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June, 1930]
STOP BATHS AND FIXING BATHS
it is desirable to process in the above bath at 70 °F. and 85°F. are given in Table IX.
TABLE IX Hardening Life of Fixing Bath with and without Revival with Acid
Nature of Film
Temperature of Solutions
Developer
Hardening Life of Bath with Revival
Without Revival
M. P. Pos.
70 °F.
D-16
600 ft. per gal.
200 ft.
M. P. Pan. Neg.
70 °F.
D-76
300 ft. per gal.
125 ft.
M. P. Pos.
85 °F.
D-16
600 ft. per gal.
200ft.
M. P. Pan. Neg.
85 °F.
D-76
300 ft. per gal.
125 ft.
If the film is rinsed before fixing or if a less alkaline developer is used the life of the bath without revival is increased and a greater quantity of film can be processed before it is necessary to revive with acid.
Revival of Bath. — With use, the hardening properties fall off rapidly as the acid is neutralized by the alkali in the developer carried over by the film. For example, when developing motion picture film with D-16 the melting point of the film drops from 212 °F. to 110°F. after processing 300 feet per gallon. The hardening properties should be revived by the addition of dilute sulf uric acid after each 50 feet of film have been processed per gallon in the case of unrinsed motion picture negative film developed in the D-76 formula, and every 75 feet per gallon with unrinsed positive film developed in the D-16 formula.
In order to determine the correct quantity of acid necessary to revive the bath, about 25 cc. or one ounce should be titrated with a 2.5 per cent solution of sulfuric acid, using brom-phenol-blue as the indicator, and sufficient acid added until the color changes to yellow, thereby restoring the acidity to a pH of 3.0. The quantity of acid necessary to revive the bath proper can then be calculated. The concentrated acid should first be diluted by pouring one part by volume into ten parts of water. Then cool to 70° F. and add slowly to the cool fixing bath while stirring the latter rapidly.
The acidity of the bath should be checked at frequent intervals in order to be certain that the pH never becomes greater than 4.0, if uniform hardening properties are desired and if sludges are to be avoided.
Since chrome alum baths tend to sulfurize readily, they should be exhausted as soon after mixing and reviving as possible. If the bath