IMPORTANCE OF WATER 



121 



concentrations of the anions and the cations of the readily diffusing 

 salt must be equal on both sides of the membrane. Schematically, 

 this may be represented as follows : in the initial state, the follow- 

 ing concentrations may be on both sides of the membrane, denoted 

 by a vertical line : 



Na+ 

 Ci 



Proteinate 

 Ci 



Na+ 

 C 2 



CI 



C 2 



Then, in the state of equilibrium, if the anions of protein cannot 

 pass through the membrane, while the other ions pass readily, we 

 shall have 



Na+ 

 Ci + X 



Proteinate 

 Ci 



X 



Na+ 

 C 2 - X 



ci- 

 c 2 - x 



According to Donnan, at equilibrium the products of the concen- 

 trations of the diffusible ions must be equal on both sides of the 

 septum. Then: 



(Ci + X)X = (C 2 - X)(C 2 - X). 



From this equation may be calculated the relation between the ions 

 of sodium and of chlorine on both sides of the septum. This cal- 

 culation shows that the higher the concentration of the sodium 

 proteinate in comparison to the concentration of sodium chloride 

 the less complete will be the equalization of the concentrations of 

 NaCl in the surrounding solution and in the osmometer, and the 

 more it is retained in the surrounding liquid. Let us give some 

 figures illustrating this "membrane equilibrium" of Donnan. 



These figures show that the large amount of sodium proteinate 

 makes the membrane appear as if impermeable to NaCl and this 

 salt ceases to penetrate into the sac of the osmometer. 



