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CRABTREE, EATON, AND MUEHLER [J. S. M. P. E.
hypo content decreased to a low value and the silver was completely removed at pH values greater than 4.9, which is the isoelectric point of gelatin (see p. 43). The pH of the bath exhausted to 300 feet per gallon was 5.1.
Further experiments indicated that after a washing time of 30 minutes the hypo content was reduced to zero when the pH. of the bath increased to 5.5, but when the pH was maintained constant at 4.1, the hypo content decreased to a certain small quantity not re
s 10 20 30
WASHING (MINUTES) 68°
I 5 10 10
WASHING (MINUTES)
3O
FIG. 5. Effect of exhaustion of fixing bath on rate of removal of silver by washing. Eastman SuperJO" Panchromatic Negative Film, Type 1232. Fixed in Kodak F-5.
Curve A — Fresh bath, pR = 4.1.
B — Exhausted to 100 feet per gallon, pR = 4.3. C— Exhausted to 200 feet per gallon, pH. = 4.7. D — Exhausted to 300 feet per gallon, pH. = 5.1. E — Exhausted to 400 feet per gallon, pH. = 5.5.
(a) pH maintained at 4.1.
(b) pH. increased with exhaustion from 4.1 to 5.5 as shown above.
movable by further washing. Silver was retained only after fixation in the baths exhausted to about 200 feet per gallon when the pYL was maintained. The quantity of this residual silver after a 30minute wash was approximately 20 per cent lower than after a 5-minute wash and remained constant even after two hours' washing. These facts are substantiated in greater detail in Fig. 4. The curves in Fig. 4(a) and (b) indicate the relative rates of removal of hypo for increasing times of washing after fixation in baths exhausted to varying degrees (1) with the pH maintained, and (2) at increasing pYL values in the range 4.1 to 5.5. An increase in pH