Kinetics and Mechanism of the Oxidation of Thiourea Dioxide by Iodine in a Slightly Acidic Medium

The thiourea dioxide (TDO)–iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at <i>T</i> = 25.0 ± 0.1 °C, and at 0.5 M ionic strength in buffered slightly acidic medium. The nitrogen- and carbon-containing products were found to be ammonium ion and dissolved carbon dioxide, respectively, while from sulfur part sulfate ion was exclusively detected, when fresh TDO solution was used. The stoichiometry of the reaction was established as 2I<sub>2</sub> + TDO + 4H<sub>2</sub>O → SO<sub>4</sub><sup>2–</sup> + 2NH<sub>4</sub><sup>+</sup> + 4I<sup>–</sup> + CO<sub>2</sub> + 4H<sup>+</sup> indicating a strict 2:1 stoichiometric ratio. However, using aged TDO solution this stoichiometric ratio is shifted to lower values suggesting the formation of elementary sulfur augmented by the 2TDO + I<sub>2</sub> + 4H<sub>2</sub>O → S + SO<sub>4</sub><sup>2–</sup> + 4NH<sub>4</sub><sup>+</sup> + 2I<sup>–</sup> + 2CO<sub>2</sub> hypothetical limiting stoichiometry. We also confirmed experimentally that in aqueous solution TDO slowly rearranges into an unindentified species. This species then produces elementary sulfur at a later stage of the aging process via subsequent reactions accounting for a loss of reducing power. The direct reaction between TDO and iodine was found to be relatively rapid and completed within seconds in absence of initially added iodide ion. Formation of the latter ion, however, strongly inhibits the oxidation process; hence, the system is autoinhibitory with respect to iodide ion. Furthermore, increase of pH markedly accelerates the reaction as well. These observations suggest that a short-lived steady-state intermediate (iodinated TDO) is produced in a rapid pre-equilibrium, where iodide and hydrogen ions are also involved. A nine-step kinetic model, to be able to describe the most important characteristics of the experimental curves with four fitted parameters, is proposed and discussed.