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July, 1943] REMOVAL OF HYPO AND SILVER SALTS 31
The F-5 (no alum), F-24, and F-23 combinations with F-5 increased the rate of removal of hypo greatly but the two baths of F-5 were no different from F-5 alone. The degree of hardening was equal in the F-5 and the F-5:F-5 combination but increased somewhat with exhaustion when using the F-5 followed by the F-23. The hardening was decreased slightly when using F-5 followed by F-24.
When exhaustion studies of the F-5: F-24 combination were made it was found that the F-24 bath began to sludge after the bath was only half exhausted. This difficulty was overcome (a) by using an intermediate water rinse, (b) by using the F-5 fixing bath with the alum omitted as the second bath, or (c) by lowering the pH value of F-24 from 5.6 to 4.5 by the addition of acetic acid. When (c) was employed the rate of removal of hypo was the same as with the F-5: F-5 (no alum) combination. In this manner sludging did not occur during the exhaustion life of the bath.
The use of F-6 as a second bath was very effective for hypo removal, the hypo content of the film being 0.04 milligram per squareinch after 10 minutes' washing. However, with this combination (F-5: F-6) it was necessary to maintain the £H of the F-6 bath at 4.9 to 5.5 or its effectiveness would be lost because of a lowering of the pH by the carry-over of the more acid F-5 fixing bath.
The silver content of the film was essentially reduced to zero throughout the exhaustion when a second fixing bath was used for 2 to 5 minutes after the first F-5 fixing bath, regardless of the composition of the second bath.
It has been shown that when the degree of exhaustion of a single bath (pH maintained) exceeded 200 feet of 35-mm. film per gallon, silver was permanently retained by the film even after prolonged washing. A second fixing bath should therefore be used and may be exhausted to the point where silver is permanently retained by the film.
When the pH of the first bath was maintained during exhaustion the pH of the second bath did not change appreciably, but if the pH of the second bath was low (e. g., F-5 at £H = 4.1) the degree of exhaustion was limited by the silver retained by the film. On the other hand, if the pH of the second bath was above £H = 4.9, the degree of exhaustion was limited only by other factors such as sludging properties.
The relationship between the quantities of silver in the fixing