682 DONNAN EQUILIBRIUM AND MEMBRANE POTENTIALS 



We therefore come to the conclusion that the depressing influence 

 of the neutral salts on the p.d. between solid gelatin and a surround- 

 ing aqueous solution runs parallel to the depressing effect of the same 

 salts on the swelling of gelatin and that the p.d. can be calculated 

 with the aid of Nernst's formula, on the assumption that the dif- 

 ference in the hydrogen ion concentration inside the gel and in the 

 outside solution determines the p.d. 



V. The Influence of the Hydrogen Ion Concentration of Gelatin Solutions 



on the P.D. 



The osmotic pressure of 1 per cent solutions of originally isoelectric 

 gelatin varies with the pH of the gelatin solution and with the valency 

 of the ion in combination with the gelatin. This is illustrated in 

 Fig. 5 where the ordinates represent the observed osmotic pressures 

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

 oxalate, and gelatin sulfate. The osmotic pressure rises steeply as 

 soon as the pH becomes less than 4.7, reaching a maximum at pH of 

 about 3.6, and then drops steeply with a further decline of pH. More- 

 over, it is obvious that the curves for gelatin chloride and phosphate, 

 both possessing a monovalent anion, are identical, that the curve for 

 gelatin oxalate, which has mainly a monovalent anion at the pH 

 under discussion, is almost but not quite as high as that for gelatin 

 chloride, but that the curve for gelatin sulfate (possessing a bivalent 

 anion) is only about half as high as that for gelatin chloride. The 

 p.d. of 1 per cent solutions of these four salts contained in collodion 

 bags were measured against outside aqueous solutions (without gela- 

 tin) after equilibrium was reached {i.e. after about 18 hours). The 

 bags contained about 50 cc. of the gelatin solution while the beaker 

 contained 350 cc. HoO with so much acid that the pH of the water 

 was at the beginning of the experiment always identical with the 

 pH of the gelatin solution; and for the outside solution the same acid 

 was used as for the gelatin solution. Fig. 6 gives the curves for the 

 value of the p.d. observed. The following points of similarity between 

 the two sets of curves for osmotic pressure (Fig. 5) and p.d. (Fig. 6) 

 are noticeable. Both sets of curves rise from the isoelectric point 

 with a lowering of the pH until they reach a maximum; this maxi- 



