohol concentrations less than 0.1 molar ratio and due to radical ions with alcohol concentrations more than 0.1 molar ratio. Thus, the study of nickel ion reduction kinetics confirmed the conclusion about influence of alcohol concentration on the decomposition mechanism of thiourea dioxide.
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