SALT EFFECTS OF PHOSPHATES 



179 



on the limiting and optimum hydrogen ion concentration used 

 by the organism, but also upon the concentration and ionization 

 constant of the acid or base generated by the organism, and the 

 proper selection can be made only after the preliminary study of 

 the generated acid or base by (a) successive titrations with suit- 

 able indicators covering the P H ranges involved or by (b) making 

 a titration curve for the resulting medium with the aid of a hydro- 

 gen electrode. A fuller discussion of these points will be given 

 in another article now completed. 



TABLE 4 



P H Correction to be added to colorimetric P H for different concentrations of 

 phosphate in order to obtain the electrometric P H 



CONCLUSIONS 



1 . It is shown that phosphate solutions varying in concentra- 

 tion from N/2 downward and showing the same hydrogen ion 

 concentration by the hydrogen electrode have marked influence 

 on the color of phenolsulfonphthalein salts. This "salt effect" 

 becomes small in phosphate solutions having low concentrations 

 from 0.05 N down. 



2. The apparent ionization constant of the phenolic group of 

 the phenolsulfonphthaleins varies with the concentration of the 

 phosphate present and averages about 2.65 X 10 -8 when uncor- 

 rected for "salt effect." 



3. When a graphical method is used for calculating the ioni- 

 zation constant freed from "salt effects" of phosphates, the value 

 is lowered to about 1.95 X 10~ 8 . 



