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Fixing Baths — Crahtree and Hart 387
ther use by the addition of developer long before the hypo is exhausted ; that is, the various properties of the bath are so impaired that the fixing bath is rendered useless for further fixation because of (1) a decrease in the hardening properties; (2) the formation of a precipitate of aluminium sulfite; or (3) a sufficient decrease in acidity to permit discoloration of the bath, while the fixing power of the hypo still remains adequate for further use. It might be argued that the concentration of the hypo would be so cut down that the hypo and hardener would be exhausted at the same time, but to meet this condition the concentration of the hypo would need to be inefficiently low.
In order to determine the most suitable method of reviving the hardening properties it was necessary to determine first, the products formed during exhaustion and, secondly, the effect of these products on the properties of the fixing bath.
1. Products of Exhaustion and their Effect on the Hardening Properties of a Fixing Bath. During exhaustion of a fixing bath, the following products accumulate therein : developer and developer oxidation products, sodium silver thiosulfate, sodium bromide and sodium iodide, sodium acetate, sodium bisulfite, sodium sulfate, basic aluminium sulfites, and occasionally, free sulfur.
The sodium acetate and sodium bisulfite are a result of interaction between the acetic acid and the sulfite and carbonate carried in by the developer while some of the sodium bisulfite is converted to sodium sulfate by aerial oxidation. The sodium bromide and iodide are products of reaction from the fixation of bromo-iodide emulsions.
The addition of sodium acetate and sodium bisulfite to a fresh bath tends to increase the hardening produced and the sulfurization life of the path. Sodium acetate does not affect the tendency of the path to precipitate aluminium sulfite, while sodium bisulfite only slightly increases this tendency. Neutralization of the sodium carbonate by the acetic acid reduces the acidity of the bath and by reference to the curves in Figs. 3, 4, and 5, it is seen that as the acidity is decreased the hardening increases up to the point at which aluminium sulfite precipitates.
It was also considered possible that the exhaustion of the bath by a given number of films might be accompanied by the absorption or removal of a given quantity of alum by virtue of the harden