Photographic fixer is a mixture of chemicals used in the final step in the

photographic processing Photographic processing or photographic development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image i ...

of film or paper. The fixer stabilises the image, removing the unexposed

silver halide A silver halide (or silver salt) is one of the chemical compounds that can form between the Chemical element, element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine wit ...

remaining on the

photographic film Photographic film is a strip or sheet of transparent film base coated on one side with a gelatin photographic emulsion, emulsion containing microscopically small light-sensitive silver halide crystals. The sizes and other characteristics of the ...

photographic paper Photographic paper is a coated paper, paper coated with a light-sensitive chemical, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then Photographic developer, developed to form ...

, leaving behind the reduced metallic silver that forms the image. By fixation, the film or paper is insensitive to further action by light. Without fixing, the remaining silver halide would darken and cause fogging of the image.

Chemistry

Fixation is commonly achieved by treating the film or paper with a solution of

thiosulfate 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, ...

salt. Popular salts are sodium thiosulfate—commonly called hypo—and

ammonium thiosulfate Ammonium thiosulfate (ammonium thiosulphate in British English) is an inorganic compound with the formula . It is white crystalline solid with ammonia odor, readily soluble in water, slightly soluble in acetone and insoluble in ethanol and diethy ...

—commonly used in modern rapid fixer formulae. Fixation by thiosulfate involves these chemical reactions (X =

halide In chemistry, a halide (rarely halogenide) is a binary chemical compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative (or more electropositive) than the halogen, to make a fl ...

, typically Br⁻):Karlheinz Keller et al. "Photography" in Ullmann's Encyclopedia of Industrial Chemistry, 2005, Wiley-VCH, Weinheim. . :AgX + 2 S₂O₃²⁻ → g(S₂O₃)₂sup>3− + X⁻ :AgX + 3 S₂O₃²⁻ → g(S₂O₃)₃sup>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 Kodachrome is the brand name for a color reversal film introduced by Eastman Kodak in 1935. It was one of the first successful color materials and was used for both cinematography and still photography. For many years, Kodachrome was widely used ...

, and chromogenic films.

Chromogenic films

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 Ethylenediaminetetraacetic acid (EDTA), also called EDTA acid, is an aminopolycarboxylic acid with the formula . This white, slightly water-soluble solid is widely used to bind to iron (Fe2+/Fe3+) and calcium ions (Ca2+), forming water-solubl ...

, a powerful chelating agent. The fixing agent reduces the silver which is then dissolved by the chelating agent.

Washing and stabilisation

After fixation,

washing Washing is a method of cleaning, usually with water and soap or detergent. Regularly washing and then rinsing both body and clothing is an essential part of good hygiene and health. Often people use soaps and detergents to assist in the emulsific ...

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

Photographic chemicals Photographic processes {{filmmaking-stub