713 



gradually the anhydride disappears by conversion into the acid and 

 so we again finish with the acid-sohibility. 



This exj)Ianati()n, howeN-cr cannot bo satisfactory, for it is assumed 

 herein that the })hase equilibria set in while the reaction equilibrium 

 is modifying the condition in the homogeneous solution. Thus we 

 have a pseudoternary system: water- an hydride-acid (W-A-Z). In 

 fig. 1 has been drawn the solubility -isotherm, the equilibrium line 

 will about coincide with the axis WZ. The position of a and b is 

 given by the assumed ditference in solubility between the acid and 

 the anhydride. If now we shake water and anhydride the solution 

 will about follow the lines Wh and Ac; in c the solubility of the 

 phthalic acid has been attained and if then a fall occurs in the 

 total solubility this means to say that solid phthalic acid huf^ depoaiteó. 

 But then the equilibrium is non- variant (p,t) and hence a fall cannot 

 take place immediately after a rise; no continuous maximum can 

 follow, but a long stop at the highest concentration must occur. A'ow 

 this was not observed by van de Stadt ; on repeating the experiment 

 at 20° I also noticed that the concentration of a solution when shaken 

 with phthalic anhydride in a rotating llask did not come to a 

 standstill, but passed continuously through a maximum value. 



In Table I this frequently repeated experiment is indicated. ^V/IO 

 sodium hydroxide was used for the titration with phenolphthalein 

 as indicator. 



The solubility of phthalic acid is 35.2 millimols p. litre; it was 

 always attained the next day. 



46* 



