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Sodium thiosulfate (Na2S2O3) (sometimes spelled thiosulphate) is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”
The thiosulfate anion is tetrahedral in shape and is notionally derived by replacing one of the oxygen atoms by a sulfur atom in a sulfate anion. The S-S distance indicates a single bond, implying that the sulfur bears significant negative charge and the S-O interactions have more double bond character. The first protonation of thiosulfate occurs at sulfur.
Additional recommended knowledge
Industrial production and laboratory synthesis
On an industrial scale, sodium thiosulfate is produced chiefly from liquid waste products of sodium sulfide or sulfur dye manufacture.
Small scale synthesis is by boiling an aqueous solution of sodium sulfite with sulfur.
As such, the anion S2O32− represents a water-soluble form of elemental sulfur.
Principal reactions and applications
Thiosulfate anion characteristically reacts with dilute acids to produce sulfur, sulfur dioxide and water:
This reaction has been employed to generate colloidal sulfur. When the protonation is conducted at low temperatures, H2S2O3 (thiosulfuric acid) can be obtained. It is a strong acid pKa = 0.6, 1.7.
In analytical chemistry, the most important use becomes from the fact thiosulfate anion reacts stoichiometrically with iodine, reducing it to iodide as it is oxidized to tetrathionate:
Due to the quantitative nature of this reaction, as well as the fact that Na2S2O3•5H2O has an excellent shelf-life, it is used as a titrant in iodometry. Na2S2O3•5H2O is also a component of iodine clock experiments.
This particular use can be set up to measure the oxygen content of water through a long series of reactions. It is also used in estimating volumetrically, the concentrations of certain compounds in solution (hydrogen peroxide, for instance), and in estimating the chlorine content in commercial bleaching powder and water.
The terminal sulfur atom in S2O32− binds to soft metals with high affinity. Thus it dissolves silver halides, e.g. AgBr, which is a component of photographic emulsions:
In this application to photographic processing, discovered by John Herschel and used for both film and paper processing, sodium thiosulfate is known as a photographic fixer.
Sodium thiosulfate is one component of an alternative lixiviant to cyanide for extraction of gold. It forms a strong complex with gold(I) ions, [Au(S2O3)2]3-. The advantage of this approach is that thiosulfate is essentially non-toxic and that ore types that are refractory to gold cyanidation (e.g. carbonaceous or Carlin type ores) can be leached by thiosulfate. Some problems with this alternative process include the high consumption of thiosulfate, and the lack of a suitable recovery technique, since [Au(S2O3)2]3- does not adsorb to activated carbon, which is the standard technique used in gold cyanidation to separate the gold complex from the ore slurry.
Sodium thiosulfate is also used:
Categories: Thiosulfates | Sodium compounds | Photographic chemicals
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Sodium_thiosulfate". A list of authors is available in Wikipedia.|