Darkroom chemical solution :

FIXER
In black and white photography, photographic film and paper both contain tiny silver halide crystals in a layer of emulsion. If the silver ion in the halide is exposed to light then it is converted from an ion into metallic silver. Visually there is no difference at this stage between the light-exposed and unexposed parts of the film/paper. When a liquid developer (such as amidol) is applied, then the metallic silver is reduced, turning the silver into metallic crystals that make up the dark areas of the exposed film or paper. A solution known as a “fixer” is then used to remove unexposed silver halide, hence rendering the image insensitive to further action by light.