The thiourea dioxide−iodate reaction has been reinvestigated spectrophotometrically under acidic conditions using phosphoric acid−dihydrogen phosphate buffer within the pH range of 1.1−1.8 at 1.0 M ionic strength adjusted by sodium perchlorate and at 25 °C. The system was found to exhibit clock behavior, having a well-defined and reproducible time lag called Landolt time, though elementary iodine may even be detected in substrate excess; hence, under these conditions, the reaction can be classified as an autocatalysis-driven clock reaction. It is clearly demonstrated that the previously proposed kinetic model suffers from serious drawbacks from both theoretical and experimental points of view. The reaction may be characterized by either sigmoidal-shaped or rise-and-fall kinetic traces, depending on the initial concentration ratio of the reactants. Iodide significantly accelerates the appearance of the clock species iodine acting therefore as an autocatalyst. The age of stock TDO solution also has a great, so far completely overlooked impact on the Landolt time. On the basis of evaluating simultaneously the kinetic curves, a 16 step kinetic model including 5 well-known rapidly established equilibria is proposed with 7 fitted rate coefficients in which the rate coefficients of both forms of TDO were determined.
The trithionate-iodine reaction has been studied spectrophotometrically in a slightly acidic medium at 25.0 +/- 0.1 degrees C in acetate/acetic acid buffer monitoring the absorbance at 468 nm at the isosbestic point of the iodine-triiodide ion system in the absence and presence of initially added iodide ion at I = 0.5 M ionic strength adjusted by sodium perchlorate. The stoichiometry of the reaction was found to be S(3)O(6)(2-) + 4I(2) + 6H(2)O --> 3SO(4)(2-) + 8I(-) + 12H(+). It is also shown that the consumption of iodine is inhibited by iodide ion, but it cannot be simply explained via the fast equilibrium formation of triiodide ion. A five-step kinetic model with four fitted and fixed kinetic parameters is proposed and discussed in detail, on the basis of which all the most important characteristics of the measured kinetic curves are adequately explained.
Indirect experimental evidence of the existence of a sluggish tautomerism of thiourea dioxide in acidic aqueous solution was discovered while studying its oxidation by chlorine dioxide. Thiourea dioxide slowly rearranges into aminoiminomethanesulfinic acid, but the reactivity of these two species towards chlorine dioxide marginally differs. It was clearly demonstrated that thiourea dioxide reacts with chlorine dioxide in a relatively slow reaction in an autocatalytic fashion with respect to the carbon(IV) species. In contrast, reaction of aminoiminomethanesulfinic acid with chlorine dioxide is orders of magnitude faster but proceeds through a normal second‐order process.
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