712 DONNAN EQUILIBRIUM AND OSMOTIC PRESSURE 



e.g., gelatin or crystalline egg albumin, lies at a pH of 3.5 or 3.4, and 

 for casein at a pH above 3.0. The maximal swelling lies at a pH of 

 gelatin of about 3.2 or 3.3. Those who state that it lies at a lower 

 pH {e.g. Michaelis^") must have mistaken the pH of the supernatant 

 liquid for the pH of the solid gelatin, thus ignoring the effects of the 

 Donnan equilibrium. The maximum for the viscosity of gelatin- 

 acid salts lies at pH of 3.0 and for the viscosity of casein chloride or 

 phosphate at pH 3.0 or above. 



Northrop^ has observed that a drop in the conductivity of gelatin 

 solutions occurs when the pH falls below 2.0, but this cannot explain 

 the drop in the osmotic pressure curves observed at pH above 3.0, 

 and Northrop's results agree entirely with my own in not having 

 noticed a drop in the conductivity curves at pH 3.0 or 3.5. 



Moreover, Fig. 7 shows that there is only a slight difTerence 

 between the conductivity curves for gelatin sulfate and gelatin chlo- 

 ride, while there is a greater difference between the conductivity of 

 gelatin chloride and gelatin oxalate. All this disagrees entirely with 

 the osmotic pressure curves in Fig. 5 in the preceding paper. Fur- 

 thermore, the idea of a noticeable hydratation of the protein ion seems 

 to be no longer tenable on the basis of Lorenz^^ and Born's^- experi- 

 ments and conclusions. 



A second colloidal hypothesis would lead us to assume that varia- 

 tions in the degree of dispersion of the protein particles are responsible 

 for the osmotic pressure curves represented in Fig. 1. We need not 

 dwell on this hypothesis since we have no way of putting it to a 

 quantitative test. 



The results of this paper show that if we assume the correctness of 

 Donnan's theory of membrane equilibrium the characteristic influ- 

 ences of pH and valency on the osmotic pressure appear as a neces- 

 sary consequence of the theory; with the exception of the two minor 

 dififerences discussed in Chapter IV. Donnan's theory leads to a 

 view radically different from all colloidal speculations since on the 

 basis of this theory the variations in osmotic pressure depend on the 



^''Michaelis, L., Praktikum der physikalischen Chemie insbesondere der Kol- 

 loidchemie, Berlin, 1921. 



1^ Lorenz, R., Z. Ekktrochem., 1920, xxvi, 424. 

 12 Born, M., Z. Ekktrochem., 1920, xxvi, 401. 



