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

György Csekö,Changwei Pan,Qingyu Gao,Attila K Horváth,Li Xu

doi:10.1021/acs.jpca.9b06207

Abstract

The thiourea dioxide-periodate reaction has been investigated under acidic conditions using phosphate buffer within the pH range of 1.1-2.0 at 1.0 M ionic strength adjusted by sodium perchlorate. Absorbance-time series are monitored as a function of time at 468 nm, the isosbestic point of the I2-I3- system. The profile of these kinetic runs follows either sigmoidal-shaped or rise-and-fall traces depending on the initial concentration ratio of the reactants. The clock species iodine appears after a well-defined but reproducible time lag even in substrate excess, meaning that the system may be classified as an autocatalysis-driven clock reaction. It is also demonstrated that the age of the thiourea dioxide solution markedly shortens the Landolt time, suggesting that the original form of thiourea dioxide (TDO) rearranges into a more reactive form and reacts faster than the original one. The behavior found is consistent with that recently observed in other oxidation reactions of TDO. To characterize the system quantitatively, a 22-step kinetic model is constructed from adapting the kinetic model of the TDO-iodate reaction published recently by supplementing it with six different reactions of periodate. By the help of seven fitted rate coefficients a sound agreement between the measured and calculated absorbance-time traces is obtained.

Highlights

Despite the widespread practical application in the paper and textile industry, the redox transformation of thiourea dioxide (TDO) in aqueous solution is still not fully understood due to its unusual structure and its complex decomposition or rearrangement reactions.[1,2] TDO is fairly stable under acidic conditions; it decomposes relatively rapidly into a sulfoxylate ion and urea via a carbon−sulfur bond cleavage depending on the pH under basic conditions.[3,4] Xu et al have recently demonstrated[5] that TDO slowly rearranges into a more reactive form, whose reactions may be considerably faster than those of the original form of TDO with the corresponding oxidizing agent
Shao et al have arrived at a conclusion supported by TOF-MS spectroscopy and quantum chemical calculations that in aqueous solution TDO exists in cyclic clusters containing various numbers of molecules of TDO and in the form of aminoiminomethanesulfinic acid (NH2NHCSO2H).[10]
These runs indicate remarkable resemblance to the kinetic curves measured in the case of the TDO−iodate reaction.[11,12]

Summary

Introduction

Despite the widespread practical application in the paper and textile industry, the redox transformation of thiourea dioxide (TDO) in aqueous solution is still not fully understood due to its unusual structure and its complex decomposition or rearrangement reactions.[1,2] TDO is fairly stable under acidic conditions; it decomposes relatively rapidly into a sulfoxylate ion and urea via a carbon−sulfur bond cleavage depending on the pH under basic conditions.[3,4] Xu et al have recently demonstrated[5] that TDO slowly rearranges into a more reactive form, whose reactions may be considerably faster than those of the original form of TDO with the corresponding oxidizing agent. 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, leaving no further place for an additional increase in the reaction rate

Methods

Results

Discussion

Conclusion