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May, 1930] STOP BATHS AND FIXING BATHS 487
Acidity Measurements. — The acidic properties of an acid solution are attributable to the presence of free hydrogen ions. In a strongly acid solution there are many more of these ions than in a weakly acid solution so that the hydrogen ion concentration may be considered as a measure of the acidity of the solution. Acidity is usually expressed by the symbol "pH" which is equal to the logarithm of
the reciprocal of the hydrogen ion concentration or pH = log H ion CQnc •
An alkaline solution has therefore a relatively high pH value as compared with an acid solution.
In the present investigation, the pH or the acidity of the various chrome alum solutions was determined by the colorimetric method which depends upon the fact that the color or light absorbing properties of solutions of certain organic compounds (indicators) vary with their degree of acidity or alkalinity (hydrogen ion concentration). For instance, methyl orange is yellow at pH values lower than 4 but the dye changes color to orange at a pH of 4.0. The indicator, brom phenol blue, is yellow at a pH of 3.0 and purple at 4.6. At intermediate pH values the dye has definite intermediate shades. Therefore, by comparing the color of any solution with a set of standards having known pH values, the pH value of the test solution is obtained.
The indicator solution used in the present investigation was made by dissolving 0.4 gram of brom phenol blue in 75 cc. of a 0.05 per cent solution of potassium hydroxide and adding enough water to make one liter of solution.
Preliminary pH measurements were made with plain 2 per cent chrome alum solutions containing increasing quantities of developer.
TABLE I. Effect of Acidity on Color of Indicator
Nature of Solution
Color
PH
2% Chrome alum
Reddish yellow
3.2
2% Chrome alum +
4%D-16
Bluish red
4.0*
2% Chrome alum +
7% D-16
Purple
4.6*
Two cc. of each solution were added to 5 cc. of distilled water containing 10 drops of the indicator solution. These solutions were placed in test tubes of equal diameter and then arranged in a single line in a rack before an illuminator containing a tungsten bulb