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Concept
Thiosulfate
Résumé
Thiosulfate (IUPAC-recommended spelling; sometimes thiosulphate in British English) is an oxyanion of sulfur with the chemical formula . Thiosulfate also refers to the compounds containing this anion, which are the salts of thiosulfuric acid, e.g. sodium thiosulfate . Thiosulfate also refers to the esters of thiosulfuric acid, e.g. O,S-dimethyl thiosulfate . The prefix thio- indicates that the thiosulfate is a sulfate with one oxygen replaced by sulfur. Thiosulfate is tetrahedral at the central S atom. Thiosulfate salts occur naturally. Thiosulfate ion has C3v symmetry, 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 salts are mainly used in dying in textiles and the bleaching of natural substances.
Sodium thiosulfate, commonly called hypo (from "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.
Thiosulfate ion is produced by the reaction of sulfite ion with elemental sulfur, and by incomplete oxidation of sulfides (e.g. pyrite oxidation). Sodium thiosulfate can be formed by disproportionation of sulfur dissolving in sodium hydroxide (similar to phosphorus).
Thiosulfate ions are stable only in neutral or alkaline solutions, but not in acidic solutions, due to disproportionation to sulfite ions and sulfur, the sulfite ions being dehydrated to sulfur dioxide:
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 light from above, due to the same mechanisms that color the sky at midday and dusk.
Thiosulfate ions react with iodine to give tetrathionate ions:
This reaction is key for iodometry.
Source officielle
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