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July, 1943] REMOVAL OF HYPO AND SILVER SALTS 23
increased appreciably the rate of removal of the hypo but, after fixation in the baths exhausted to about 200 feet of 35-mm film per gallon and with the pH maintained, the hypo content could not be reduced entirely to zero by any amount of washing.
The curves in Fig. 5 (a) and (b) show a similar effect of pH on the rate of removal of silver, namely, that above a degree of exhaustion of approximately 200 feet per gallon the silver can not be removed entirely by washing when the £H is maintained.
It is therefore apparent that after a certain degree of exhaustion, if the pH of the fixing bath is maintained constant, neither the hypo nor the silver is completely removable by washing. A study of the relative quantities of hypo and silver retained after fixation in baths exhausted to different degrees in the range from 250 to 500 feet per gallon showed that the retention of silver approximately paralleled that of the hypo.
The curves in Figs. 4 and 5 show that an increase in the £>H of the fixing bath to a value above 4.9 permitted the complete removal of the hypo and silver. Complete removal may also be accomplished by bathing the film in an 0.03 per cent ammonia solution (1 cc of 28 per cent ammonia per liter) following washing of the film or by the use of a fresh second fixing bath as described later.
In general, similar results were obtained when a chrome alum fixing bath (Kodak F-23) was used. The hypo content decreased with time of washing much more rapidly than as indicated in Fig. 4, except for a small quantity retained by the film following fixation in exhausted baths, when the residual silver and hypo contents were somewhat less than those retained after fixation with exhausted potassium alum baths. However, prolonged washing removed some of the residual silver and hypo retained after chrome alum fixation but had little effect on that retained after potassium alum fixation when the pH of the bath was maintained.
With the bisulfite-sulfite non-hardening fixing bath, there was no retention of hypo or silver following any degree of exhaustion, and the hypo was removed somewhat more rapidly than after fixing with chrome alum baths.
Data from experiments with finer-grained and more thinly coated films, such as Eastman Motion Picture Release Positive Film, Type 1301, and Fine-Grain Release Positive Film, Type 1302, indicated that the same general effects were obtained but to a lesser degree than with the high-speed negative film, Type 1232. A factor