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1949 COLOR PROCESSING CONTROL 17
This is conveniently done by providing two hard-rubber or ceramic mixing tanks for the bleach with pumps so that the solution may be pumped from the machine tank to either mixing tank for the rejuvenation treatment while machine operation is continued using the other tank of bleach solution.
A bleach exhaustion of 25 feet per liter normally corresponds to a potassium-ferrocyanide concentration of 4.5 to 5.0 grams per liter. With most technical grades of bromine, roughly 1.05 grams or 0.33 cubic centimeter per liter would be required to rejuvenate the bleach completely. In practice, however, in order to avoid the danger of adding an excess of bromine which would give excessive fuming and would be dangerously active both on the color film and on the tanks, spool banks, and so forth, it is desirable to retain a small amount of ferrocyanide in the bleach, normally 1.0 gram per liter or equivalent to the exhaustion produced by 5 feet of film per liter.
The addition of bromine should be made in a well-ventilated room or with a hood over the tank. Protective clothing and goggles should be worn as contact with the bromine will cause bad burns. The addition should be made slowly with vigorous stirring continued for a minimum of 30 minutes after the bromine addition is complete. The bromine will be assimilated more rapidly and with less fuming if it is first dissolved in 5 to 10 tunes its own volume of cold methanol and the mixture then added to the bleach mixing tank.
A second potentiometer titration should be made after the bromine addition to check the accuracy of the replenishment.
8. Replenishment of Diluted Bleach
The dilution of the bleach by the wet film can be corrected by making additions of the original chemicals in the same proportion as they were originally compounded. The degree of dilution can be detected by specific-gravity measurements using a hydrometer. No chemical additions are necessary unless the dilution exceeds 10 per cent. The specific gravity of fresh bleach No. 713 is approximately 1.110 at 20 degrees centigrade. Upon dilution, the specific gravity is reduced. An estimate of the degree can be made from the following calculation :
Specific Gravity of Fresh Bleach Minus
Specific Gravity of Diluted Bleach X 100 „ „
-rf j-= — , p. . , . ::. = Per Cent Loss of Dry .
^n™ ?JS* lt^°fr/re?*1 BJeaf Minus ^ f Ori^nal Bleach 1.000 (Specific Gravity of Water)