Thiosulfate
The thiosulfates are the salts of the hypothetical acid H2S2O3.
The chemical formula of thiosulfate anion is S2O32-. They are stable in media with basic and neutral pH and decompose under the formation of elemental sulfur, hydrogen sulfide (H2S), sulfur oxide (IV) (SO2) and traces of other sulfur compounds in the presence of an acid. Its structure is tetrahedral with a central sulfur atom surrounded by three oxygen atoms and another sulfur atom. Thiosulfates are easily oxidized. They can be formed with mild oxidants such as elemental iodine tetrathionates (O3S-S-S-SO32-). The prefix thio indicates the presence of a sulfur atom which analogously corresponds to an oxygen atom in sulfates.
Application
They are mainly used as fixatives in photography where they form soluble complexes (Na3[Ag(S2O3))2]) with the silver salts that can be removed from the photosensitive layer in this way. It is also used to eliminate chlorine from solutions, or as a bleach in different industries, such as ivory, the manufacture of paper pulp or in the leather industry; and in the textile industry it is used as a color fixer and as a dye. In pharmaceutical laboratories it is used as a reagent or volumetric solution (with a normality of 0.10584) for quantitative analysis or valuations. In the medical field, it is applied as a treatment against certain parasitic worms; and its use as an antidote for cyanide poisoning is known. It is widely used in silver extraction mines.
Summary
Sodium thiosulfate is obtained by refluxing a solution of sodium sulfite (Na2SO3) with elemental sulfur. The solution is filtered, concentrated and cooled, obtaining sodium thiosulfate pentahydrate (Na2S2O3 5 H2 O) in the form of colorless prisms. Mechanistically, it is a nucleophilic attack of the free electron pair of sulfite sulfur on an elemental sulfur atom, similar to the formation of polysulfides and disulfides from sulfides.