A Simple Kinetic Model for Description of the Iodate–Arsenous Acid Reaction: Experimental Evidence of the Direct Reaction

Abstract

The autocatalytic iodate-arsenous acid reaction was investigated by a stopped-flow instrument under strongly acidic medium (pH ≤ 1) by monitoring the absorbance-time profiles at 468 nm. The kinetic traces were found to exhibit a perfect sigmoidal shape in stoichiometric excess of iodate with a well-defined and reproducible induction period that depends on the initial concentration of the reactants as well as on the pH. All the experimental curves can be globally fitted by a simple kinetic model involving the direct reaction between the reactants to produce iodide ion, the Dushman and the Roebuck reactions, and two rapid equilibria. Our measurements along with simultaneous evaluation of the kinetic traces clearly support that indeed the initiation reaction exists at strongly acidic conditions and contributes to the overall kinetics. The measured traces cannot be described adequately by the iodide ion impurity-driven Dushman and Roebuck reactions with assuming no direct reaction at all.

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

“…Mambo and Simoyi also argued that their iodate solution was contaminated with iodide and the concentration of this species may be as high as 4 μM in their experimental conditions. In contrast to this, our study, however, showed that even if the stock iodate solution contained a trace amount of iodide its concentration could not be higher than approximately 0.005% of the iodate concentration, in agreement with our previous study . It thus corresponds to 5 × 10 –8 M, a value that is significantly smaller than the smallest initial iodide concentration used in our experiments.…”

“…In contrast to this, our study, however, showed that even if the stock iodate solution contained a trace amount of iodide its concentration could not be higher than approximately 0.005% of the iodate concentration, in agreement with our previous study. 19 It thus corresponds to 5 × 10 −8 M, a value that is significantly smaller than the smallest initial iodide concentration used in our experiments. Consequently, in our case, the inherent iodide contamination of the iodate solution could not play any decisive role in determining the characteristics of the kinetic curves.…”

“…This feature immediately poses quite important questions on developing a reliable kinetic model. Does the direct reaction between TDO and iodate exist at all, or is the system initiated by trace iodide impurities inherently involved in stock iodate solution? − If there is indeed, then which form of TDO does really react with iodate, the initial one or the aged one or even both? Because aging of TDO solution is a well-documented feature, it is crucial to have reliable information on the rate coefficient of the equation described below under the present experimental conditions …”

Imperfect mixing as a dominant factor leading to stochastic behavior: a new system exhibiting crazy clock behavior

“…To unambiguously answer this question, the pH is set to nearly neutral conditions (pH = 6.51) where the arsenous acid-iodate reaction is vanishingly slow. 12 The reactor was a standard quartz cuvette with a 1 cm pathlength. The stirring process was carried out using a 0.9 cm long stirring bar.…”

“…10 It is therefore expected that a simpler real chemical system may facilitate the study of the theoretical background of the emergence of stochastic behavior. This expectation, along with the fact that the arsenous acid-iodate reaction exhibits crazy-clock behavior (CCB) as reported recently by our research group, 11 has led us to elucidate the comprehensive kinetic model of the arsenous acid-iodate system, 12,13 including the well-known Dushman 14 and Roebuck 15 reactions. Very recently, Pagnacco et al reported 16 that the state I (low iodide and iodine concentration) to state II (high iodide and iodine concentration) transition of the Briggs-Rauscher reaction 17 after leaving the strongly reproducible oscillatory state was found to be irreproducible as well.…”