108 THEORY OF COLLOIDAL BEHAVIOR 



series are chiefly due to the failure to measure the influence 

 of the salts on the hydrogen ion concentration of the gelatin 

 solutions. This neglect has given rise to the statement that 

 salts like sodium acetate have the same depressing effect on 

 the physical properties of proteins as the sulphates. 



Neutral salts, when added in low concentrations i.e., below 

 M/16 affect the physical properties of proteins in two different 

 ways: first, by an exchange of one of the ions of the salt for the 

 ion with which the protein is in combination. Thus by adding 

 K 2 SC>4 to a solution of gelatin chloride, gelatin sulphate is formed 

 resulting in a diminution of osmotic pressure, viscosity, etc., of the 

 protein solution; or if KC1 is added to gelatin sulphate the reverse 

 chemical and physical changes take place. If the protein is on 

 the alkaline side of its isoelectric point, e.g., in the case of Na 

 gelatinate, the addition of a salt with bivalent cation, e.g., 

 MgCl2 or CaCl2, etc., results in the formation of Mg or Ca 

 gelatinate with the consequence that the osmotic pressure, vis- 

 cosity, and swelling of the gelatin is diminished. By mixing 

 two different salts, e.g., NaCl and MgCh, the antagonistic effects 

 so well known in biology can be imitated. 



The second effect of the addition of a neutral salt to a solution 

 of a protein is a general depressing effect on the physical proper- 

 ties of a solution of a protein salt and this depression is caused 

 by that ion of the salt which has the opposite sign of charge to 

 that of the protein ion. Thus all anions regardless of valency 

 depress the osmotic pressure, viscosity, and swelling of gelatin 

 chloride and the depressing effect increases with the concentra- 

 tion and valency of the anion of the salt added. All cations 

 depress the viscosity, swelling, and osmotic pressure of Na gela- 

 tinate and the more so the higher the concentration and valency 

 of the cations added. 



This effect is similar to the depression of electrolytic dissocia- 

 tion of one electrolyte caused by the addition of a second electro- 

 lyte with a common ion, but, nevertheless, the salt effects just 

 mentioned are not (or only to a negligible degree) due to a depres- 

 sion of the degree of electrolytic dissociation of the protein salt, 

 but are due to Donnan's membrane equilibrium. 



Previous authors had already observed that only electrolytes 

 have a depressing effect on the physical properties of protein 



