The removal of unexposed silver halides from the film during processing (1983).
Fixation consists in the removal of the silver halide which has not been used in the formation of the image.
While there are a number of solvents of the silver halide, only one, sodium thiosulfate or hypo, is in common use for fixing purposes. In the process of fixation with hypo, thiosulfate ions combine with silver ions to form various soluble complex ions, although it will dissolve silver halide, a solution of hypo does not form a satisfactory fixing bath, as the film carries over into it a certain amount of developing solution, which oxidises, and the coloured oxidation products stain the gelatin.
This may be prevented by adding to the fixing bath a substance, such as sodium sulfite, which prevents the oxidation of the developing solution in the fixing bath.
The addition of an acid neutralises the alkali of the developer assists the sodium sulfite in preventing oxidation of the developing agent.
Happe, L. B., 1983, Your Film and the Lab, 2nd Edition, Media Manuals, Focal Press, London, Boston