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June, 1930] STOP BATHS AND FIXING BATHS 669
(1) Sulfuric acid is superior to acetic and citric acids or sodium bisulfite for use in a chrome alum fixing bath because with a sulfuric acid bath the hardening properties do not fall off so rapidly with age. The formulas containing 3 cc. of sulfuric acid per liter liberated so much sulfur dioxide gas that they were considered unsatisfactory for use. Those containing 1.0 cc. of sulfuric acid per liter appeared to have too low an acidity to be practical. A concentration of 2 cc. per liter was considered the most satisfactory concentration.
Baths containing citric acid gave no hardening whatsoever and in this respect the behavior of citric acid in a chrome alum bath is similar to its effect with potassium alum baths.
(2) Chrome alum baths containing sodium bisulfite have poor keeping properties unless a relatively high concentration of chrome alum is present.
(3) An increase in the concentration of sodium sulfite lowers the hardening life with age and increases the sulfurization life while a decrease prolongs the hardening life and causes the bath to sulfurize more readily.
(4) With high concentrations of chrome alum the degree of hardening increases, the hardening properties are maintained better on storage, while the sulfurization life is decreased. The reverse is true if the concentration of chrome alum is lowered.
(5) The hardening properties of some baths fall off more rapidly with age when stored in an open tray.
(6) With a fixed proportion of chrome alum and sodium sulfite, the hardening produced with alkaline film increases with the acid concentration of the bath within the limits tested. Apparently with the higher concentrations of acid tested the conditions are most suitable for the formation of a basic chromium sulfate which produces the maximum degree of hardening. This observation agrees with the fact that it was possible to restore the hardening properties of some partially used chrome alum fixing baths by the addition of further quantities of acid.
(7) The formulas having the best hardening properties have an optimum ratio of 3:5 by weight of sodium sulfite to chrome alum. Less sulfite hastens sulfurization while the addition of more sulfite lowers the degree of hardening produced.
Further tests were made with the most promising formulas as well as others which were suggested from a study of the previous experiments. These tests were all made by storing the solutions in trays