Kinetics Of The Thiosulfatolysis Of Tetrathionate And The Sulfitolysis Of Pentathionate

The kinetics of reactions of thiosulfatolysis of tetrathionate and sulfitolysis of pentathionate were studied concurrently at 25 ± 0.1 ℃ by simultaneous high-performance liquid chromatograph online tracking of polythionates（including tirthionate,tetrathionate,pentathionate and hexathionate）,thiosulfate and sulfite.The tetrathionate-thiosulfate reaction system was studied in the phosphate buffer solution（pH 6.70 and 7.30）,the acatate buffer solution（pH 4.70 and 5.60）and the citrate buffer soluton（pH 2.70,3.00,3.30,3.60 and 3.90）.The reactions were found to be first-order with respect to the reactants of tetrathionate and thiosulfate.At the pH 6.70,7.30,4.70 and 5.60,the tetrathionate-thiosulfate reaction was not affected by the pH of the reaction solution.Thiosulfate acts as a catalyst and the concentration of thiosulfate remained essentially uncharged.In an acidic solution with p H = 2.70 3.90,the concentration of sulfite was decreased due to the protonation of sulfite.So the consumption of tetrathionate and thiosulfate in the reaction begins to depend on the change in pH.The consumption of tetrathionate and thiosulfate were increased.The pH dependence during the reaction can be explained by the reactivity of sulfite and bisulfite with the β-S atom of tetrathionate.The pH dependence is derived from the reactivity of sulfite and bisulfite with the β-S atom of tetrathionate.The pentathionate-sulfite reaction system was studied in the acatate buffer solution（pH 4.00,4.25,4.50,4.75 and 5.00）.This reaction was found to be first-order with respect to the pentathionate and sulfite.The concentration of the sulfite was increased with the increase of the pH value,which can accelerate the reaction rate of the sulfitolysis of pentathionate ion.When [S₅O₆^2-]0 / [SO₃^2-]0 ≥ 1,the intermediate hexathionate can be detected during the reaction.The sulfur chain of pentathionate contains polarity divalent sulfur atoms.So the cleavage of sulfur-sulfur bond has selective.Therefore,the complexity of the reaction mechanism increases.In combination with the kinetics of thiosulfatolysis of tetrathionate and sulfitolysis of pentathionate,a five-step reaction mechanism model was proposed.The reaction kinetics curves were fitted at the same time.The fitting results were consistent with the experiment.And the rate constant of each reaction was determined.