A Possible Candidate to Be Classified as an Autocatalysis-Driven Clock Reaction: Kinetics of the Pentathionate–Iodate Reaction

Abstract

The pentathionate-iodate reaction has been investigated by spectrophotometrically monitoring the formation of the total amount of iodine at 468 nm in the presence of phosphoric acid/dihydrogen phosphate buffer. We noticed that iodine forms only after a fairly long time lag, and the inverse of time necessary to produce a certain amount of iodine is linearly proportional to the initial concentration of iodate ion and the square of the hydrogen ion concentration, while depending complexly on the concentration of substrate pentathionate. This reaction can therefore be treated as a clock reaction but differs from the original Landolt reaction in the sense that substrate pentathionate and the clock species iodine coexist for a relatively long time--due to their relatively slow direct reaction--depending on the experimental circumstances. Furthermore, we also provided experimental evidence that iodide ion acts as an autocatalyst of the system. A 14-step kinetic model is proposed in which the mechanisms of the pentathionate-iodine, bisulfite-iodate, and the well-known Dushman reactions are combined. A thorough analysis revealed that the direct pentathionate-iodate reaction plays a role only to produce iodide ions via a finite sequence of reactions, and once its concentration reaches a certain level, the reaction is almost exclusively governed by the pentathionate-iodine and the Dushman reactions. As expected, a strong catalytic effect of the buffer composition is also found that can readily be explained by its well-known catalytic influence on the original Dushman reaction.

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. − …”

The Key Heterolysis Selectivity of Divalent Sulfur–Sulfur Bonds for a Unified Mechanistic Scheme in the Thiosulfatolysis and Sulfitolysis of the Pentathionate Ion

“…Moreover, the iodide ion produced by the reaction strongly inhibits the oxidation. The oxidation of pentathionate by periodate is an iodide autocatalysis‐driven clock reaction, as the iodine clock species appears after a well‐defined reproducible time lag that depends on the initial concentration range of the reactants . In the pentathionate–chlorine dioxide reaction, two limiting stoichiometries exist, and more complex kinetics can be displayed with increasing sulfur chains.…”

Antibacterial activities of polythionates enhanced by carbonates

“…36 Furthermore, the formation of sulfur species does occur not only during the decomposition of polythionate but surprisingly in the case of the oxidation of pentathionate by mild oxidants as well. 41,42 This case also appeared in the MIC test for both hexathionate and pentathionate. Furthermore, the pH of the solution of 1000 μg mL −1 potassium hexathionate decreased from 6-7 to about 3-4 after the solution was aged for 24 hours at 37 °C.…”

“…The MIC values of potassium hexathionate against P. aeruginosa and S. aureus are 1.95 and 125 μg mL −1 , respectively. 41,42 This case also appeared in the MIC test for both hexathionate and pentathionate. Furthermore, for other pure potassium polythionates, only potassium pentathionate displayed a weak antibacterial effect, that is, the MIC value of 62.5 μg mL −1 against P. aeruginosa.…”