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J. I. CRABTREE AND H. D. RUSSELL [J. S. M. p. E.
From these tests it would appear that 2.5 parts by weight of sodium sulfite produce the same effect as one part by weight of sodium carbonate, which ratio corresponds to the relative alkalinity of the two salts using phenolphthalein as an indicator. It is apparent, therefore, that the falling off in hardness with immediate use is largely a result of a change in the acidity of the bath.
TABLE IV
Effect of Addition of Sodium Sulfite and Sodium Carbonate on Hardening Properties of Chrome Alum Fixing Bath, No. I
Grams Sodium Sulfite (des.) per 100 Cc. Fixing Solution
Melting Points
Fresh
2 Days
5 Days
0.4
>212°F.
180 °F.
170 °F.
0.8
>212°F.
145 °F.
140 °F.
1.2
174 °F.
106 °F.
106 °F.
1.6
134 °F.
97 °F.
97 °F.
2.4
116°F.
97 °F.
97 °F.
Grams Sodium Carbonate (des.) per 100 Cc. Fixing Solution
0.2
>212°F.
182 °F.
180 °F.
0.4
172 °F.
P— 96 °F.
0.6
124 °F.
P— 96 °F.
0.8
106 °F.
P— 96 °F.
1.0
100 °F.
P— 95 °F.
P = Precipitate of Chromium Hydroxide.
(3) Effect of Exhaustion and Revival with Acid. — A practical test was made by exhausting gallon quantities of the fixing bath with Eastman motion picture positive and panchromatic negative film (Type2) using the developers D-16 and D-76, respectively, at 70°F. and 85°F. The film was processed as follows: The positive film was developed for five minutes in D-16, placed in the fixing bath without rinsing for five minutes, and washed twenty minutes. Ten per cent of sodium sulfate (desiccated) was added to D-16 at 85 °F. in order to prevent excessive swelling of the gelatin. The negative film received similar treatment in D-76 but was developed for ten minutes and likewise fixed for ten minutes.
The method used for the determination of the acidity at each revival point will be described later. The average quantity of concentrated sulfuric acid added to the baths at each revival point was