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May, 1930] STOP BATHS AND FlXING BATHS 485
When considering the advisability of using an acid hardening stop bath, the efficiency of the processing must be considered in terms of the total time required to develop, harden, and fix a given film, that is, in the case of a hardening fixing bath, the film is hardened and fixed simultaneously, while in the case of a hardening stop bath used in conjunction with a non-hardening fixing bath, hardening and fixing are independent. It is therefore evident that the use of a hardening stop bath increases the time required for processing a given film and for this reason the hardening fixing bath is most desirable under normal conditions because of the time saved.
Desirable Properties of Stop Baths. — (a) The bath must be distinctly acid and to be efficient should remain acid during the life of the fixing bath. Although the stop bath should be sufficiently acid to insure long life, when the film leaves the bath it is more or less acid and will therefore increase the free acid content of the fixing bath which in turn will increase the tendency of the latter to sulfurize. The choice of acids therefore depends on the propensity of the acid to precipitate sulfur in a hypo solution.
(b) A stop bath should not be sufficiently acid to cause blisters, and the limit of acidity in this case is much less than in the case of a fixing bath because the alkalinity of the film is much greater when removed from the developer than after a slight rinse and previous to immersion in the fixing bath.
(c) A hardening stop bath should have properties similar to those of a plain acid stop bath and in addition should produce satisfactory hardening throughout its life. Since the hardening produced by alum mixtures varies with the quantity of developer or alkali added, it is apparent that an acid hardening stop bath will not produce uniform hardening except for a limited time unless it is suitably revived.
Choice of Acids and Hardening Agents for Use in Stop Baths. — Since the acid contained in the stop bath is carried into the fixing bath, it is important that the acid should have properties similar to those of the acid used in the fixing bath, that is, it should have the least possible tendency to precipitate sulfur with hypo and should not produce blisters readily. Solid organic acids, such as citric, tartaric, malic, and maleic, are not suitable because they decrease the hardening properties of a fixing bath by virtue of the formation of non-hardening complexes. At the present time, acetic