Photographic fixer is a chemical, or mixture of chemicals, used as the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed light-sensitive material from the media, and making it insensitive to further action by light.
In the gelatin silver process, the basis for all film photography, exposed silver halides are reduced to metallic silver by the developer, forming the image. Once development is complete, remaining silver halides are removed from the film or photographic paper by the fixer, typically sodium or ammonium thiosulfate. Without fixing, the remaining silver halide would darken and cause fogging of the image.
Chemistry
[edit]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.[1] Fixation by thiosulfate involves these chemical reactions (X = halide, typically Br−):[2]
- AgX + 2 S2O32− → [Ag(S2O3)2]3− + X−
- AgX + 3 S2O32− → [Ag(S2O3)3]5− + X−
In addition to thiosulphate the fixer typically contains mildly acidic compounds to adjust the pH and suppress trace amounts of the 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.
Chromogenic films
[edit]In chromogenic films, the remaining silver must be removed by a chemical mixture called a bleach fix, sometimes shortened to blix. This mixture contains ammonium thiosulfate and ferric EDTA, a powerful chelating agent. The fixing agent reduces the silver which is then dissolved by the chelating agent.
Washing and stabilisation
[edit]After fixation, washing is important to remove the exhausted chemicals from the emulsion. Otherwise they cause image deterioration. Other treatments of the remaining silver-based image are sometimes used to prevent "burning".
References
[edit]- ^ Sowerby, A. L. M., ed. (1961). Dictionary of Photography: A Reference Book for Amateur and Professional Photographers. London, England: Illife Books Limited. pp. 324–326.
- ^ Karlheinz Keller et al. "Photography" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. doi:10.1002/14356007.a20_001.
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