Photographic fixer

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Photographic fixer is a chemical used in the final step in the photographic processing of film or paper. The fixer removes the unexposed silver halide remaining on the negative or photographic paper, leaving behind the reduced metallic silver that forms the image. By removing the unexposed silver halide, the fixer prevents any further reaction of the silver salts and ensures a permanent image. If the film or paper was not fixed, the remaining silver halide would quickly darken and cause severe fogging of the image. The film or paper can be exposed to light after fixing.

Fixer is used for processing all commonly used films, including black and white films, colour negative films (C41), colour reversal films (E6), and chromogenic films.

Most fixers are based on the thiosulfate ion, especially ammonium thiosulfate. Up until the 1970s, sodium thiosulfate or 'hypo' was the commonly used fixer. Both fixers work best in acid conditions and this is usually created using small quantities of acetic acid.

In the case of colour films, the elemental silver left behind after development must be subsequently removed by solution in a chemical cocktail called a Blix or bleach-fix. This contains a mixture of ammonium thiosulfate and ferrous EDTA, a powerful chelating agent.

One disadvantage of the use of thiosulfate as a fixer is its ability to dissolve elemental silver at a very slow rate. If films or papers are inadequately washed after fixing, any residual fixer can slowly bleach or stain the photographic image. For prints on high grade fibre papers, a period of continuous washing in clean, cold water for up to 40 minutes may be required. For modern plastic (resin) coated papers, washing for as little as 2 minutes in warm water can be sufficient to eliminate residual fixer.