JACQUES LOEB 709 



be calculated from the observed osmotic pressure (Pi) by the follow- 

 ing equation, 



El = C^ + Ci 

 Po Ci + 2C2 



assuming that gelatin is monovalent, which is improbable, where 

 Ci is the concentration of NaCl and C2 that of NaR. If Ci is 

 small compared with C2, P\ = h Po\ if, however, d is small in com- 

 parison with Ci, Pi = Po. It follows from this that the greater the 

 concentration of NaCl (or of any neutral salt) added to a colloidal 

 solution, the smaller the observed osmotic pressure of the colloidal 

 solution becomes, and this is what actually happens. It follows also 

 from this theory that the observed osmotic pressure cannot be 

 depressed below a certain minimum. This seems to agree with the 

 observation that when the solution of the salt reaches the value 

 m/8 a further increase in the concentration of the salt can no longer 

 increase the depressing effect of the salt (see Figs. 1 and 2 in the pre- 

 ceding paper of this series) } To this extent Donnan's theory accounts 

 for the depressing action of the salt on the osmotic pressure. A 

 discrepancy between theory and observed value of osmotic pressure 

 arises, however, in the fact that according to the theory the maximal 

 depression caused by a salt should be | the real osmotic pressure of 

 the colloidal solution. The observed osmotic pressure of a 1 per cent 

 solution of gelatin chloride of pH 3.5 is about 425 mm. H2O while the 

 osmotic pressure in the presence of m/8 NaCl is only about 30 mm. 

 According to the theory the latter value should be at least \ of 425; 

 i.e., 212. 



Donnan's theory can only give approximate results in this case 

 since in his theory complicating factors were intentionally ignored 

 for the sake of simplification. Thus the depressing effect of the addi- 

 tion of a neutral salt on the electrolytic dissociation of the electrolyte 

 is not taken into consideration. 



Northrop^ has shown by conductivity measurements that the 

 degree of ionization of gelatin chloride is noticeably depressed when 

 the pH is 2.0 or below and is considerable when the pH is 1.0. There 

 may be other complicating factors besides this depression of ionization. 



^Northrop, J. H., /. Gen. Physiol, 1920-21, iii, 211. 



