Xu Li, Valkai László, Kuznetsova Alena A, Makarov Sergei V, Horváth Attila K
Department of Inorganic Chemistry, University of Pécs , Ifjúság útja 6, Pécs H-7624, Hungary.
Department of Food Chemistry, State University of Chemical Technology , 7 Sheremetevsky Avenue, Ivanovo 153000, Russia.
Inorg Chem. 2017 Apr 17;56(8):4680-4688. doi: 10.1021/acs.inorgchem.7b00326. Epub 2017 Mar 24.
The thiourea dioxide (TDO)-iodine reaction was investigated spectrophotometrically monitoring the consumption of total amount of iodine at 468 nm, at T = 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 + TDO + 4HO → SO + 2NH + 4I + CO + 4H 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 + 4HO → S + SO + 4NH + 2I + 2CO 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.
在25.0±0.1°C的温度、0.5M离子强度的缓冲微酸性介质中，通过分光光度法监测468nm处碘总量的消耗，对二氧化硫脲（TDO）-碘反应进行了研究。当使用新鲜的TDO溶液时，发现含氮和含碳产物分别为铵离子和溶解的二氧化碳，而从硫部分仅检测到硫酸根离子。反应的化学计量关系确定为2I + TDO + 4HO → SO + 2NH + 4I + CO + 4H，表明化学计量比严格为2:1。然而，使用老化的TDO溶液时，该化学计量比会向较低值偏移，这表明通过2TDO + I + 4HO → S + SO + 4NH + 2I + 2CO这一假设的极限化学计量关系，单质硫的形成增加。我们还通过实验证实，在水溶液中TDO会缓慢重排为一种未鉴定的物质。该物质随后在老化过程的后期通过后续反应产生单质硫，这导致了还原能力的损失。发现TDO与碘之间的直接反应相对较快，在没有初始添加碘离子的情况下，几秒钟内即可完成。然而，后者离子的形成会强烈抑制氧化过程；因此，该体系对碘离子具有自抑制作用。此外，pH值的升高也会显著加速反应。这些观察结果表明，在一个快速的预平衡中会产生一种短寿命的稳态中间体（碘化TDO），其中碘离子和氢离子也参与其中。提出并讨论了一个九步动力学模型，该模型能够用四个拟合参数描述实验曲线的最重要特征。
