Journal of the Society of Motion Picture and Television Engineers (1950-1954)

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tion using only slightly more than the theoretical current consumption, the loss in current efficiency being due to the evolution of heat. However, reaction (5) never goes to completion and in reducing 85 to 90% of the trivalent vanadium to the divalent state, current efficiencies of 50% or less prevail. This severe drop in current efficiency is the result of competition between the following reaction and reaction (5) at the cathode, 2H+ + 2e = H2. (6) Normally, it would be expected that reaction (6) would be predominant, since the standard emf's of the two systems would indicate that hydrogen ion would more readily accept an electron than would trivalent vanadium ion. However, owing to the fact that the lead cathode has a very high overvoltage to the hydrogen-forming reaction, trivalent vanadium will be more readily reduced. When about 70% of the vanadium has been reduced to the divalent state, hydrogen gas begins to form at the cathode, and the rate of formation increases quite rapidly as electrolysis proceeds. It has been found impractical to carry the electrolysis beyond the point where 85% of the vanadium has been reduced, since current efficiency at this point is extremely low. From a photographic standpoint, further electrolysis would be unnecessary since the developing capacity of the solution at this point is ample. During the early stages of the electrolysis, oxygen forms at the anode, 2H20 = 02 + 4H+ (7) but, as electrolysis proceeds, a certain amount of bromide ion diffuses through the porous cup from the cathode compartment and is immediately oxidized to bromine. 2Br~ = Br2 + 2e. (8) The amount of diffusion of bromide ion that occurs during the preparation 0.5 15 0 0.50 1.00 Log E Fig. 1. Characteristic curves produced by vanadium development of Eastman Fine Grain Release Positive Film, Type 5302. Development times of 25, 30, and 35 seconds at 60 F are represented by Curves 1, 2, and 3. These are compared with development times of 3, 5, and 8 minutes at 68 F in Kodak D-16 Developer, producing Curves 1A, 2A, and 3A. of small batches of solution does not significantly change the composition of the developer. In the case of a longterm electrolysis, proper balancing of the concentrations of the starting components can be made to balance the effect of the electrolysis. Properties of Vanadium Developer The solution thus prepared is an extremely active developer for emulsions such as Eastman Fine Grain Release Positive, Type 5302. As shown in Fig. 1, developing for times ranging from 25 to 35 sec will produce sensitometric curves very similar to those produced in 3 to 8 min in a conventional Metolhydroquinone developer, such as Kodak D-16. The vanadium development gives substantial increases in speed, amounting to more than twice that January 1954 Journal of the SMPTE VoL 62