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18 CRABTREE, EATON, AND MUEHLER [J. s. M. p. E.
Other ingredients (exclusive of alums) had no effect provided they did not change the pH of the bath.
Under the experimental conditions described on p. 13, the silver content of the washed film was essentially zero.
(4) pH of Fixing Baths.— The pH value of a fixing bath is significantly related to its composition which results from a consideration of sulfurization life, hardening properties, rate of fixation, exhaustion life, and sludging properties.10-11 Various fixing bath formulas therefore give different initial pH values as, for example,
Kodak F-23 — pH. = 3 . 1 — Chrome Alum Kodak F-5 — pH = 4 . 1 — Potassium Alum-Boric Acid Kodak F-10 — pU. = 4 . 6 — Potassium Alum-Boric Acid Kodak F-6 — pU = 4 . 9 — Potassium Alum-Boric Acid
These baths were compared to determine whether or not a difference in initial pH affects the rate of elimination of hypo from film. It is considered that the slight differences in the chemical constituents of the potassium alum baths were unimportant as compared with the £H differences.
TABLE II
Effect ofpH of Fixing Baths on Rate of Removal of Hypo (Eastman Super -XX 1232}
Time of Hypo Content (Mg per Sq-In)
Washing F-5 F-10 F-6 F-23
(Min) pH = 4.1 pH = 4.6 pH = 4.9 pH •» 3.1
10 0.20 0.12 0.02 0.02
25 0.08 0.06 Nil Nil
The data show that with potassium alum baths the rate of removal of hypo is greatest with baths having a high pH value. It was also found that by adjusting the initial pH of a fixing bath to a higher or lower value by the addition of alkali or acid, a corresponding change in the rate of removal of hypo resulted, but this procedure may impair the optimum properties of the fixing bath. Although chrome alum baths have low initial pH values there is no appreciable retention of hypo after washing.
(5) Degree of Exhaustion of Fixing Bath: (a) Effect of pH Change on Hypo Removal. — The F-23 chrome alum, F-5, F-6, and F-10 potassium alum fixing baths were exhausted to approximately 500 feet per gallon with Eastman Super-JY^ Panchromatic Negative Film, Type 1232, and the pH, degree of hardening, and rate of removal of hypo determined at different stages of the exhaustion.