The thiosulfatolysis and sulfitolysis of pentathionate were studied in the pH ranges 4.50-8.0 and 4.0-5.0 at T = (25.0 ± 0.1)°C by HPLC monitoring of the time-dependence and distribution of different sulfur species. The two systems are first order with respect to both reactants. In contrast to tetrathionate, pentathionate and higher polythionates contain inequivalent divalent sulfur atoms; therefore, the complexity of the mechanism increases. The selective heterolytic cleavages of [a]
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.
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