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June, 1930]
STOP BATHS AND FIXING BATHS
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bath to blister was also greatly increased and the odor of the gas evolved was very objectionable.
Two types of chromium salts are known which although of similar chemical composition differ widely in their chemical properties. Recoura4 and other workers have investigated the properties of the two types of chromium salts (violet and green) and it has been shown that boiling a solution of the violet modification of chrome alum converts it to the green modification which on standing slowly changes back to the violet one.
Tests were made to determine whether it was possible to obtain suitable hardening with the green modification of chrome alum. Samples of violet chrome alum were boiled for two hours, then cooled, and the hardening compared with that of the original violet solution as follows. Test strips were developed in D-16 for five minutes, treated in the chrome alum solution for three minutes, bathed in a 30 per cent solution of hypo for five minutes, washed twenty minutes, and the melting points determined. The results are given in Table VII.
TABLE VII
Effect of Boiling on Hardening Properties of a Chrome Alum Solution
% Chrome Alum
Melting Points
Fresh
Boiled 2 Hours
After Boiling and Adjusting Acidity to pH 3.2
0.2
120 °F.
110°F.
170 °F.
0.4
>212°F.
110°F.
>212°F.
0.8
>212°F.
110°F.
>212°F.
1.6
>212°F.
110°F.
>212°F.
3.0
>212°F.
110°F.
>212°F.
4.0
>212°F.
110 °F.
>212°F.
From the above table it is seen that the hardening properties of chrome alum solutions towards alkaline gelatin are almost completely destroyed on boiling. A determination of the acidity of these solutions after boiling showed that it had increased to a pH of 2.0 which accounts for the low degree of hardening produced. After adjusting the acidity to a pH of 3.2 by adding alkali, the hardening properties were restored, although the green color still remained. This is not conclusive evidence that the green modification hardens because there is always a gradual change from the green to the violet taking place