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394 Transactions of S.M.P.E., Vol XIII, No. 38, 1929
2. A series of arbitrary tests has been devised whereby it is possible to determine the merits of any particular fixing bath with relation to the criterion as outlined above.
3. For fixing Eastman negative motion picture film a 40% solution of hypo is most efficient and for Eastman positive motion picture film a 25% solution gives maximum efficiency. When a bath is used for both negative and positive film, a concentration of 30% is satisfactory.
4. The hardening produced by potassium alum solutions of various concentrations is affected by the addition of acetic acid and sodium sulfite. For a given quantity of alum and acid the hardening increases to a maximum on the addition of sodium sulfite, then diminishes and finally a precipitate of aluminium sulfite is formed. With fixed concentrations of alum and sodium sulfite the hardening decreases as the acid concentration is increased.
From a series of curves showing the effect of variation in the proportions of alum, sulfite, and acid on the sulfurization and hardening life and the tendency of the bath to precipitate aluminium sulfite, it is possible to compound fixing baths having any desired properties.
5. Sodium sulfite may be replaced by sodium acetate in a fixing bath but sodium acetate is only one-fifth as efficient as sodium sulfite as a sulfurization inhibitor.
6. In certain cases, the acidity of a potassium alum fixing bath cannot be sufficient to keep the bath clear during the addition of a large quantity of developer because the acidity is limited by the propensity of the bath to blister and sulfurize. Under certain conditions where the efficiency of the fixing bath is determined by the quantity of developer which may be added before aluminium sulfite is precipitated, it has been shown that the addition of acid at frequent intervals during exhaustion of the bath is a material advantage.
X. Practical Eecommendations
In motion picture work the nature of the most satisfactory fixing bath to be used depends on the particular photographic procedure adopted. For instance,, if an acid stop bath is used after developing, high acidity of the fixing bath is not necessary, while