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16 BATES AND RUNYAN July
TABLE V BLEACH REPLENISHMENT
No. 713 Bleach
Mono Sodium Dibasic Potassium Ferricyanide 100 grams Potassium Bromide 15
Dibasic Sodium Phosphate 40
Sodium Bisulfate 25
Water to make 1 liter
It is recommended that No. 713 bleach be rejuvenated intermittently with bromine additions.
During normal bleaching operations, the bleach exhausts caused by depletion of ferricyanide and bromide ions as well as from dilution of the bleach solution by water carried into the tank by the wet film. The accumulation of ferrocyanide ions slows the rate of bleaching to a much greater extent than would be predicted from the depletion of ferricyanide. In practice, a concentration of potassium ferrocyanide greater than 5 grams per liter should be avoided. These concentrations can be detected using either the potentiometer method described by Brunner, Means, and Zappert2 or, if desired, the colorimetric method described by Varden and Seary.7
The ferrocyanide can then be reoxidized to ferricyanide by the direct addition of liquid bromine to the solution. This reaction produces bromide ions equivalent to the number of reoxidized ferricyanide ions and thus effectively regenerates the bleach bath. The chemical reactions of bleach exhaustion and rejuvenation are shown in Table VI.
TABLE VI
BLEACH EXHAUSTION
4Ag + 4K3Fe(CN)6 - > Ag4Fe(CN)6 Ag4Fe(CN)6 + 4KBr — > K<Fe(CN)6 + 4AgBr
BLEACH REJUVENATION
4K4Fe(CN)« + 4Br° > 4K3Fe(CN)e + 4KBr
It is recommended that a bleach bath be rejuvenated at intervals corresponding to 25 feet of 35 millimeters per liter of tank solution.