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(582 LEVENSON December
fiuence on the rate of autoxidation. On circulating D76d in the S-R Spray system, with the air-return tube in position, it was found that the oxygen content of the air did not deviate much from that of normal air (top curve, Fig. 10) . On flushing the cabinet with cylinder nitrogen the oxygen content of the air was reduced from the normal 20 per cent to 4 per cent and maintained at this level by a nitrogen flow of 2 liters per minute. The rate of autoxidation of the developing agents was the same at this low oxygen level as it was in normal air. The reason for this emerged later, and will be described below.
In carrying out runs with the positive developers, the solutions were replenished, because of their much more rapid rate of autoxidation, in order to maintain them at substantially constant composition, especially in respect to developing-agent concentration and pH.. Fig. 10 shows the change in oxygen content of the air in 3-hour runs. When the air-return tube was in position, the oxygen content fell toward equilibrium at about 15 per cent at pR 10.0. At pR 10.5 the oxygen content reached 10 per cent, and was still descending rapidly. When the air-return tube was closed the reduction in oxygen content was much less. At pH. 10 the equilibrium value would appear to be at about 18 per cent, and perhaps 17 per cent at pH. 10.5.
These values would be markedly affected in practice if a compressed-air knife were used in the developing cabinet to prevent carryover of developer. -The imperfect correlation between the relative rates of developer oxidation and rates of oxygen depletion, probably due to changes in the degree of closure of the cabinet, makes it necessary to regard the curves in Fig. 10 as being of illustrative significance only.
Influence of Degree of Aeration on Rate of Autoxidation
On considering the fact that, in view of the high degree of aeration produced in the spray cabinet, some of the rates of loss of developing agents are lower than might be expected, and the fact that reducing the oxygen content of the air to 4 per cent had no influence on the rate of autoxidation of D76d, it seemed likely that the degree of aeration involved was sufficient to supply the developer with oxygen faster than the oxygen could be removed by reaction with the reducing agents. Thus, under the conditions of spray processing, the rate-determining factor would no longer be the degree of exposure of the solution to air. This hypothesis was checked experimentally and was found to be in keeping with the experimental results.