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May, 1930] STOP BATHS AND FlXING BATHS 491
taining 0.1 per cent sulfuric acid and 2 per cent chrome alum was determined, using the same procedure as above for neutral film.
From Fig. 2 it is seen that as the concentration of sodium sulfite increases, the acidity decreases and the hardening effect increases. On standing for three weeks, a decrease in hardening occurred due to an increase in acidity of the solution. From curves A and B it is seen that maximum hardening is obtained when the solution has a pH of 4.0 on ageing. Curve C was obtained by adjusting the acidity of the solutions to a pH of 4.0 each time before measuring the hardening. The results indicate that the hardening action is largely dependent upon the acidity or pH value of the solution.
It was also noticed that on the addition of sodium sulfite to the chrome alum solution the original violet color changed to a deep green by transmitted light and it was considered that this color change was correlated in some way to the decrease in the hardening properties of the bath which resulted. From extensive tests, however, it was concluded that the hardening properties were determined largely by the degree of acidity or pH of the solution. This matter will be discussed more fully in Part II of this paper.
Effect of Addition of MQ-25 Developer on the Hardening Properties of Chrome Alum Stop Baths. — To test further the factors favoring the hardening action of chrome alum solutions, increasing quantities of MQ-25 developer* were added to several stop bath formulas and the hardening properties tested in the same manner as above with neutral and alkaline film over a period of several days.
In the case of a plain 2 per cent chrome alum stop bath (Fig. 3) the hardening properties with alkaline film and the acidity of the solution decreased as the quantity of developer added increased. A minimum degree of hardening was obtained with neutral film in the plain solution but the hardening increased on the addition of a small quantity of developer and then decreased as the proportion
*MQ-25 Developer
Metric Avoirdupois
Elon 1.25 grams 18 grains
Sodium sulfite (desiccated) 75.0 grams 2!/2 ounces
Hydroquinone 3.75 grams 54 grains
Sodium carbonate (desiccated) 25.0 grams 365 grains
Potassium bromide 1.5 grams 22 grains
Water to make 1 .0 liter 32 ounces