THE INFLUENCE OF ELECTROLYTES ON THE SOLUTION 

 AND PRECIPITATION OF CASEIN AND GELATIN. 



By JACQUES LOEB and ROBERT F. LOEB. 

 (From the Laboratories of The Rockefeller Institute for Medical Research.) 



(Received for publication, September 12, 1921.) 

 I. INTRODUCTION. 



The modem concepts of colloid chemistry originated largely from 

 a study of the precipitation of colloids by electrolytes, and this ac- 

 counts, perhaps, for the fact that the tendency to form aggregates 

 was considered the chief characteristic of colloids in solution. As a 

 consequence it is assumed in the text-books of colloid chemistry that 

 the ultimate units of a colloid in solution are not isolated molecules 

 or ions but larger aggregates of molecules or ions, the so called micella 

 (small crumbs) of Naegeli, which are supposed to be kept in a stable 

 solution or suspension through forces of repulsion due to the fact that 

 they are electrically charged. The precipitation of the particles by 

 electrolytes is ascribed to a diminution of the charge of the micella 

 through the adsorption of the ions of the electrolyte. It was noticed 

 by Hardy that the active or precipitating ion of the electrolyte was 

 always that ion which had the opposite sign of charge as the colloidal 

 particle, and Picton and Linder had noticed that the precipitating 

 efficiency of ions increased rapidly with their valency.^ 



The experiments pubhshed by one of us have led to results which 

 are compatible with some but not all of the assumptions just enumer- 

 ated. In the first place, experiments on the osmotic pressure and the 

 viscosity of protein solutions have made it probable that the ultimate 

 units in certain protein solutions, such as crystalline egg albumin, are 

 essentially isolated protein ions or molecules, though such solutions 

 may, in a secondary way, also contain aggregates of ions or molecules.* 



^Zsigmondy, R., KoUoidchemie, Leipsic, 2nd ed., 1918. 

 2Loeb, J., J. Gen. Physiol., 1920-21, iii, 827; 1921-22, iv, 73, 97. 



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