Autocatalysis-Driven Clock Reaction III: Clarifying the Kinetics and Mechanism of the Thiourea Dioxide–Iodate Reaction in an Acidic Medium

Abstract

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.

Autoinhibition by Iodide Ion in the Methionine–Iodine Reaction

“…The ratio of these rate coefficients provides the p K a1 of phosphoric acid to be 1.8 . This value worked consistently well in a number of previous systems investigated. − …”

On the Kinetics and Mechanism of the Thiourea Dioxide–Periodate Autocatalysis-Driven Iodine-Clock Reaction

“…The pH of the reacting solutions was regulated between 1.1 and 2.0 by phosphoric acid/dihydrogen phosphate buffer taking the p K a1 of phosphoric acid as 1.8 . This value worked consistently well in a number of previous studies. ,,, The dihydrogen phosphate concentration was kept constant at 0.25 M. The temperature of the reaction vessel was maintained at 25.0 ± 0.1 o C.…”

“…The apparent irreproducibility of the kinetic curves is also highlighted in the TDO–iodine reaction when a fresh or an aged TDO solution is used to perform the experiments . The TDO–iodate system was shown to display a clock feature first by Mambo and Simoyi, though the authors failed to report that the age of the stock TDO solution significantly affects the Landolt time . In contrast to this, in the case of the TDO–bromate reaction, no aging effect was observed as reported by Csekö et al The lack of aging effect was explained by the fact that the key TDO–bromine reaction is so rapid in the presence of fresh TDO that it hits almost the diffusion control limit, thus leaving no further place for an additional increase in the reaction rate.…”

Kinetics on the Oxidation of Aminoiminomethanesulfonic Acid by Hypochlorous Acid: A Novel Product in the Chlorination of Aminoiminomethanesulfonic Acid

“…Moreover, the formation and stability of the oxidation product were simply discussed. The new findings in this work not only contribute to the understanding of AIMSOA oxidation but also provide insight into the oxidation mechanism of thiourea and thiourea dioxide when halogen‐containing species are used as oxidants [29–31] …”