Photographic fixer is a chemical or a mix of chemicals used in the final step in the photographic processing

Photographic processing

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

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

Silver halide

A silver halide is one of the compounds formed between silver and one of the halogens — silver bromide , chloride , iodide , and three forms of silver fluorides. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX...

 remaining on the photographic film

Photographic film

Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...

 or photographic paper

Photographic paper

Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints.Photographic paper is exposed to light in a controlled manner, either by placing a negative in contact with the paper directly to produce a contact print, by using an enlarger in order to create a...

, leaving behind the reduced metallic silver that forms the image, making it insensitive to further action by light. Without fixing, the remaining silver halide would quickly darken and cause fogging

Fogging

Fogging may refer to:*Distance fog, a technique used in 3D computer graphics*Fogging , adverse impacts in photography*Fogging , a technique for visual censorship...

 of the image. The most common salts used are sodium thiosulfate

Sodium thiosulfate

Sodium thiosulfate , also spelled sodium thiosulphate, is a colorless crystalline compound that is more familiar as the pentahydrate, Na2S2O3•5H2O, an efflorescent, monoclinic crystalline substance also called sodium hyposulfite or “hypo.”...

 — commonly called hypo — and ammonium thiosulfate

Thiosulfate

Thiosulfate is an oxyanion of sulfur. The prefix thio indicates that thiosulfate ion is a sulfate ion with one oxygen replaced by a sulfur. Thiosulfate occurs naturally and is produced by certain biochemical processes...

 — commonly used in modern rapid fixer formulae.

Fixer is used for processing all commonly used films, including black-and-white films, Kodachrome

Kodachrome

Kodachrome is the trademarked brand name of a type of color reversal film that was manufactured by Eastman Kodak from 1935 to 2009.-Background:...

, and chromogenic

Chromogenic

Chromogenic refers to color photographic processes in which a traditional silver image is first formed, and then later replaced with a colored dye image.- Description :...

 films. In chromogenic

Chromogenic

Chromogenic refers to color photographic processes in which a traditional silver image is first formed, and then later replaced with a colored dye image.- Description :...

 films, the remaining silver must be removed by a chemical called a bleach fix, sometimes shortened to blix. This contains a mixture of ammonium thiosulphate and ferrous EDTA

EDTA

Ethylenediaminetetraacetic acid, widely abbreviated as EDTA , is a polyamino carboxylic acid and a colourless, water-soluble solid. Its conjugate base is named ethylenediaminetetraacetate. It is widely used to dissolve limescale. Its usefulness arises because of its role as a hexadentate ligand...

, a powerful chelating agent.

After fixation, washing

Washing (photography)

In photography, washing is an important part of all film processing and printmaking processes. After materials have been fixed, washing removes unwanted and exhausted processing chemicals which, if left in situ, may cause deterioration and destruction of the image.A disadvantage of the use of...

is important to remove the exhausted chemicals from the emulsion, which cause image deterioration if left in place.