OSMOTIC PRESSURE 181 



was made at the beginning 3.0 to accelerate the establishment 

 of the equilibrium. The osmotic pressure was read after about 

 20 hours. The temperature was (as always in these cases) 

 24C. 



In Fig. 46 the abscissae are the initial concentrations of the salt 

 solutions while the ordinates are the osmotic pressures. The Don- 

 nan equilibrium caused a change of pH as well as of the dis- 

 tribution of the neutral salts on the opposite sides of the 

 membrane. The change of pH in this experiment has already 

 been discussed in Tables XVIII, XIX, and XX of Chapter 

 VIII. Figure 46 shows that the depressing effect of NaCl and 

 NaN0 3 is practically the same, that the depressing effect of an 

 equimolecular concentration of CaCl 2 is not very far from twice 

 as great as that of NaCl, but that the effect of Na 2 SO 4 where 

 the anion is bivalent is about eight times as great as that of a 

 NaCl solution of the same molecular concentration. This leaves 

 no doubt that the depressing effect is due to the anion and that 

 the cation is seemingly without any influence (it has certainly 

 not any influence in the opposite direction from that of the 

 anion). This depressing influence of the anion of a neutral salt 

 on the osmotic pressure of protein-acid salts can be derived from 

 the Donnan equilibrium equation. 



Omitting that share of the osmotic pressure of the solution 

 which is due to the protein molecules and ions, the share due to the 

 Donnan equilibrium is expressed by the term 



2y + z - 2Vy(y + z) (1) 



Suppose the gelatin be gelatin chloride and the salt added NaCl. 

 Then z is the concentration of Cl in combination with gelatin, 

 while y is the sum of the concentration of the Cl ions combined 

 with the H ions of the free HC1 present in the gelatin solution 

 and the Cl ions of the NaCl contained in the gelatin solution at 

 equilibrium. We can ascertain the total concentration of Cl 

 ions inside the gelatin solution, i.e., the value of y + z in term 

 (1) by tit ration. This term 2y -\- z 2\/y(y + z) will become 

 the smaller, the more closely 



approaches the value 1. 



