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June, 1930] STOP BATHS AND FlXING BATHS 687
even when observing the above precautions, this may be prevented largely by thorough agitation when the film is first immersed in the fixing bath.
(b) Stains. — Stains similar to those outlined in Part I under "Stop Bath Troubles" are likewise apt to be produced with chrome alum fixing baths, especially when fixing in baths containing a high concentration of chrome alum at high temperatures. The remedy is to use a bath containing a lower concentration of chrome alum and to renew this more frequently.
DISCUSSION OF THE HARDENING PROPERTIES OF CHROME ALUM IN VARIOUS BATHS
The anomalous hardening properties of chrome alum solutions may be summarized as follows:
1. The addition of an alkali, sodium bisulfite, or developer to a plain chrome alum solution causes a decrease in the hardening properties and acidity of the bath and a change of color from purple to green.
2. The addition of borate, acetate, citrate, and other organic acid radicles causes a decrease in hardening properties similar to that by the above salts, but does not give a corresponding change in color. The presence of 5 per cent sodium acetate in a chrome alum bath changes the color to a deep purple which is similar to the color of a chromic acetate solution.
3. Successful revival of the hardening properties of exhausted baths is possible with sulfuric acid, the life of the bath after revival depending upon the amount of developer already in the bath.
4. A maximum degree of hardening results when the solution in contact with the gelatin film has a definite basicity or alkalinity resulting after the acid in the fixing bath has neutralized the alkali in the developer that is retained by the film. Since the acidity of a fixing bath or stop bath changes with use, its hardening properties change also.
5. The acidity of a plain chrome alum solution must be maintained within certain limits before a maximum degree of hardening can be obtained. For the hardening of neutral or well washed film, the acidity range lies between a pH of 3.8 to 4.0 and for alkaline film (as defined in Part I) between a pH of 3.0 and 3.8. Since the acidity of 2 per cent and 3 per cent chrome alum solutions is about 3.2. these solutions do not harden neutral gelatin.
6. The hardening life of chrome alum fixing baths can be prolonged