128 



Scientific Proceedings (112). 



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 trans- 

 mitted horizontally through the box thus presents the complete 

 range of color change of the indicator. For purposes of calibrating 

 the color scale in terms of P H , buffer solutions of known hydrogen 

 ion concentration and containing the same indicator concentration 

 were placed in a small glass box having the same fluid diameter 

 as the large box. For any given buffer solution within the range 

 of the indicator an exact color match is obtained. A scale along 

 the lower edge of the comparator is divided into 100 parts and 

 graduated from left to right. If the acid color of the indicator 

 occupies the left end of the comparator, the readings of this scale 

 will thus represent the percentage of alkaline indicator color 

 present in the color blend observed at that point. The colors are 

 best viewed against an oblique plate mirror reflecting the sky. 

 The buffer solutions used were the phthalate, phosphate and 

 borate mixtures of Clark and Lubs, 4 and their P H values were 



1 Conn, H. J., Harding, H. A., Kliglcr, I. J., Frost, W. D., Prucha, M. J., and 

 Atkins, K. N., 1919, " Methods of pure culture study," Jour. Bad., iv, No. 2, 107-132. 



1 Gillespie, Louis J., 1920, " Colorimetric determination of hydrogen ion concen- 

 tration without buffer mixtures, with special reference to soils," Soil Science, ix, 

 No. 2, 115-136. 



'Clark, W. M. and Lubs, H. A., 1917, "The colorimetric determination of 

 hydrogen ion concentration and its applications in bacteriology," Jour. Bad., i. 



4 Clark, W. M. and Lubs, H. A., 1916, "Hydrogen electrode potentials of 

 phthalate, phosphate and borate buffer mixtures," Jour. Biol. Chem., 1916, xxv, 479- 

 Sto. 



