RECENT ADVANCES IN SCIENCE 213 



Typical mono-, di- and tri-basic acids were investigated, as well 

 as typical monohydric, dihydric, and trihydric phenols. The 

 results are most conveniently shown by means of a graph in 

 which the abscissse denote the normality of the " solvent ' 

 acid (HC1 etc.), the ordinates denoting the corresponding 

 solubility (expressed in normality) of the dissolved acid or 

 phenol. The curves obtained are of two main types according 

 as the " solvent " acid is a mineral acid or an organic acid, but 

 in each case the assumption of oxonium salt formation is 

 sufficient to account for the observed results. In the first 

 type — e.g., phenyl acetic acid in HC1 solution — the solubility of 

 the organic acid diminishes rapidly at first, reaches a mini- 

 mum, and afterwards increases steadily with increasing con- 

 centration of the solvent acid. The decrease in solubility is 

 naturally ascribed to the mass action effect brought about by 

 the presence of the common ion, the hydrogen ion. At the 

 same time the oxonium salt is formed, and its greater solu- 

 bility eventually more than compensates for the common ion 

 effect, with the result that the solubility curve turns upward. 

 When the " solvent " acid is an organic one, a different type 

 of curve is obtained. Two factors have to be taken into 

 account : (1) the weakness of the organic acids in general ; and, 

 (2) the wide difference between the solubilities of the " dis- 

 solved " acid in the water and the organic acid " solvent." 

 As regards the first factor — since both the " solvent " acid and 

 the !.! solute " acids are weak, the diminution in solubility, 

 due to the common hydrogen ion, is too small to be detected, 

 except in the case of oxalic acid, which exhibits a slight initial 

 diminution in solubility. The other " solute " acids investi- 

 gated show an increase in solubility from the beginning, except 

 tartaric acid dissolved in acetic acid solution, and oxalic acid 

 in lactic acid solution, in which cases no evidence of union was 

 obtained. As regards the second factor — if the dissolved acid 

 is more readily soluble in the " solvent " organic acid than in 

 water — the resulting curve shows a steady rise, and no definite 

 conclusion can be drawn from it. This is realised, for ex- 

 ample, in the case of suberic acid in acetic acid solution. 

 Other cases occur in which the solubility of the dissolved 

 acid rises, passes through a maximum, and then falls. This is 

 clear evidence of oxonium salt formation, the initial increase 

 in solubility being due to the formation of the salt, which after- 



