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510 J. I. CRABTREB AND H. D. RUSSEU, [J. S. M. p. B.
The life of baths exhausted with other developers containing equivalent quantities of sodium carbonate and sodium sulfite would be analogous to those given above. The quantity of acid to be added at each revival point can be readily determined by the method described and enough acid should be added to the exhausted bath to bring the acidity to a pH value of 3.0, at which point maximum hardening is obtained with unrinsed film when using developers containing an average quantity of alkali.
Sludging Life. — The addition of excess alkali or developer to a chrome alum solution may cause the precipitation of a chromium sludge although its precipitation is greatly retarded if an excess of sodium sulfite is present. The composition of the precipitate varies with the quantity of sulfite present. No test for sulfite was found with the precipitate obtained with the D-16 developer but a stop bath precipitated with D-76 gave evidence of sulfite even in the well washed precipitate. No sludges were obtained providing the solutions had a pH less than 4.0.
After adding 15 per cent of D-76 developer (without revival) to a 2 per cent chrome alum bath a sludge was obtained after storing for two months at 110°F. No precipitate was formed at 70°F. With the 3 per cent bath, no precipitate was obtained under any conditions up to a concentration of 20 per cent of D-76. With the addition of 15 per cent of D-16 (without revival) to a 2 per cent or 3 per cent chrome alum bath a sludge was obtained after storing for 1 day at 70°F.
The film should be agitated when first immersed in the bath, otherwise in view of the high concentration of alkali in the vicinity of the film a sludge of chromium hydroxide is apt to deposit on the film surface. Its formation can be entirely prevented by reviving the bath with acid at intervals.
When to Discard the Bath. — The stop bath should be discarded either when (1) a sludge forms in the bath or on the film, or (2) the bath ceases to harden after revival with acid. The addition of further quantities of chrome alum to a bath which has lost its hardening properties will revive it more or less in proportion to the quantity added but this procedure is not to be recommended.
Effect of Temperature and Time on Hardening. — If the film is agitated when immersed in the fresh 2 per cent stop bath, maximum hardening with positive film is secured in thirty seconds at 70°F. and