194 



SOLUTION AND PRECIPITATION OF PROTEINS 



forces of cohesion in the case of casein sulfate and casein trichlorace- 

 tate seem to be so great that they cannot be overcome by the osmotic 

 pressure due to the Donnan equilibrium; and hence no swelling (and 

 as a consequence no solution) of solid casein is possible in H2SO4 or 

 trichloracetic acid. 



CD 



3 5.5 



"OH 



0^ 



i 



g 



5.0 



4.5 



4.0 



3.5 



3.0 



2.5 



M 



15^ 





M 

 B 



Concentration of NaCl 



Fig. 3. Depression of swelling of casein particles in m/100 HCl by the addition 

 of NaCl. A low concentration of acid suffices already for a noticeable depression 

 of the swelling. The lower curve represents swelling after 1 hour; the upper 

 curve swelling after 24 hours. Solution of the swollen particles occurs when the 

 average diameter of the particles exceeds the value 5 to 6 in our scale. 



Procter and Wilson have shown that the theory of the Donnan 

 equilibrium explains the depressing effect of a salt on the swelHng of 

 solid gelatin.^ Microscopic measurements of the influence of NaCl 

 on the rate of swelling of individual grains of casein particles in 

 m/100 HCl were made at 24°C. and the results plotted in Fig. 3. 



