694 DONNAN EQUILIBRIUM AND OSMOTIC PRESSURE 



HCl solution (free from protein) of a pH of about 3.0, both solu- 

 tions being separated by a collodion membrane. 



Let y be the concentration of the H and CI ions of the free HCl 

 inside a gelatin chloride solution (containing 1 gm. of originally 

 isoelectric gelatin in 100 cc), z the concentration of the CI ions held 

 by the gelatin ions, and a the sum of the gelatin ions and non-ionized 

 molecules of gelatin. For the sake of simplification we assume com- 

 plete electrolytic dissociation of the gelatin chloride and of the HCl. 

 In this case the real osmotic pressure of the inside solution is deter- 

 mined by 



2y -t- z + a 



Since, however, the outside solution is at equilibrium not HoO but 

 a HCl solution — in the example selected of about pH 3.0 — the observed 

 osmotic pressure is the difference between the osmotic pressure of the 

 inside solution against H2O and the osmotic pressure of the outside 

 solution. 



Let X be the concentration of the H ions in the outside solution, 

 then the osmotic pressure of the outside solution is determined by Ix. 



Hence the observed osmotic pressure Po of the gelatin chloride 

 solution is determined by 



Po = 2^ + z + fl - 2* 



Po is observed experimentally, y can be calculated from the pH 

 inside, and x from the pH outside. 



z can be calculated from I)onnan's equilibrium equation in the 

 form given it by Procter^ 



x^ = y{y^z) (1) 



(« + 3') (a; — y) 



where x, y, and z have the significance stated above. We have seen 

 in the preceding paper that this equation leads to correct results in 

 regard to the p.d. 



^Procter, H. R., /. Chem. Soc, 1914, cv, 313. Procter, H. R., and Wilson, 

 J. A., J. Chem. Soc, 1916, cix, 307. 



