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July, 1943]
REMOVAL OF HYPO AND SILVER SALTS
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chrome alum fixing bath10 causes an increase in the rate of hypo removal as compared with the use of a fresh potassium alum fixing bath. This was clearly demonstrated when Plus-X Panchromatic Negative Film, Type 1231, was washed for 10 minutes in a vertical glass tube with different quantities of water flowing through the tube per unit of time. The quantity of residual hypo was plotted against the rate of flow as shown in Fig. 2, from which it is apparent that the rate of removal from chrome alum hardened film far exceeds that from potassium alum hardened film.
With a chrome alum fixing bath apparently the hypo is not so strongly held as in the case of potassium alum baths and is very
30 60 100
180 225
GALLONS PER HOUR
FIG. 2. Effect of rate of flow on rate of removal of hypo from A : film fixed in fresh potassium alum fixing bath (Kodak F-25) \ B: fresh chrome alum fixing bath (Kodak F-23). Eastman Plus-Jf Panchromatic Negative Film, Type 1231. Time of washing, 10 minutes.
readily washed from the film. The relatively thick Eastman Motion Picture Films, Types 1231 and 1232, when washed under the extreme conditions of 5 minutes in the experimental apparatus (Fig. 1) at 41 °F, following the chrome alum fixing bath, retained only very small quantities of hypo.
The use of a non-hardening fixing bath (Kodak F-24) without pre or supplementary hardening was slightly more effective than the use of a chrome alum fixing bath with respect to hypo removal but was impractical because of swelling of the emulsion and the tendency for reticulation. A non-hardening fixing bath is practical only when used in combination with a suitable hardening bath or with films which are sufficiently hardened in manufacture
The variations in concentration of hypo in the fixing baths studied had no effect on the rate of removal of hypo from processed film.