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16 CRABTREE, EATON, AND MUEHLER [J. S. M. P. E.
sulfide spot when the transmitted light was picked up by a photoelectric cell, the output current of which was amplified and indicated on a microammeter.
The pH of solutions was determined with the Beckman Industrial pH meter standardizing with an acid phthalate buffer solution at £H = 3.97.
The relative quantities of silver in prints were determined by spot testing the emulsion side with 0.2 per cent sodium sulfide solution and visually judging the intensities of the silver sulfide stains.
A further check was made on the relative silver contents by subjecting samples of the prints to accelerated fading conditions (over water at 110°F) which caused the silver thiosulfate complexes to decompose to produce a silver sulfide stain.
The concentration of silver in fixing baths was determined by using the Argentometer described by Weyerts and Hickman.9
The relative rates of hypo and silver removal were determined for chrome alum (Kodak F-23), potassium alum (Kodak F-25 or Kodak F-5), and non-hardening (Kodak F-24) fixing baths, and the effect of increasing £H in each case was studied. Two-bath combinations of these three types and modifications of them were also investigated.
The effect of a series of wetting agents upon the removal of both hypo and silver was also studied. The tests indicated that the use of wetting agents had no practical value in this connection.
EFFECT OF COMPOSITION OF PROCESSING BATH
(1) Composition of the Developer.— Tests made under the same conditions of rinsing, fixing, and washing with Kodak D-76, DK60a, D-72 (1:2), and D-7 (pyro) developers showed that there were no differences in the rate of removal of hypo or silver from SuperXX Panchromatic Negative Film, Type 1232, when using the Kodak F-5 fixing bath.
(2) Composition of Stop Baths. — Although acid stop baths are not invariably used when processing negative or positive films, a comparison between water, a 2 per cent solution of acetic acid, and a 3 per cent solution of chrome alum (Kodak SB -3) showed that none of these baths had any appreciable effect in retarding or hastening washing when the Kodak F-5 fixing bath was employed.
(3) Composition of Fixing Baths. — Experiments by the authors with paper prints and roll films have shown that the use of a fresh