Editing Photographic fixer (section)

== Chemistry ==
Fixation is commonly achieved by treating the film or paper with a solution of [[thiosulfate]] salt. Popular salts are [[sodium thiosulfate]]—commonly called hypo—and [[ammonium thiosulfate]]—commonly used in modern rapid fixer formulae.<ref name="Sowerby">{{cite book |title=Dictionary of Photography: A Reference Book for Amateur and Professional Photographers |publisher=Illife Books Limited |year=1961 |editor-last=Sowerby |editor-first=A. L. M. |location=London, England |pages=324–326}}</ref> 
Fixation by thiosulfate involves these chemical reactions (X = [[halide]], typically Br<sup>−</sup>):<ref name="Ullmann">Karlheinz Keller et al. "Photography" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. {{doi|10.1002/14356007.a20_001}}.</ref>
:AgX + 2 S<sub>2</sub>O<sub>3</sub><sup>2−</sup> → [Ag(S<sub>2</sub>O<sub>3</sub>)<sub>2</sub>]<sup>3−</sup> + X<sup>−</sup>
:AgX + 3 S<sub>2</sub>O<sub>3</sub><sup>2−</sup> → [Ag(S<sub>2</sub>O<sub>3</sub>)<sub>3</sub>]<sup>5−</sup> + X<sup>−</sup>

In addition to thiosulphate the fixer typically contains mildly acidic compounds to adjust the pH and suppress trace amounts of the [[Photographic developer|developer]]. This compound is often an alkali hydrogen sulfite ([[bisulfite]]) which also serves to preserve the thiosulphate. Less commonly it may also contain other additives e.g. for the hardening of gelatin. There are also non-thiosulphate fixers, at least for special purposes. Fixer is used for processing all commonly used films, including black-and-white films, [[Kodachrome]], and [[chromogenic]] films.