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mCi Transactions of S.M.P.E., Vol. XJII, No, 38, 1929
1. Non-hardening Fixing Baths
A plain solution of hypo is quite satisfactory for fixing purposes provided little or no developer is carried over to it by the films. It is therefore necessary either to wash the film thoroughly after developing and before fixing Avhich is usually impractical, or to use an acid stop or rinse bath. In such a case, some sodium sulfite must be added to the hypo to prevent sulfurization by the acid carried over by the films from the stop bath.
Many motion picture laboratories employ non-hardening fixing baths with their processing machines. A satisfactory formula is as follows :
Metric Avoirdupois
Hypo 250 grains 100 lbs.
Sodium sulfite (desiccated) 10 grams 4 lbs.
Sodium bisulfite 25 grams 10 lbs.
Water to make 1 liter 50 gals.
The function of the bisulfite is to neutralize the sodium carbonate in the developer carried over by the films while the sodium sulfite serves to prevent possible sulfurization in hot weather. It is necessary to maintain the acidity of the bath either by allowing a continuous stream of sodium bisulfite solution to flow into the bath, or hj adding a definite volume of a stock solution at intervals. The precise quantity to add depends on the alkalinity of the developer, the degree of rinsing between developing and fixing, and the quantity of film processed, but this can be determined easily by titrating a small quantity of the fixing bath with alkali at intervals to ascertain the loss in acidity.
An alternative method of maintaining the acidity is to pass sulfur dioxide gas into the fixing bath either continuoush' or intermittently through a perforated hard rubber tube leading to the bottom of the tank. There is little to choose between sulfur dioxide gas or a sodium bisulfite solution as regards efficiency but there is a danger of contaminating the darkroom atmosphere with the objectionable sulfur dioxide gas if an excess of gas is accidentally passed into the solution.
2. Acid Hardening Fixing Baths
An acid hardening fixing bath contains the following ingredients: (a), a silver halide solvent (b), an anti-staining agent (e), a preservative or sulfurization inhibitor (d), a hardening agent.