A New Method of Determining Solubilities Based on Stability of Phthalate Buffers of Low pH at Low Temperatures.

The disconcerting finding that buffers of potassium acid phthalate-HC1 become more alkaline on standing led to a consideration of the cause of this change and to the discovery of a new method of determining solubilities.

The buffers were made according to the directions given by Clark. 1 The immediate determination of the pH of these buffers by the quinhydrone electrode gave values close to those expected, but subsequent pH determinations showed that a change had taken place on standing. This was particularly true at 13°C., whereas at 20°C. the changes were not great. A deposition of crystals could be observed accompanying the change in the pH of the buffers and this crystallization seemed to be proportional to the degree of change in the pH values. After the effect had been noticed in one set of buffers a new series was made up and the pH values determined by the quinhydrone electrode within 2 hours after making it up. Each buffer in the series was then divided into 2 pyrex flasks. One set was placed at 20°C. and the other at 13°C. for 48 hours after which the pH values were again determined. The results are shown in Table I.

The results show that the pH changed appreciably at 20°C. in buffers of pH 2.26 and 2.44 but not in the buffers of higher pH value. At 13°C. the pH changed considerably more in each of the above mentioned buffers as well as changing definitely in the buffers up to pH 2.80. The exact change in C_H+ after 48 hours is shown in column 3 of the table. The buffers showing crystal formation are indicated in the table by the asterisk (∗) and seem to coincide with high values for change in C_H+.