Among the common acids and acid salts, boric acid and bisulphite of soda alone cause no decomposition of hyposulphite solutions ; or at any rate, the decomposition is so slow as not to occur during normal times of storage.
This property of bisulphite of soda in relation to hyposulphite causes sulphite of soda to behave as a protector of hyposulphite against acids. The latter are not able to attack the hyposul phite until all the sulphite has been converted into bisulphite.
Neutral or acidified solutions of hyposulphite attack a great many metals, the action being particularly rapid in the case of zinc, so that zinc washing trays are quickly corroded when used continually. Nickel is only slightly at tacked, and tanks of this metal may therefore be used for fixing baths, provided that the solutions are removed when fixing has been finished. For commercial use, tanks of wood, glazed earthenware, or even of lead, lapped and self-soldered, are employed. I Neutral solutions of hyposulphite only attack metallic silver very slowly, even when it is in such a finely divided condition as in photographic negatives ; even then, free access to the air must be allowed for the action to occur (Haddon and Grundy, 1896). Thus, there is no harm in leav ing photographic images for several hours in a neutral fixing bath, provided that the negatives or prints are fully covered with solution ; an appreciable reduction would appear on any portions sticking up from the liquid or lying on the surface.
Acid solutions of hyposulphite of soda, on the other hand, slowly attack metallic silver (A. Lainer, 1890) without the intervention of atmospheric oxygen, the solution of all the silver in a negative sometimes being complete in 48 hours. Leaving negatives or prints for long periods in acid fixing baths is therefore to be avoided.
Alkaline solutions of hyposulphite have prac tically no action on the silver of photographic images.
400. The Chemistry of Fixation. When a very small quantity of hyposulphite comes in contact with a large excess of a salt of silver it tends to form hyposulphite of silver ; this salt is unstable, like almost all hyposulphites, and decomposes quickly into brown or black insoluble sulphide of silver and sulphuric acid, which remains in solution.
This explains the stains which are formed. on sensitive surfaces, particularly on print out papers (containing soluble salts of silver), when they are touched by fingers soiled with hyposulphite.
It is quite otherwise when the salt of silver comes in contact with a large excess of hypo sulphite of soda in sufficiently concentrated solution. Again, hyposulphite of silver tends to be formed, but this salt combines at once with excess of hyposulphite of soda, forming complex hyposulphites of silver and sodium. These salts are comparatively stable and are sharply differentiated in all their properties from a mixture of the two simple hyposulphites. For example, these complex hyposulphites have a sweet whilst hyposulphite of soda has a bitter and sulphurous taste, and the salts of silver generally have a very disagreeable metallic taste.
The composition of these complex salts ap pears to vary according to the nature of the salt of silver which is caused to react with hyposulphite of soda (Gaedicke, 1903; Lumiere and Seyewetz, 1907). Whatever the original salt of silver, however, according to the proportions of hyposulphite and of the silver salt, one may always obtain either a soluble complex hyposulphite or an insoluble complex hyposulphite. 1 It is obvious that it is the formation of the soluble complex hyposulphite which must be aimed at during fixing, because only a soluble salt can be removed by washing. The soluble complex hvposulphites contain, for a given quantity of silver hvposulphite, a higher pro portion of hvposulphite of soda than the in soluble hyposulphite; their formation requires the presence of a large excess of available hyposulphite of soda, which is not already saturated with salts of silver.
These complex salts decompose spontaneously in the cold, depositing silver sulphide; this decomposition is very rapid in the absence of an excess of hvposulphite of and is acceler ated by heat and light. It is well-known that used fixing baths deposit in time a black sludge consisting mainly of sulphide of silver.