DETEBMINATION OF H-ION CONCENTRATION 



593 



more correct measurement of an unknown when it matches or 

 nearly matches pair no. 1, or pair no. 7, of the "color standards." 

 The actual pH which the "acid" and "alkaline" tubes repre- 

 sent cannot be assigned to an imknown: because the unknown 

 may be much more acid or much more alkaline than the "acid" 

 or "alkaline" tubes at either end of the set although it does not 

 show a different color. The value of these end tubes therefore 

 lies in the fact that there is a distinct difference in color between 

 the "acid tube" and the first pair, of the set on one side, and the 

 "alkaline tube" and the last pair (pair no. 7), at the other end. 

 The color of an imknown can be better judged when found to be 

 between the "acid tube" and the ^rs< pair, or between the laU 



T.\BLE 2 



Clark's table giving the amounts in cubic centimeters of N/iO NaOH per decigram 

 (0.1 gram) of indicator powder 



pair and the "alkaline tube," but such judgment should be con- 

 firmed by the use of another indicator next to the one used. 



It was thought best to bring together all the data necessary 

 for the preparation of the "color standards" with the additional 

 "end tubes" in the form of a table, which would also show the 

 pH values which these "color standards" represent. This will 

 facilitate the use of the colorimetric detennination by means of 

 the "color standards" described in the previous article and the 

 additional colors described in the present article. These data 

 may be found in table 1. 



The author's attention has been called to certain minor 

 difficulties which various workers have experienced in using the 

 "color standards" described in the previous article already 



