Fixer is the chemical solution used in the fixation stage of traditional photographic processing, applied to both film and photographic paper after development to permanently stabilise the image and make it safe for handling in normal light. It is one of the three fundamental chemicals used in standard black and white processing, alongside developer and stop bath.
Fixer solution is most commonly based on either sodium thiosulphate - historically known as hypo, a term still widely used as an informal name for fixer - or the faster acting ammonium thiosulphate, which is the active ingredient in rapid fixers. Both compounds work by reacting with and dissolving the unexposed silver halide crystals remaining in the emulsion after development, converting them into soluble silver complexes that are subsequently removed from the emulsion during the washing stage. The result is a clean, stable image that will not continue to react to light.
Fixer is available in liquid concentrate or powder form, both of which are diluted with water to produce a working solution. It can be used as a one-shot solution discarded after a single use, or as a replenished bath that is topped up and maintained over multiple processing sessions. The capacity of a fixer solution diminishes with use as it becomes saturated with dissolved silver, and exhausted fixer should be replaced to ensure consistent and reliable fixing times. Spent fixer contains significant concentrations of dissolved silver and should be disposed of responsibly through appropriate silver recovery or chemical waste disposal processes rather than being poured directly down the drain.