Dr. F. Tinker on Osmotic Pressure. 



43L 



It is to be noted also that these assumptions are only valid 

 for the interior of the fluid * ; but it is immaterial whether 

 the fluid be the solution itself, the vapour phase proper, the 

 membrane or any other medium in contact with the solution. 

 To prevent the subject from becoming unduly complicated, 

 it is also assumed that neither the solvent nor the solute is 

 either associated or dissociated, and that no solvates are 

 formed. 



(a) The Partial Pressures of Solvent and Solute 

 inside the Solution. 



Let N mols. of a solvent X having an internal fluid 

 pressure ir\ be mixed with n mols. of a solute Y having a 

 fluid pressure tt 2 so as to give a mixture of (N + w) mols. 

 having a total fluid pressure ir. During the mixing, both 

 of the constituents X and Y will expand into one another 

 until the pressure throughout the mixture is uniform, i. e. 

 until "the mean free space F per molecule of either kind " 

 is the same for each molecule, whether of X or of Y t- 

 Or, using an alternative phraseology, on mixing, the molecular 

 volumes V l and V 2 of the pure solvent and solution both 

 alter themselves in such a way that (V 1 — b{) and (V 2 — b 2 ) 

 readjust themselves to a common volume which can be 

 represented either by Vi —hi or V 2 ' — b 2 , or by the letter F. 



* This is, of course, one Gf the fundamental Dieterici assumptions. 

 The Dieterici equation for the pressure in the interior of a pure fluid 

 is7r(V-&) = RT. 



t For this phrase I am indebted to Prof. A. W. Porter, F.R.S. The 

 conditions before and after mixing can be represented graphically, as 

 below : — 











XXX 

 XXX 

























tit 





mols. of pure solve 

 at press, wi. 



n m< 



)ls. of pure £ 

 at press. 7r 2 . 



olute 



(N+w) mols. of solution at press, ir. 

 Mean free space F the same for each molecule. 



2 G2 



