Decomposition of Thiourea Dioxide under Aerobic and Anaerobic Conditions in an Aqueous Alkaline Solution
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ICAL CHEMISTRY OF SOLUTIONS
Decomposition of Thiourea Dioxide under Aerobic and Anaerobic Conditions in an Aqueous Alkaline Solution K. S. Nikitina, Yu. V. Polenova,*, and E. V. Egorovaa aIvanovo
State University of Chemistry and Technology, Ivanovo, 153000 Russia *e-mail: [email protected]
Received November 18, 2019; revised March 26, 2020; accepted April 17, 2020
Abstract—The kinetics and mechanism of the decomposition of thiourea dioxide in an aqueous alkaline solution under aerobic and anaerobic conditions are established. It is discovered that along with the decomposition of thiourea dioxide molecules with C–S bond cleavage and the subsequent formation of sulfoxyl acid anions, there is a reversible stage of the formation of thiourea and peroxide anions. The rate constants of the indicated stages are determined via mathematical modeling using the experimental data. Keywords: thiourea dioxide, kinetic model, complex reaction mechanism, electronic absorption spectra DOI: 10.1134/S0036024420100209
INTRODUCTION Thiourea dioxide (TDO) or formamidine sulfinic acid in aqueous solutions is used as a reducing agent for both organic [1, 2] and inorganic [3–5] compounds. In concentrated aqueous-alkaline solutions, its activity is so high that it allows the reduction of cadmium cations to a metal with a fairly high negative redox potential [6]. It is known from numerous literature sources that the high reducing activity of TDO molecules is explained by their decomposition with breaking of the weak bonds between carbon and sulfur and the formation of intermediates: sulfoxyl acid i anions (SO22 − ), radical anions (SO2− ), and dithionite anions (S2O24 − ) [7–12]. These particles that have strong reducing properties. It should be noted that the stoichiometric mechanism of decomposition of aqueous-alkaline solutions of thiourea dioxide has been studied in a number of works [13–15]. However, they did not determine the rate constants of the individual stages of the process nor perform a comparative analysis of the kinetics of the decomposition of TDO in air and inert atmospheres. One of the aims of this work was to solve these problems. EXPERIMENTAL Thiourea dioxide was synthesized from thiourea by oxidizing it with an aqueous solution of hydrogen peroxide, according to the procedure described in [16]. The content of the main substance in the final product, determined iodometrically, was 99.1% [17]. The IR absorption spectrum corresponded to that of aminoiminomethanesulfinic acid (another name for TDO),
which was listed in the NIST Chemistry WebBook database. All reagents were of chemically pure grade. Spectrophotometry was used to study the kinetics of TDO decomposition, since (according to the data of [18]) the initial material (thiourea dioxide) displays maximum absorption at λ = 270 nm in the electronic spectrum, but the reaction products do not absorb in this region of the spectrum. A LEKISS 2110 UV spectrophotometer (Finland) was used to measure electronic absorption spectra. In our kinetic experiment, 1 g of NaOH was
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