﻿632 Mr. Bernard Oavanagh on 



Since, further, at these middle concentrations the density 

 of the solution is expressible as a linear function of the con- 

 centration, so that the " partial-molal volume V v of the 

 solute is a constant (at constant temperature), 



V = V + u, (90) 



where V is the volume of solution containing one gm. -molecule 

 of solute, and (89) can take the form 



P[V-6]=RT, (91) 



where b is constant at constant iemperature, and 



6=[*+^ + l/8aT] (92) 



Equation (91) was put forward by 0. Sackur *, in analogy 

 with van der Waal's equation for imperfect gases, and was 

 shown to represent the data for middle concentrations. We 

 see that for a perfect solute this equation can be predicted 

 for such concentrations. 



Now consider a slightly soluble solute s in a solution con- 

 taining also a " second " solute in concentration C, relative 

 to which c s is negligible. 



If Gr s is negligible, we have 



!^=<k-Rlogc. + J, (93) 



on s 



In solubility measurement we have the solution in equi- 

 librium with a phase consisting entirely of the solute s at 

 constant temperature and pressure, so that 



log c s — jj J s ( = log c,7«) = constant, . . (94) 



where c s is the solubility in presence of the concentration C 

 of the "second" solute. If c 8o be the solubility in absence 

 of a " second " solute, we have 



logc, =logc,-=gj„ (95) 



or log^(=log 7s ) = -^J s . .... (96) 



For concentrations at which the " second approximation " 

 suffices, we expect then to find 



log^=^C=(i + gC, . . . . (97) 



where t/ is a constant, since t depends practically entirely 

 * Zeitschr.J. Phys. Chem. (1910). 



