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July, 1943] REMOVAL OF HYPO AND SILVER SALTS 29
fixing bath and the relationship of this time to the degree of hardening of the film should explain this anomaly.
The apparent time to clear in fresh F-5 was approximately 30 seconds for the Release Positive Film, and 3 minutes 15 seconds for the Super-XX" Negative Film. Therefore, four and ten times the time to clear the films, Release Positive and Super-^O Negative, in fresh and exhausted -F-5 baths were :
Release Positive Super-XX Negative
Type 1301 Type 1232
Fresh bath 4X 2 Min 13 Min
10 X 5 Min 32V2 Min
Exhausted bath 4X 4 Min 26 Min
10 X 10 Min 65 Min
The degree of hardening produced in an F-5 fixing bath increased from about 150°F to near the maximum in 1 to 10 minutes and reached the maximum in about 20 minutes (Fig. 10). It is believed that the gelatin-hardener complex is responsible for the retention of the silver and that as the degree of hardening increases the retention of silver increases provided the £H of the bath is below 4.9. This would suggest that the quantity of residual silver does not increase with SuperXX Negative Film, because the maximum degree of hardening is obtained in four times the apparent time to clear in a fresh bath. However, it can increase with Release Positive Film because the degree of hardening increased rapidly during the periods of "four" and "ten" times the "time to clear."
Thus, from the standpoint of residual silver, extended times of fixation in potassium alum baths at ^>H's below 4.9 have no advantage especially in exhausted or partially exhausted baths. To remove the silver completely, the use of multiple fixing baths must be considered.
When a fresh chrome alum (F-23) or fresh bisulfite-sulfite (F-24) fixing bath was used, the rate of removal of hypo was so great as compared with the rate when a potassium alum fixing bath (F-5) was used that all of the hypo was removed in approximately 10 minutes following any time of fixation up to 30 minutes.
After fixation in exhausted chrome alum baths for the recommended times, both silver and hypo as a complex were retained by the film in approximately the same ratio and quantity as following fixation in a potassium alum bath. However, with prolonged