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July, 1943]
REMOVAL OF HYPO AND SILVER SALTS
27
positive film, Type 1301, except that the quantities of silver retained were somewhat less.
The actual difference in the quantities of silver retained after fixing for twice and four times the "time to clear" in the F-5 fixing bath at £H = 4.1 is admittedly small and may be of no practical significance. However, since removal by washing of the last traces of silver was desired, a fixing time of four times the apparent time to clear was used in both fresh and exhausted baths throughout this investigation.
When the times of fixing were extended to six and ten times the apparent "time to clear," the Super-^JY Negative film, Type
0.09 0.08 GOT
0.05
002 001
EXHAUSTION pH VALUES
FRESH 4-. I 1 25 FT /GAL. 4.5O 25OFT/GAL. 4.9O 375 FT/ GAL. 5.25 'Ox;
500 FT/ GAL. S.fcO
'EXHAUSTION pHs.
125 25O 375 BOO
DEGREE OF EXHAUSTION: FEET 35mm FILM PER GALLON
FIG. 8. Silver retained after increasing times of fixation in exhausted baths. Kodak Fixing Bath F-5. Film washed 15 minutes. Eastman Super-JOf Panchromatic Negative Film, Type 1232.
1232, retained an almost constant quantity of silver after fixing in an exhausted bath with the £H maintained at 4.1, as shown in Fig. 7, but retained no silver when the £H was greater than 4.9.
It was of interest to determine the extent of the retention of silver at various stages of the exhaustion life of a fixing bath when the £>H was maintained at 4.1 and when it was allowed to increase by addition of "carry-over" developer. The data and curves for tests made with Eastman Super-JO Negative Film, Type 1232, are given in Fig. 8 in which the relative silver contents are plotted in density units obtained by reading silver sulfide spots as previously described. Three facts are evident, namely, (a) when the pH was below 4.5, the silver retained increased with exhaustion, (b) when the p*K of the baths reached 4.9, the silver content was almost zero and at pH