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June, 1930] STOP BATHS AND FlXING BATHS 691
such as the addition of acid, the addition of chrome alum, and the oxidation of the sulfite, would increase the hardening action.
Although the addition of sodium sulfite to a chrome alum solution reduces the acidity and therefore tends to increase hydrolysis, a secondary reaction undoubtedly takes place and apparently a compound is formed which does not tend to dissociate. This would explain the fact that in the presence of an excess of sodium sulfite a chrome alum fixing bath has a minimum tendency to sludge on the addition of developer. The extent of the reaction with sodium sulfite apparently increases with age since the hardening properties of some fixing baths or a used chrome alum stop bath fall off with time especially with baths containing a low concentration of chrome alum. With a high concentration of alum, the rate of decrease in hardening is slower but it is necessary to have a minimum concentration of 2 per cent to insure reasonably good keeping properties.
Since the formation of basic chromium salts or chromium hydroxide is necessary for the hardening of gelatin, solutions of chrome alum containing a relatively high concentration of sodium acetate would not be expected to harden film because the formation of basic chromium salts is prevented in the presence of acetates. This explains why organic acids such as acetic, citric, and tartaric cannot be used as acidifiers in chrome alum solutions.
PERMANENCE OF HARDENING PRODUCED BY ALUMS
As indicated in the experimental procedure, the degree of "hardening" as expressed by melting point determinations is merely relative and the results should not be considered as being directly applicable to darkroom practice. For instance, if the melting point or relative hardening is indicated as 150°F. or more, this does not mean that the film hardened in the manner indicated could be washed indefinitely at that temperature without affecting the emulsion. The following experiments were made to determine the relative permanence of the hardening produced by chrome alum in relation to the experimentally determined hardening value.
Strips of film were fixed in hardening fixing baths which produced varying degrees of hardening and the melting points were determined in the usual manner. Duplicate strips of each of the films were washed at varying temperatures and the time noted for the film to become soft or swollen to such an extent that it was unsatisfactory for practical purposes. The results are given in Table VIII.