The Staining Properties of Ammonium Thiosulphate Fixers

Paper presented at a Symposium on “Processing Technology” organized by the Science Committee on 12-14 May 1965. in London.

Abstract

Spots of exhausted ammonium thiosulphate fixers on cellulose fibre degrade to give dark stains of silver sulphide because of loss of ammonia and sulphite accompanied by a drop in pH. Some ammonia is preferentially absorbed by the cellulose though most is lost by volatilization during exposure to air. In the first stage of exposure the loss of ammonia is paired by a loss of weak acid anion. Further loss of ammonia in the presence of silver ion and air leads to a drop in pH and to the loss of bisulphite and thiosulphate, with the generation of elementary sulphur and the formation of a strong sulphur acid (sulphate). When the thiosulphate content has fallen to the point where the stability of the silver thiosulphate complexes is affected, silver ion is increasingly freed and reacts with the cellulose. The finely divided silver thus produced reacts with the sulphur to give silver sulphide. On an inert support, the spot degrades mostly to silver halide and sulphur.

Notes

Paper presented at a Symposium on “Processing Technology” organized by the Science Committee on 12-14 May 1965. in London.