THE pH O F GELATIN SOLUTIONS J. GILBERT MALONE
AND
MARY G. MALONE
Department of Chemistry, University of Michigan, Ann Arbor, Michigan Received August 5, 1931
The authors are led by the recent paper of Hartman and Fleischer (1) to make the following comments. The pH of a solution is theoretically rather poorly defined, since the idea of concentration has lost its early significance. The usual accepted methods of determination and comparison are useful and satisfactory in many cases, but as is well-known they are subject to influences which more or less invalidate the results obtained. Thus, the hydrogen electrode is usually considered the most fundamentally sound, with the colorimetric methods subject to errors due to influences such as salt, alcohol, and proteins. It is therefore surprising to come upon an article where the colorimetric methods are given preference over the electrometric methods. I n such a case substantial proof would appear to be necessary before the conclusions can be accepted. Hartman and Fleischer have determined the pH of gelatin solutions by several methods, These included the use of two sets of colorimetric buffers and indicators, the use of the hydrogen and quinhydrone electrodes, and the use of the glass electrode, although in this latter case the pH values were not determined, as sufficient data was lacking. With the colorimetric methods the values agreed, but with the electrometric methods, especially in the case of the hydrogen electrode, variation with time was enormous. With the quinhydrone electrode the values obtained a t different times agreed well, but did not agree with the values obtained colorimetrically. The results obtained with the glass electrode varied with time, but not to the same degree as shown by the hydrogen electrode. The authors have had occasion to determine the pH of gelatin solutions over a rather wide range in connection with a study of the dielectric constant and dispersion in gelatin solutions. They have used the glass electrode as a means of making these determinations and have no reason to doubt the accuracy of the records obtained. A vacuum tube voltmeter using the FP 54 tube and following the general lines of Hill’s apparatus (2), was constructed through the cooperation of Prof. H. H. Willard. The apparatus was shielded by means of metal boxes and the grid circuit was 219
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J. GILBERT MALONE AND MARY G. MALONE
insulated by means of amber. The electrodes were constructed after the specifications of MacInnes and Dole, using for the membranes the special Corning glass .015 manufactured for this purpose. Saturated calomel was used as a reference electrode. In all cases two glass electrodes were used and the connection to the grid was changed from one to the other by means of a long glass rod so as to avoid polarization effects while the electrode shielding cage was open. The electrodes were calibrated against standard buffers and it was found that a linear relation was approximated. The readings on any solution could be changed to pH by means of this calibration. That no drifting occurs may be seen from the determinations (see table 1) made of the pH of a solution of Eastman’s isoelectric gelatin and distilled water at widely different times. These values were taken only with the idea in mind of getting an accuracy of about 0.1 pH. Therefore recalibration of the electrodes each day was not made as is essential TABLE 1 The pH of a solution of isoelectric gelatin GlL
per cent
I
pH
A0 OIVEN BY ELECTRODl
I
1
4.69 4.60 4.61 4.65
4.70 4.70 4.68 4.69
0.5 1.0 1.0 0.5
2
in very accurate work. However, if the glass electrodes vary as stated by Hartman and Fleischer, it is hardly conceivable that they will vary together, therefore the authors give the following values, taken consecutively and without the added refinements that would go with determinations of pH in other connections. These are stated without regard for the content of gelatin acid or alkali, the pH having been changed by the addition of hydrochloric acid or sodium hydroxide. 2.72 5.67 4.67 5.86 8.20 4.19 3.68 3.96
2.71 5.75 4.70 5.88 7.29 4.20
3.64 3.91
It would be possible to continue, as the authors have some hundred such determinations. In the list, there appears to be only one case where a pH disagreement of more than .1is found, namely, with 7.08 and 6.96.
THE
pH
OF GELATIN SOLUTIONS
22 1
It has been found by Ferguson (4)and his students as well as by others that the pH values determined by the hydrogen electrode are reproducible and constant. The authors are fully aware that mere agreement does not constitute a sound basis establishing the fundamental accuracy of a method. They do not a t this time contend that the values obtained in gelatin solutions give the proper measure of the hydrogen-ion activity. They do however contend that, properly used, the glass electrode will give definite and reproducible values in gelatin solutions and, as contrasted with colorimetric determinations, the presumption of accuracy is in favor of the electrometric methods. REFERENCES (1) HARTMAN AND FLEISCHER: J. Phys. Chem. 36, 1136 (1932).
(2) HILL:Science 73,529 (1931). (3) MACINNES AND DOLE:Ind. Eng. Chem., Anal. Ed. 1,57 (1929). (4) FERQUSON A N D SCHLUCHTER: J. Gen. Physiol. 16,463-75 (1932).
