696 DONNAN EQUILIBRIUM AND OSMOTIC PRESSURE 



From equation (2) follows 



y- 



The osmotic pressure of the gelatin solution should therefore be cal- 

 culated from the following values (omitting the share of the osmotic 

 pressure due to the gelatin) 



3 3 3 



2-^2 2 



///. The Calculated Curves. 



Solutions containing 1 gm. of originally isoelectric gelatin in 100 

 cc. and containing different quantities of acid were prepared. Col- 

 lodion bags cast in the form of Erlenmeyer flasks of 50 cc. volume 

 were filled with the 1 per cent solution of a gelatin-acid salt and put 

 into a beaker containing 350 cc. of H2O. In order to accelerate the 

 establishment of the equilibrium between inside and outside solutions 

 a certain amount of acid was added to the outside water {e.g.^ HCl 

 in the experiments with gelatin chloride, H3PO4 in the experiments 

 with gelatin phosphate, etc.). Each Erlenmeyer flask was closed 

 with a rubber stopper perforated by a glass tube serving as a 

 manometer. All this was described in more detail in previous 

 publications. 



In Fig. 2 are plotted the values of the calculated osmotic pressures 

 for 1 per cent solutions of gelatin chloride, gelatin phosphate, and 

 gelatin sulfate, and Tables I, II, and III give the data on the basis 

 of which the curves in Fig. 2 are calculated. The experiments from 

 which these calculations were made are identical with the experi- 

 ments from which the curves for the observed osmotic pressures in 

 Fig. 1 were plotted. The abscissae in Fig. 2 are the pH in the inside 

 solution at the point of equilibrium, the ordinates are the values for 

 osmotic pressure calculated on the basis of the Donnan equilibrium 

 as discussed before. The reader will notice that the three curves 

 plotted in Fig. 2 show not only the same qualitative characteristics 

 as the curves for the observed osmotic pressures in Fig. 1, but show 

 them almost quantitatively; except that a correction for the value of 

 osmotic pressure due to the gelatin particles itself may have to be 



