COLLOIDAL ELECTROLYTES 409 



is due to physical adsorption or to chemical reaction ; thus, it 

 has been found that a relatively larger amount of the total 

 substance in solution is withdrawn from a dilute than from a 

 strong solution. 



COLLOIDAL ELECTROLYTES. 



The proteins can function as acids to form salts with the 



+ - 

 alkali metals of the type NaPr, in which Pr stands for a colloidal 



ion ; compounds of this type are known as colloidal electro- 

 lytes. Such substances, composed as they are of a diffusible 

 metallic ion and a non-diffusible colloidal ion, are comparable 

 with such a salt as sodium ferrocyanide, which manifests a 

 characteristic behaviour when separated from a solution of 

 a completely diffusible electrolyte by a membrane. Theoretical 

 considerations led Donnan to conclude that a freely diffusible 

 electrolyte, having a common ion with the colloidal electro- 

 lyte, if placed on the opposite side of a membrane would not, 

 at equilibrium, be distributed uniformly on both sides of the 

 membrane ; on thermodynamic grounds there must be a 

 greater concentration of the diffusible electrolyte on the side 

 opposite to the non-diffusible ion. The same condition of 

 affairs would result if the colloidal electrolyte were mixed 

 with a diffusible salt and separated by a membrane from pure 

 water. This is known as the Donnan Equilibrium ; the 

 correctness of the theoretical conclusions have been experi- 

 mentally verified.* 



Certain writers, notably Loeb,t unduly strain this theory in 

 the endeavour to make it explain the entire colloidal behaviour 

 of proteins to the exclusion of all other considerations. It will 

 be apparent that the living cell, consisting of protoplasm com- 

 posed of colloidal electrolytes both in contact with and separ- 

 ated from diffusible electrolytes by a semi-permeable membrane 

 (which semi-permeability may vary with altering conditions), 

 presents just those conditions as are suitable for the establish- 

 ment of a Donnan Equilibrium. Naturally these conditions 



* Donnan : " Zeit. Elektrochem.," 191 1, 17, 572. Donnan and All- 

 mand: "J. Chem. Soc," 1914, 105, 1963. Harwood : id., 1923, 123, 2254. 



t Loeb : " Proteins and the Theory of Colloidal Behaviour," London 

 and New York, 1922. 



