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Fixing Baths — Crahiree and Hart ' 367
(a) Although many solvents of silver halides are available, the most satisfactory for fixing purposes are sodium and ammonium thiosulfate. A solution of ammonium thiosulfate of a given concentration fixes more rapidly than a corresponding solution of sodium thiosulfate^ but ammonium thiosulfate is more expensive and commercial grades are apt to contain free sulfur. For most purposes sodium thiosulfate fixes sufficiently rapidly and is entirely satisfactory.
(b) Practically any acid will function as an anti-staining agent because it has merely to neutralize the alkali in the developer carried over by the films and thereby retard oxidation of the developing agent. Organic acids such as citric, tartaric, oxalic, lactic, malic, maleic, and acetic, are more suitable than the inorganic acids because the}^ are less dissociated and therefore have a correspondingly less tendency to precipitate sulfur from the hypo. Practically all of the solid organic acids yield with aluminium salts, complex aluminium ions which are not effective hardening agents.*^ Acetic acid has been found to be the most generally efficient acid for use in fixing baths but much is still to be desired from the standpoint of a perfect acid for this purpose.
Of the acid salts, sodium bisulfite and potassium metabisulfite are the most generally used but they are not suitable for compounding a hardening fixing bath containing alum because the reserve acidity of these salts is not sufficient to prevent the precipitation of aluminium sulfite on the addition of alkali.
(c) It is imperative when compounding an acid fixing bath to have a quantity of free acid present in order to prevent discoloration of the bath by developer oxidation products and also to prevent the precipitation of the hardening agents by the alkali in the developer. This free acid tends to cause the precipitation of sulfur from the hypo especially at temperatures above 70° F. It is therefore necessary to add some substance which will prevent the precipitation of sulfur without impairing the anti-staining properties of the acid.
Two types of substances act in this manner. The first of these consists of the alkaline sulfites of wiiich sodium sulfite is the most common. Since hj^po decomposes in the i)resencc of acid to form sodium sulfite and sulfur, it is considered that the addition of sulfite to an acid fixing bath retards the decomposition of the hypo