Colorimetric determination of hydrogen ion concentration by means of a double-wedge comparator

Abstract

In a former paper 2 a method was outlined for determining hydrogen ion concentration colorimetrically without the use of buffer solutions, and data for making such determinations for P_H values between 6.7 and 8.1 were given, using phenolsulphonephthalein as an indicator. The method consisted in the partition of a constant amount of indicator solution between pairs of test tubes of equal caliber, one tube of each pair containing 5 c.c. of weak acid, and the other tube 5 c.c. of weak base. When such pairs of tubes are viewed by transmitted light in the comparator of Hurwitz, Meyer and Ostenberg a series of colors is observed covering the range of the indicator, each color corresponding to a definite P_H value. A committee of the American Association of Bacteriologists later investigated and reported on the method, 1 and published a similar series of hydrogen ion exponents for brom thymol blue. More recently Gillespie 2 has extended the method to all of the indicators described by Clark and Lubs, 3 determining the P_H values of his tube pairs by comparison with buffer solutions whose hydrogen ion concentrations were checked by the gas chain method. The present work was in progress when Gillespie's paper appeared, and is largely a confirmation of his results.

Instead of dividing the indicator between two tubes, however, use has been made of a comparator consisting of a long narrow rectangular glass box containing a diagonal glass partition dividing it into two equal wedge-shaped compartments placed base to apex. One wedge is filled with acid indicator solution, and the other with alkaline indicator solution of the same concentration. Light transmitted horizontally through the box thus presents the complete range of color change of the indicator.