Comprehensive Simultaneous Kinetic Study of Sulfitolysis and Thiosulfatolysis of Tetrathionate Ion: Unravelling the Unique pH Dependence of Thiosulfatolysis

Abstract

The kinetics of the reactions of tetrathionate with S(IV) species and with thiosulfate in slightly acidic and neutral media were studied concurrently at 25.0 ± 0.1 °C by simultaneous high-performance liquid chromatography monitoring of the concentrations of polythionates (including trithionate, tetrathionate, and pentathionate), thiosulfate, and sulfite. The tetrathionate-sulfite and tetrathionate-thiosulfate reactions were found to be first-order with respect to both reactants. The tetrathionate-sulfite reaction was found to be pH-dependent under the conditions studied. In contrast, the tetrathionate-thiosulfate reaction was experimentally demonstrated to be pH-independent at neutral medium, where the pKa2 value of sulfurous acid plays a key role, whereas under slightly acidic conditions, between pH 4 and 5 the consumption of tetrathionate during the course of reaction was found to become pH-dependent. We show that the pH dependencies in both systems can be readily explained by the reactivity difference between sulfite and bisulfite toward the β-sulfur of the tetrathionate. A simple two-step kinetic model incorporating the protonation equilibrium of sulfite is proposed on the basis of the simultaneous evaluation of the kinetic curves of the two systems, which allowed us to determine reliable rate coefficients for both the forward and backward reactions. Furthermore, the powerful ability of simultaneously evaluating the two chemical systems to yield reliable rate coefficients of the kinetic model is demonstrated.

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

“…Varga and Horváth measured the rate coefficient of this reaction as (1.94 ± 0.08) × 10 –4 m –1 s –1 . In our previous work on the thiosulfatolysis of tetrathionate, we obtained a value of (3.01 ± 0.13) × 10 –4 m –1 s –1 for the rate of the reaction . Here, we obtained a fitted k –M2 value of (2.01 ± 0.12) × 10 –3 m –1 s –1 , which gave a better fit to our experimental curves, although the fitted rate coefficient is slightly higher than the average value reported previously.…”

“…Reaction (M7) is the reversible sulfitolysis of tetrathionate and the thiosulfatolysis of trithionate, which was previously studied in detail . We used k M7 = (1.05 ± 0.15) m –1 s –1 and k –M7 = (3.01 ± 0.05) × 10 –5 m –1 s –1 , which is consistent with the value used previously.…”

“…The model that we first used to fit our experimental data was similar to that for tetrathionate proposed in our previous work, only with the addition of the reversible step of thiosulfatolysis of pentathionate and sulfitolysis of hexathionate. However, it was not sufficient to describe our measured kinetic data for the decrease of hexathionate concentration.…”

“…In the forward Reaction (M2), the sulfite generated from Reaction (M1) reacts with the main reagent pentathionate to produce tetrathionate and thiosulfate. The constant k M2 is kept at the same value as that used in our previous work, that is, 101 m –1 s –1 . The nucleophilic attack of the SO 3 2– ions at the γ‐S atom rather than the β‐ or β′‐S atom of pentathionate to produce lower polythionates (tetrathionate and trithionate) has been reviewed by Wagner et al, and 35 S‐labelled polythionates were synthesized.…”

“…Thiosulfate can catalyze the rearrangement of polythionates . In our recent study of the sulfitolysis and thiosulfatolysis of tetrathionate, we presented a general mechanism for these two processes . However, in pentathionate or higher polythionates, inequivalent divalent sulfur atoms (β, γ, etc.)…”