Hydrogen Ion Concentration. 



131 



intervals of 0.1 on each side of the mid-point. This may be done 

 by giving to the quantity log x/(ioo — x) successive values from 

 — 0.9 to +0.9 in intervals of 0.1. From these equations corre- 

 sponding values of x are readily obtained : 



Log — - — 









100 — X 

 (P a Difference). 



_*r(Scale Reading). 



Log — — — . 



100 — X 



X 



-0.9 



II. 2 



+0.1 



55-7 



-0.8 



13-7 



+0.2 



61.3 



-0.7 



16.6 



+0.3 



66.6 



-0.6 



20.1 



+0.4 



7i-5 



-0.5 



24.O 



+0.5 



76.0 



-0.4 



28.5 



+0.6 



79.9 



-0.3 



33-4 



+0.7 



83-4 



— 0.2 



38.7 



+0.8 



86.6 



—0.1 



44-3 



+0.9 



88.8 



0.0 



50.0 







If these values are indicated on the scale by the figures repre- 

 senting P H differences, we may read the P H value of an unknown 

 solution by adding to the mid-P H value (dissociation constant) 

 of the indicator the differential quantity indicated by the scale 

 reading obtained. Thus, if a scale-reading midway between 

 +0.4 and +0.5 is obtained, using methyl red, the P H of the 

 solution will be 5.01+0.45, or 5.46. With reasonable care the 

 error of such a reading is certainly not greater than 0.02 P H , 

 especially in the region of the mid-point, where the indicators are 

 most used. Compensation for colored or turbid solutions is made 

 by placing a small glass compartment of the same fluid diameter 

 behind that portion of the comparator in which the match is to 

 be obtained. 



Summary. 



1. The method of determining hydrogen ion concentrations 

 colorimetrically without the use of buffer solutions is extended to 

 the group of indicators described by Clark and Lubs. Values of 

 the dissociation constant at 20 0 of each of the indicators are given. 



2. A double glass wedge comparator is described for making 

 such determinations. 



