198 SOLUTION AND PRECIPITATION OF PROTEINS 



are adsorbed by the casein and that this adsorption of ions annihi- 

 lates the electrical charges on the particles of casein. 



Emulsions of oil drops in water are prevented from coalescing by 

 their electrical charges (and not by osrnotic forces). The investiga- 

 tions of Beutner^^ have shown that the p.d. at the boundary of water 

 and substances immiscible in water is determined by an unequal 

 distribution of crystalloidal ions between the two phases, and the 

 writer ventures to suggest that this distribution may be regulated 

 by Donnan's theory, owing to the fact that, e.g., the oleic acid anion 

 in oil cannot diffuse into the water. The similarity of the behavior 

 of the p.d. between oil and water and between solid gelatin and 

 water is rather striking. The depression of the charge on the particles 

 through the addition of salt can in both cases be explained without the 

 adsorption hypothesis. 



III. The Precipitation and Solution of Na Caseinate. 



When we prepare solutions of Na caseinate of pH 11.0 containing 

 1 gm. of originally isoelectric casein in 100 cc, we notice that it 

 requires enormous concentrations of NaCl or LiCl to precipitate the 

 casein. NaCl as concentrated as 2| m and LiCl as concentrated 

 as 3| M were not able to precipitate Na caseinate from its solution, 

 while m/8 NaCl was sufficient for instantaneous and complete precipi- 

 tation of casein chloride from its solution. Hence Na caseinate 

 belongs to the other group of colloidal solutions ; namely, those which 

 require high concentrations of electrolytes. 



When a grain of isoelectric casein is put into a solution of NaOH 

 Na caseinate is formed at the surface of the solid granule. This dis- 

 solves in water, as any crystalloidal substance does (i.e., by the forces 

 of residual valency), except for the accidental fact that the solution 

 of Na caseinate seems to lower the surface tension of the watery solu- 

 tion at the interface, as is suggested by the rather violent spreading 

 of the Na caseinate solution at the interface, which can be observed 

 under the microscope. As a result projecting particles at the surface 

 of the granule are torn away from the surface of the solid casein, collect- 



^^ Beutner, R., Die Entstehung elektrischer Strome in lebenden- Geweben, 

 Stuttgart, 1920. 



