Abstract
Reactions of hexathionate with thiosulfate and sulfite have been investigated by high-performance liquid chromatography via monitoring the concentration-time series of tetrathionate, pentathionate, hexathionate, and thiosulfate simultaneously within the pH range of 4.0-5.0. In both reactions, elementary sulfur forms; more significant sulfur precipitation may be observed in the case of the hexathionate-thiosulfate reaction, but slight turbidity in the other system means that elementary sulfur also appears in a detectable amount in the hexathionate-sulfite reaction. Initial rate studies have revealed that the formal kinetic orders of both reactants in both systems are clearly unity but pH-dependence can only be observed in the case of the hexathionate-sulfite reaction. The proposed kinetic model appears to suggest that nucleophilic attack of sulfite and thiosulfate may also occur on the β- or γ-sulfur of the polythionate chain and breakages of the α-β, β-γ, and γ-γ' bonds are all conceivable possibilities to drive the reactions. Consequently, the generally accepted sulfur-chain elongating effect of thiosulfate on longer polythionates is also proven to be accompanied by sulfur-chain shortening pathways, eventually leading to the formation of elementary sulfur.
Highlights
IntroductionPolythionates are well-known long-lived intermediates produced from the oxidation of thiosulfate by different oxidants, leading eventually to sulfate ion.[1,2] Their further reactions with the corresponding oxidants sometimes display fascinating kinetics[3] and may contribute to the appearance of a rich variety of exotic nonlinear dynamical phenomena.[4−9] Hexathionate, being one of the important intermediates in redox transformations or metabolism of sulfur-containing compounds in various environmental, industrial, and biological systems,[10,11] is often reported to be produced in numerous oxidation processes of thiosulfate.[1,2] It is well-known that hexathionate is unstable in weakly alkaline conditions and degradation of the sulfur−sulfur bond eventually leads mainly to the formation of elemental sulfur and thiosulfate.[12]Compared to tetrathionate and pentathionate, significantly less information is available in the literature about redox transformation of hexathionate
In the case of the thiosulfatolysis of hexathionate because of the presence of γ and γ′-sulfur it is expected that this feature becomes more profound at the beginning stage, which may lead to the appearance of significant amounts of sulfur precipitation
It is generally accepted that thiosulfatolysis of polythionates leads eventually to the formation of higher polythionates, whereas sulfitolysis results in the formation of lower polythionates in a slightly acidic medium
Summary
Polythionates are well-known long-lived intermediates produced from the oxidation of thiosulfate by different oxidants, leading eventually to sulfate ion.[1,2] Their further reactions with the corresponding oxidants sometimes display fascinating kinetics[3] and may contribute to the appearance of a rich variety of exotic nonlinear dynamical phenomena.[4−9] Hexathionate, being one of the important intermediates in redox transformations or metabolism of sulfur-containing compounds in various environmental, industrial, and biological systems,[10,11] is often reported to be produced in numerous oxidation processes of thiosulfate.[1,2] It is well-known that hexathionate is unstable in weakly alkaline conditions and degradation of the sulfur−sulfur bond eventually leads mainly to the formation of elemental sulfur and thiosulfate.[12]Compared to tetrathionate and pentathionate, significantly less information is available in the literature about redox transformation of hexathionate. Hexathionate can readily be degraded by the attack of nucleophilic reagents such as cyanide, sulfide, and sulfite ions via heterolytic cleavage of sulfur−sulfur bonds These reactions are generally offered for quantitative determination of hexathionate.[13,14] The sulfur chain of polythionates may as well be attacked by thiosulfate;[12,15−21] decomposition of polythionates is usually accompanied by catalytic rearrangement of polythionates even in a nearly neutral medium.[19,22−24] Heterolysis selectivity of the divalent sulfur−sulfur bond plays a key role in modulating the distribution of sulfur species in the case of thiosulfatolysis and sulfitolysis of pentathionate as the attack of thiosulfate or sulfite may take place on the inequivalent divalent sulfur atoms of hexathionate formed.[25]
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