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1949 SPRAY PROCESSING
ory. On the other, hand, additions of potassium chloride to the following developer:
Elon 1.72 grams
Hydroquinone 3.3 grams
Sodium sulfite, anhydrous 20.0 grams
Sodium Carbonate 21.2 grams
Water to 1 liter
pH adjusted to 10.0
up to 100 grams per liter seemed to cause an initial increase in the rate of autoxidation of the hydroquinone. Increasing the potassium chloride concentration up to 200 grams per liter caused the autoxidation rate to fall back to its initial value. Similar results were obtained when potassium nitrate was used to load the developer. If it is assumed that loading the developer in this way does reduce the solubility of oxygen, then these results for the positive-type developers favor the view that under these circumstances it is not the solubility of oxygen but some other factor that sets the maximum rate of autoxidation on increasing the degree of aeration. To deal with this matter satisfactorily it will be necessary to measure the solubility of oxygen in the developer solutions.
The fact that, at pH 10.5, no maximum rate of autoxidation was found on increasing the degree of aeration, and the fact that the slope of the curve is much steeper than it is at pH levels up to 10.0, indicate that the pR of the developer should not be raised above 10.0* in systems involving considerable aeration.
Influence of Sulfite Concentration on Rate of Oxidation
The effect of varying the sulfite concentration was studied in the case of the positive developer at pH. 10. No special study of the negative developers was made in this connection because a very high sulfite concentration is maintained in them for reasons other than the prevention of autoxidation. However, it will be seen from the results in the previous sections that, at an air flow of 1250 cubic centimeters per minute, 5 per cent of the hydroquinone was oxidized in 30 minutes when the sulfite concentration was 100 grams per liter. The same percentage loss of hydroquinone occurred in 15 minutes when the sulfite concentration was 20 grams per liter. Thus the rate of oxidation w% halved on increasing the sulfite concentration from 20 to 100 grainy per liter. n;,
* See note on page 674. •• •