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Fixing Baths — Crahtree and Hart 399
cessfully at intervals by adding a definite quantity of acetic acid. It has been found best to add 500 c.c. or one pint of glacial acetic acid after every 7,500 feet of film (positive and negative) are processed in 120 gallons of the bath. If more acid than the quantity recommended is added, blisters and sulfurization are liable to occur.
These figures refer only to the above fixing bath (formula F-2) when using either developer formula D-16 or the Eastman borax formula D-76 and rinsing thoroughly in v^ater after development. With developers containing more or less alkali or sulfite than these developers, the quantity of acid added should be adjusted accordingly.
If a footage record of the film processed is not kept, the acidity of the bath should be tested by. titrating with alkali, using phenolphthalein as indicator. When the total acidity has dropped to twothirds of the original acidity, enough acid should be added to restore the original acidity. The best way to revive a fixing bath would be to add acid gradually at a very slightly greater rate than that at which the acid is neutralized, it having been found that with use the acidity of the fixing bath should be increased slightly in order to maintain its hardening properties. However, the method of revival by adding acid after treating a definite number of feet of film has been found to work very satisfactorily in practice and is to be recommended.
If the bath has deposited a sludge of aluminium sulfite through neglect in reviving with acid, it is not advisable to try to dissolve this sludge by adding acid, although if the precipitate is freshly formed, it will usually dissolve after revival with acid on standing over night.
5. Revival of Fixing Baths hy the Addition of Hypo. With use, the rate of fixation falls off as a result of (a) removal of active thiosulfate ions by virtue of the formation of complex silver thiosulfate ions, (b) dilution of the bath as a result of the addition of developer or rinse water and removal of thiosulfate by the films to the wash water, (c) accumulation of sodium iodide formed as a by-product in the conversion of the silver iodide in the emulsion to silver thiosulfate.
It is quite possible to revive the bath at intervals by the addition of a concentrated solution (60% to 70%) of hypo and still retain the hardening properties. The frequency of revival is deter