Thiosulfate (S2O32−) is an oxyanion of sulfur produced by the reaction of sulfite ions with elemental sulfur in boiling water. Thiosulfate occurs naturally in hot springs and geysers, and is produced by certain biochemical processes. It instantly dechlorinates water, and is notable for its use to halt bleaching in the paper-making industry. Thiosulfate is also useful in smelting silver ore, in producing leather goods, and to set dyes in textiles. Sodium thiosulfate was widely used to fix black and white photography negatives after the developing stage.

Thiosulfates are stable only in neutral or alkaline solutions, but not in acidic solutions, due to decomposition to sulfite and sulfur, the sulfite being dehydrated to sulfur dioxide:

S2O32−(aq) + 2H+(aq) → SO2(g) + S(s) + H2O

This reaction may be used to generate a temporary aqueous suspension of particles and demonstrate the Rayleigh scattering of light in physics. If white light is shone from below, blue light is seen from sideways and orange from above, due to the same mechanisms that color the sky at mid-day and dusk.

Thiosulfates react with halogens differently, which can be attributed the decrease of oxidizing power down the halogen group:

2S2O32−(aq) + I2(aq) → S4O62−(aq) + 2I(aq)

S2O32−(aq) + 4Br2(aq) + 5H2O(l) → 2SO42−(aq) + 8Br(aq) + 10H+(aq)

S2O32−(aq) + 4Cl2(aq) + 5H2O(l) → 2SO42−(aq) + 8Cl(aq) + 10H+(aq)

Uses of thiosulfates include the dissolving of silver salts from photographic film. In this context, sodium thiosulfate is often referred to as hypo.

Thiosulfate is also a very aggressive species for the metal corrosion. Steel and stainless steel are particularly sensitive to pitting corrosion induced by thiosulfate. Addition of molybdenum to stainless steel is needed to improve its resistance to pitting (AISI 316L hMo). Thiosulfates are also often produced by incomplete oxidation of sulfides (pyrite oxidation) or partial reduction of sulfate (Kraft paper).

The natural occurrence of the thiosulfate group is practically restricted to a very rare mineral sidpietersite, Pb4(S2O3)O2(OH)2, as the presence of this anion in the mineral bazhenovite was recently disputed.


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