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090 J. I. CRABTREE AND H. D. RUSSELL [ J. S. M. P. E.
and the hardening produced by the green modification may have been due to the presence of a small quantity of violet salt which formed during cooling after boiling.
This change from violet to green, according to Vallance and Eldridge,3 is accompanied by an increase in acidity and is apparently due to hydrolysis together with a change in structure, probably as follows:
2Cr2(S04)3 + H20;=±[Cr40(S04)4]S04 + H2SO4
This conclusion is based upon considerations of cryoscopic and conductivity measurements, a study of absorption spectra, and upon observations regarding precipitation with barium chloride in aqueous solutions.
When in solution, chrome alum hydrolyzes forming basic salts which polymerize on heating and more slowly at ordinary temperature on standing. Such a reaction may be represented by the following equation:
+ «H2O ^ Cr,(OH)» (SO4)a
where a + b = y
from which it may be concluded that the hardening effect of chrome alum is probably a result of the precipitation of chromium hydroxide in the gelatin or at least of a reaction between the basic salt and the gelatin so that it is to be expected that the repression of the hydrolysis such as by the addition of sulfuric acid would diminish the hardening towards neutral gelatin while an increase in the degree of hydrolysis by a reduction of the acidity would increase the hardening.
From a study of hardening and pH curves during the practical exhaustion tests, it is seen that the acidity and hardening produced changes with the age of the baths. The acidity of the fresh solutions, which are alkaline to brom -phenol -blue, increases, whereas the acidity of the solutions which are acid to the indicator decreases. This can be accounted for by the hydrolysis equation since increasing quantities of developer lower the acidity, causing more hydrolysis to take place, which in turn causes a decrease in the number of chromic ions, and also a decrease in the hardening properties. The addition of acid causes the opposite reactions to take place. This indicates that the hardening action of chrome alum solutions is due to the presence of chromic ions and that conditions favoring their formation