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May, 1930] STOP BATHS AND FIXING BATHS 497
quantity of developer retained by the film, the concentration of alkali in the developer, and the hydrogen ion concentration of the bath; (8) the hydrogen ion concentration of the bath must be maintained between certain limits in order to obtain satisfactory hardening properties; (9) a maximum degree of hardening is produced with neutral film when the acidity of the bath is maintained between a pH of 3.8 and 4.2 and with alkaline film (as defined) when the acidity lies between a pH of 3.0 and 4.0; (10) the concentration of chrome alum and the age of a plain chrome alum bath containing developer has but little effect on the hardening properties if the acidity of the bath is maintained between the above limits by the addition of sulfuric acid or caustic soda.
For practical purposes it was concluded that a 2 per cent chrome alum solution is the best stop bath. In cases where dilute developer is used and the alkalinity of the film is sufficiently low to reduce to a minimum the tendency of the bath to blister, the addition of 0.1 per cent sulfuric acid is an advantage in increasing the developer life of the bath, but it is not considered advisable to increase the quantity of acid above this amount because of the accompanying propensity of the bath to blister and to fail to harden weakly alkaline film.
Fixing Baths for Use with Acid Stop Baths. — Gelatin which has been hardened in a suitable chrome alum stop bath is sufficiently hardened to withstand wash water at relatively high temperatures and therefore further hardening of the film in the fixing bath is not necessary. However, a non-hardening fixing bath suitable for use in conjunction with a chrome alum hardening stop bath should not precipitate sulfur in the presence of an appreciable quantity (50 per cent) of stop bath since a relatively large quantity of acid is continually being carried into the fixing bath. Therefore, the fixing bath must contain an appreciable quantity of an anti-sulfur protective so as to prevent the precipitation of sulfur as the fixing bath becomes exhausted and at the same time the chrome alum must not precipitate as a sludge in the fixing bath.
Since sodium sulfite is the most efficient anti-sulfur protective, experiments were made to determine the quantity of sodium sulfite necessary to prevent the precipitation of sulfur from a 30 per cent hypo solution containing 50 per cent of stop bath (a quantity of stop bath which would normally be carried into the fixing bath). At normal temperatures, sodium sulfite was found to be satisfactory