Thiosulfate (S2O32−) (IUPAC-recommended spelling; also thiosulphate in British English) is an oxyanion of sulfur. The prefix thio- indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes. It rapidly 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, commonly called hypo ("Hyposulfite"), was widely used in photography to fix black and white negatives and prints after the developing stage; modern 'rapid' fixers use ammonium thiosulfate as a fixing salt because it acts three to four times faster. Some bacteria can metabolise thiosulfates.
- S2O32− (aq) + 2 H+ (aq) → SO2 (g) + S (s) + H2O
This reaction may be used to generate an aqueous suspension of sulfur 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 to the decrease of oxidizing power down the halogen group:
- 2 S2O32− (aq) + I2 (aq) → S4O62− (aq) + 2 I− (aq)
- S2O32− (aq) + 4 Br2 (aq) + 5 H2O(l) → 2 SO42− (aq) + 8 Br− (aq) + 10 H+ (aq)
- S2O32− (aq) + 4 Cl2 (aq) + 5 H2O (l) → 2 SO42− (aq) + 8 Cl− (aq) + 10 H+ (aq)
In acidic conditions, thiosulfate causes rapid corrosion of metals; 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). In alkaline aqueous conditions and medium temperature (60°C), carbon steel and stainless steel (AISI 304L, 316L) are not attacked, even at high concentration of base (30%w KOH), Thiosulfate (10%w) and in presence of Fluoride ion (5%w KF).
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.
The systematic additive IUPAC name is trioxido-1κ3O-disulfate(S—S)(2-), the name of its corresponding acid is dihydroxido-1κ2O-oxido-1κ1O-disulfur(S—S).
See also 
- Sowerby (Ed.), A. L. M. (1961). Dictionary of Photography: A Reference Book for Amateur and Professional Photographers (19th ed.). London: Illife Books Ltd.
- C.Michael Hogan. 2011. Sulfur. Encyclopedia of Earth, eds. A.Jorgensen and C.J.Cleveland, National Council for Science and the environment, Washington DC
- http://www.handbookofmineralogy.org/pdfs/sidpietersite.pdf Mineral Handbook