Hydrogen Ion Concentration. 



129 



checked with the hydrogen electrode, following the method of 

 Clark, 1 and using the tables of Schmidt and Hoagland. 



The comparator was made in the laboratory from the glass of 

 discarded X-ray plates jointed with balsam. Since only extremely 

 weak acid and base have been used there have been no leakage 

 difficulties. The boxes are carefully dried with filter paper after 

 each day's use. 



Dimensions: Inside length 35 cm. 



Fluid diameter 15 mm. 

 Height 2.5 cm. 



Gillespie has shown that the indicator transformations follow 

 the law of mass action within the limits of error of the method, 

 and has calculated "apparent dissociation constants" for each 

 of his observations from the modified mass-law equation 



K = P H + log 



H & 100 - x 



where x/(ioo — x) represents the partition ratio of the indicator 

 in per cent. Similar values are given in the tables below. This 

 constant is the P H value of the mid-point of the indicator, i.e. 

 the P H value at which the indicator is half transformed from acid 

 to salt form. 



It will be noted that the constants above calculated show 

 somewhat less deviation than those of Gillespie, probably because 

 the method permits an exact color match, no interpolation being 

 necessary. That our values are slightly higher than those of 

 Gillespie is doubtless due to the fact that our measurements were 

 made in the close neighborhood of 20 0 , instead of at the higher 

 temperatures he used. The greatest discrepancies are with brom 

 phenol blue and brom cresol purple, with which we have had some 

 difficulty in obtaining a perfect color match. 



In order to determine the P H of unknown solutions we may 

 construct a table or curve for each indicator, giving the theoretical 

 value of the P H for scale readings at convenient intervals. It is, 

 however, more convenient to graduate the scale directly in P H 



1 Clark, William Mansfield. 1915, "A hydrogen electrode vessel," Jour. Biol. 

 Chem., 1915, xxiii, 475-486. 



2 Schmidt, Carl L. A. and Hoagland. D. R., 1919. "Table of Ph, H, and OH 

 values," Univ. California Publications in Physiology, Vol. 5, No. 4, pp. 23-69. 



