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502 J. I. CRABTRBE AND H. D. RUSSELL [J. S. M. p. E.
A comparison between the hardening properties of the 2 per cent and 3 per cent chrome alum stop baths indicated that the hardening life of the 3 per cent bath is about twice as great as that of the 2 per cent bath. It also seems plausible that a further increase in hardening properties might be obtained with higher concentrations of chrome alum, but concentrated solutions of chrome alum terid to stain the film, especially at high temperatures. For this reason, it was not considered desirable to increase the concentration of chrome alum beyond 3 per cent for use as a stop bath.
Effect of Exhaustion and Revival with Acid on the Hardening Properties of Stop Baths. — Practical exhaustion tests during which the stop baths were suitably revived were made with 2 per cent and 3 per cent chrome alum solutions as follows. Lengths of Eastman positive motion picture film were processed in D-16 at 70°F. and 85°F. in a manner similar to that outlined previously. Ten per cent of sodium sulfate (desiccated) was added to the D-16 developer at 85 °F. to prevent excessive swelling. One hundred cc. samples of the solution were taken after every 50 feet of film had been processed and after revival with acid. They were then stored at 70°F. and their hardening properties measured at regular intervals. Lengths of Eastman panchromatic negative motion picture film were developed in D-76 for 10 minutes and subsequently given a treatment similar to the positive film in D-16 above.
The acidity of the baths was revived at regular intervals by adding a quantity of acid so as to restore the acidity of the bath to a pH of 3.0, at which point previous experiments had indicated that maximum hardening was obtained. The condition of the bath was determined by titrating a 25 cc. sample (in 50 cc. of distilled water containing 5 cc. of brom phenol blue indicator) with a 2.5 per cent sulfuric acid solution until the solution was just acid to the indicator. From these results the quantity of concentrated acid necessary to add to the bath was calculated. The pH of the bath was always measured after the concentrated acid had been added so as to be sure that it was correct. The average quantity of concentrated acid which it was necessary to add at each revival point with the D-16 developer was 1 cc. per liter or 0.13 ounce per gallon and with film developed in D-76 the quantity was 1.5 cc. per liter or 0.19 ounce per gallon. The results of the exhaustion tests are given in Figs. 9, 10, 11, 12, and 13.
With D-16 at 70°F. (Fig. 9), the 2 per cent bath, revived with