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498 J. I. CRABTREE AND H. D. RUSSELL [J. S. M. p. E.
but on incubation a distinct gray-green precipitate occurred similar to that formed in an aluminum alum fixing bath.
In further experiments it was found that 1 per cent sodium sulfite together with 0.5 per cent sodium bisulfite prevented the precipitation of both sulfur and the hardening agent from a 30 per cent hypo solution containing 50 per cent of a 2 per cent solution of chrome alum. Comparative experiments were made with a 2 per cent chrome alum stop bath containing 0. 1 per cent sulf uric acid and it was found that the increased acidity was not sufficient to cause any material difference in the sulfurization life of the fixing bath.
PRACTICAL TESTS WITH CHROME ALUM STOP BATHS
At the outset, practical tests were made with the following formula:
Metric Avoirdupois
Potassium chrome alum 20 grams 8% Ibs.
Water to make 1 liter 50 gallons
Effect of Addition of Developer on Hardening Properties. — In order to gain a preliminary idea of the behavior of the stop bath on exhaustion with film, increasing quantities of the developers D-16 and D-76* were added to a definite volume of the bath and samples stored at 70°F. and 110°F. Hardening tests were then made at regular intervals after storing the mixtures for increasing periods of time.
The tests with the D-16 developer were made according to the procedure described on page 486 for alkaline film. With the D-76 developer a similar procedure was followed, except that strips of Eastman motion picture panchromatic negative film were developed in D-76 for ten minutes. The fixing bath for these tests contained 30 per cent hypo and 1.0 per cent sodium bisulfite.
The hardening tests were made by determining both the reticulation and melting points of the film. Since a hardening stop bath is used mostly for high temperature work, the reticulation point is of the
* Formula D-76
Metric
Avoirdupois
Elon
2 grams
13 ounces
Hydroquinone
5 grams
2 pounds
Sodium sulfite (anhydrous E. K. Co.)
100 grams
42 pounds
Borax
2 grams
13 ounces
Water to make
1 liter
50 gallons