HETEROGENEOUS EQUILIBRIA 



259 



assumption that the initial depression in the solubility of the acid is due 

 to the depression of its ionization. Acetic acid is so weak that, even at 

 fairly high concentrations, it has no appreciable effect on the ionization 

 of salicylic acid, and consequently the resulting curve merely measures 

 the increase in the solubility of the un-ionized fraction. In the case of 

 hippuric and salicylic acids in formic acid, the added acid is sufficiently 

 strong to practically completely repress the ionization of salicylic acid 

 present in solution. In these cases, therefore, there is an initial decrease 

 in the solubility, while finally, when the ionization is completely repressed, 

 the solubility is increased, owing, presumably, to the increased solubility 

 of the un-ionized molecules of the first acid on addition of the second. 

 By extrapolating the linear solubility curves backwards, until they inter- 



Per Cent of Added Acid. 

 O I e y 4 s 6 7 s 9 to 



o.ois 



02 



O.Olf 



FIG. 50. Solubility of Weak Organic Acids in Water in the Presence of Other 



Organic Acids at 25. 



sect the axis of solubility, the intercepts on this axis correspond approxi- 

 mately to the solubility of the un-ionized fraction in pure water. 



It will be noted that the solubility of salicylic acid and of orthonitro- 

 benzoic acid is depressed according to the requirements of the mass-action 

 law not only on addition of weak acids, but also on addition of hydro- 

 chloric acid. In this case, the solubility of the un-ionized fraction in the 

 more concentrated solutions decreases slightly with increasing concen- 

 tration of hydrochloric acid. The initial depression effect is marked, 

 particularly in the case of orthonitrobenzoic acid, which is a fairly soluble 

 acid. Apparently, the addition of a strong acid to a solution of a weak 

 acid, as well as the addition of a weak acid to a solution of a weak acid, 

 does not greatly alter the ionization constant of weak acids. The ioniza- 

 tion constant of salicylic acid at 25 is 1.02 X 10~ 3 ; that of hippuric acid 

 is 2.22 X 10- 4 ; and orthonitrobenzoic acid 6.16 X 10" 3 . 



