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386 Transactions of SM.P.E., Vol. XIII, No. 38, 1929
centratioii so reduced that few or no gas bubbles are formed when immersed in the fixing bath.
5. The degree of swelling of the gelatin when first placed in the fixing bath which, in turn, depends upon the nature of the gelatin and temperature and composition of the developer, and the time of development.^^
6. The acidity of the fixing bath. It was at first thought that the active acidity as represented by the hydrogen ion concentration measurements might be a measure of the tendency of a fixing bath to blister. The hydrogen ion concentration was determined for fixing baths containing widely varying quantities of sodium sulfite, acetic acid, and potassium alum, but in all cases the pH values were approximately constant varying only from 4.5 to 4.7 although the tendency of the baths to produce blisters varied considerably. It is apparently not possible to correlate the hydrogen ion concentration figures with the propensity of a given bath to blister because a stable bath is sufficiently buffered to maintain a constant hydrgen ion concentration regardless of the concentration of acid used.
It was observed that the tendency of a bath to cause blisters increased as the total acidity was raised above the value at which blisters were found under average practical working conditions. The total acidity was determined by the quantity of a known alkali required to neutralize a given quantity of the fixing bath. Since the developer (MQ25) represents a known alkali and since it was used in all other cases in the testing of the properties of a fixing bath, it was also used to determine the total acidity of a fixing bath. The results are expressed in terms ^'the number of c.c. of MQ25 developer required to neutralize 100 c.c. of fixing bath using phenolphthalein as indicator. ' ' It was considered that for practical purposes when a normal rinse is employed between developing and fixing and when the temperature does not exceed 70° F. to 75° F. the acidity of a fixing bath should not be greater than 150 c.c. For temperatures above 75° F. to 85° F. an acidity of 30 c.c. to 50 c.c. only is permissible.
VIII. The Eevival of the Hardening Properties op Fixing Baths Even the most suitable acid hardening fix:ing bath compounded according to previous discussion is rendered unsatisfactory for fur