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C. L. BRIGHTMAN, MEACHEM AND ACREE 



The average ratio of these two values was computed, and from 

 this ratio we computed the value of the change in hydrogen ion 

 concentration when the salt concentration changes from 0.166 N 

 to 0. These changes were measured off on the curve sheet, the 

 points being indicated by A, and a curve E was drawn through 

 these points. The slope of this line can be called the ionization 

 constant of the phenolsulfonphthalein. The value as determined 

 from this curve is 1.93 X 10 -8 (table 2). In the same way Curve 

 D was obtained by taking given values of hydrogen ion concen- 



TABLE 1 



tration between 10 ~ 7 and 2.6 X 10 _7 and using the changes in 

 (1 — a) /a freed from "salt effects." The ionization constant 

 obtained from curve D is 1.98 X 10 ~ 8 (table 3). 



If this graphical method is approximately correct, curves D 

 and E should coincide and the corresponding ionization constants 

 should be identical. That such is found to be approximately 

 true gives confidence in this method as a provisional one. The 

 decrease in ionization constant is about that expected from our 

 theory of these indicators and justifies the use of the simplified 

 equations given above in place of the more complicated ones 

 actually applying to the dibasic sulfonphthalein series. The 

 decrease in ionization constant with decrease in sodium and other 



