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426 Transactions of SM.P.E., Vol. XIII, No. 38, 1929.
(b) Sodium sulfite is used in most developers to protect the developing agents from aerial oxidation. When the developer is exposed to the air, a small quantity of the sulfite is converted to sulfate which in this concentration has no effect upon the development process. Some sulfite is also used up in forming the hydroquinone sulfonates, but the quantity involved is relatively small so that the effective concentration of sulfite in the borax developer remains practically constant with use.
(c) Sodium sulfite is also a solvent for the silver bromide in the emulsion and forms a silver bromide-sodium sulfite complex salt which is soluble in developing solutions. This solvent action goes on as a side reaction during development and the silver complex is then slowly reduced or developed to metallic silver which settles out on the walls of the developing tank and precipitates in the developer as a gray sludge. This sludge formation is a secondary reaction which both depletes the supply of developing agent and adds more restraining products to the developer.
2. Effect of Use on the Rate of Development. The experimental developers were exhausted in a miniature system with racks holding 50 inches of motion picture film, and tanks holding one-half gallon of developer. The data on exhaustion and revival, however, which are given below were obtained with the developer under commercial working conditions in a 120-gallon tank. The life history of several of these developers was followed by H. & D. methods with special emphasis placed on the changes that occurred when the developer was revived. Fig. 6 shows H. & D. curves obtained by developing for fifteen minutes at various stages throughout the life of a 120-gallon tank of borax developer.
An exhaustion curve for another batch of developer is given in Fig. 7. The gamma values and relative speed values are plotted against the quantity of film processed per gallon.
During use, the supply of developing agents in the borax developer is gradually depleted as a result of the development process itself and the process of sludge formation so that obviously the rate of development decreases with use. Referring to Fig. 6, for a given exposure when developing for 15 minutes, the density dropped from 1.90 to 1.64 after processing 80 feet of film per gallon. It was then necessary to increase the development time to 18 minutes to produce the density of 1.90 produced originally in 15 minutes.