78 HYDROGEN ION CONCENTRATION 



In the supernatant solution we have molecules of undissoeiated 

 benzoic acid, H-ions and benzoate-ions. (In addition to the H-ions 

 there must also be present OH-ions in accordance with the laws of 

 dissociation. But since these play no part in this consideration 

 they may be ignored). Two interpretations may then be given to 

 the expression "solubility of solid benzoic acid." Either it may 

 mean the concentration of those molecules of undissoeiated benzoic 

 acid which are present in the saturated solution, i.e., the concen- 

 tration in the saturated supernatant solution of the molecular 

 species of which the sediment is composed. Secondly, the term 

 "solubility" in this case may be regarded as being denoted by the 

 concentration of all the benzoic acid molecules regardless of the form 

 in which they are present in the saturated solution. We shall 

 designate the first as the "partial solubility," or X, of benzoic acid 

 and the second as the "total solubility," or A. Furthermore, one 

 may also speak of the "solubility of the benzoate ions." It is 

 apparent that the benzoate ions are much more soluble in water 

 than the free benzoic acid, which is easily demonstrated by the 

 greater solubility of alkali benzoates. It is also occasionally stated 

 that in a pure benzoic acid solution the state of saturation exists 

 only for the undissoeiated acid, and that, on the other hand, the 

 benzoic acid solution cannot be saturated in respect to the benzoate 

 ions. But this statement is not in accord with the true concep- 

 tion of solubility. Solubility is represented by the concentration 

 reached by a substance in solution, when the solution is in contact 

 with the solid phase. Thus the definition of solubility includes two 

 contiguous phases, and solubility represents the state of equi- 

 librium between the two phases. Without the contact with the solid 

 phase the solubility of a substance cannot be properly defined. 

 Under certain conditions a metastable supersaturated solution may 

 be obtained without the presence of a solid phase. But as soon 

 as a small amount of the solid substance is added to such a super- 

 saturated solution, the excess of the solute separates out until the 

 true state of saturation is reached. Therefore, it cannot be properly 

 stated that a saturated solution of benzoic acid is not saturated in 

 respect to the ions of the acid. For we are dealing here with the 

 definition of the solubility of the undissoeiated acid alone of which 

 the solid phase is composed. As for the acid anions, the amount of 



