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1949
SPRAY PROCESSING
687
liter of sulfite is about the optimum concentration for positive developers in highly aerated systems. To halve this rate of loss of developing agents by autoxidation it would be necessary to raise the sulfite concentration to about 100 grams per liter, a concentration that probably would cause considerable development fog and decrease the maximum gamma obtainable. Whether or not any economy can be effected by increasing the sulfite to this level depends, of course, upon the relative cost of sulfite and hydroquinone. The ratio between the prices of these products fluctuates from time to time and varies considerably from one country to another. To illustrate this problem, the case of the L-R Spray system will be considered, with the developer at
It is assumed, as before, that this machine will process 1000 feet of film per hour. The autoxidation cost in terms of hydroquinone is shown by Fig. 8 to be 41 grams per hour. In this time, 5 liters of replenisher would be required and this replenisher would contain the extra 41 grams of developing agent plus, among other things, the sulfite. Analysis showed that in the
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Fig. 13 — These curves, showing the cost of sodium sulfite and developing agent in the positive bath replenisher, indicate that unless sodium sulfite (anhydrous) costs much less than l/n of an equal weight of hydroquinone, it is not worth while increasing its concentration above 60 to 70 grams per liter.
system in question, the loss of sulfite was roughly equal to the two moles per mole of hydroquinone that theory, based on the evidence of Tausch, would require. Thus the concentration of the sulfite in the developer, by influencing the rate of autoxidation of the developing agent, affects, in turn, its own rate of loss.
Using the linear relationship between the sulfite concentration and the rate of loss of developing agent given in Fig. 12, the total cost, in arbitrary units, of sulfite and developing agent per hour, is shown in Fig. 13 for four different hydroquinone-sulfite price ratios. The absolute levels of these curves will depend upon the absolute prices of these chemicals. It will be seen that increasing the sulfite concentration in the bath from 38 to 100 grams per liter, to halve the rate of loss