Thiosulfate
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.[1] Some bacteria can metabolise thiosulfates.[2]
Formation
Thiosulfate is produced by the reaction of sulfite ion with elemental sulfur, by incomplete oxidation of sulfides (pyrite oxidation), or by partial reduction of sulfate (Kraft paper).
Reactions
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) + 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 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 alcaline acqueous contitions 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,[3] as the presence of this anion in the mineral bazhenovite was recently disputed.[4]
Nomenclature
The systematic additive IUPAC name is trioxidosulfidosulfate(2-), the name of its corresponding acid is dihydroxidooxidosulfidosulfur.
See also
References
- ↑ Script error
- ↑ 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
- ↑ http://www.minsocam.org/msa/AmMin/Toc/Abstracts/2005_Abstracts/Oct05_Abstracts/Bindi_p1556_05.pdf
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