274 COLLOIDS AND ELECTROLYTES 



bivalent cation causes a more rapid drop than the addition of a salt 

 with monovalent cation. These phenomena bear so striking a resem- 

 blance to the action of the concentration of electrolytes upon the 

 osmotic pressure, the swelling, and other properties of colloids that 

 a discussion of the similarity may seem of interest. 



It was found by Pauli^ that the addition of a little acid to blood 

 albumin which had been dialyzed for weeks (and which was therefore 

 approximately isoelectric) caused an increase in the viscosity of the 

 protein, which at first was the greater the more acid was added. Very 

 soon, however, a point was reached where the addition of more acid 

 caused again a diminution in the viscosity. The same phenomenon 

 occurs when acid is added to isoelectric gelatin. The addition of a 

 sHght amount of acid causes an increase in the osmotic pressure until 

 finally a point is reached where the further addition causes a dimi- 

 nution (Fig. 1). The increase in the osmotic pressure of isoelectric 

 gelatin when a slight quantity of HCl is added, is due to the formation 

 of gelatin chloride, but the depressing effect of the addition of an 

 excess of acid is not so easy to explain. According to PauH we should 

 ascribe it to the diminution of the degree of electrolytic dissociation 

 of protein chloride due to the increase in the concentration of the 

 common anion CI. The writer's measurements of conductivity do 

 not support this idea.^ Another suggestion made by colloid chemists 

 is that the addition of more acid causes an aggregate formation of the 

 gelatin particles and therefore a diminution of osmotic pressure. This 

 suggestion rests only on the phenomenon which it is supposed to 

 explain, but it may, nevertheless, be correct. If so, it remains to be 

 explained why an increase in the concentration of electrolytes causes 

 a formation of aggregates. 



In a preceding pubhcation* the writer has shown that a 1 per cent 

 solution of gelatin-acid salt, e.g. gelatin chloride or gelatin citrate, 

 etc., has its maximal osmotic pressure when the pH is about 3.4 or 

 3.3. When to gelatin chloride of this pH acid or neutral salt is added, 

 the osmotic pressure (as well as the swelling, viscosity, etc.) falls and 

 the more so the more acid or salt has been added. When we add 



^ Pauli, W., Fortschr. naturwiss. Forschung, 1912, iv, 223. 

 3 Loeb, J., /. Gen. Physiol., 1918-19, i, 483, 559. 

 ''Loeb, J., /. Gen. Physiol, 1918-19, i, 559. 



