90 



DONNAN EQUILIBRIUM AND VISCOSITY 



which the casein was at 40°C. The maximum swelHng occurred at 

 about pH 2.3. At this time the amount of casein dissolved into 

 separate casein ions was negligible. The curve resembles the 1 

 hour curve in Fig. 9 except that the depression due to the solution 

 of some casein chloride in Fig. 9 is lacking in Fig. 10. After 1^ hours 

 the second measurements of viscosity were taken, and the reader 

 will notice from Fig. 10 that the viscosity had dropped considerably 

 in the neighborhood of pH. 2.2, where the solubility of casein chloride 

 is the greatest, and the maximum depression is at pH 2.1 where also 

 the solubility is a maximum. With a further lowering of the pH 

 the viscosity rises again. The maximal viscosity in the 1| hours 



1.7 

 .2 '« 



to 1.5 



i 1.3 



o 



53 1.2 

 > 



1.1 

 1.0 



pHl4 1.6 1.8 20 2,Z 2.4 2.6 26 3.0 3.2 3.4 3.6 3.8 40 4.2 



Fig. 10. Diminution of viscosity through solution of solid particles of casein 

 chloride. 



series is now at pH of about 2.7 or 2.8 where it was also in 22 hours 

 series in Fig. 9. The later viscosity measurements, after 3 and 6 

 hours (Fig. 10), confirm these conclusions. 



5. These experiments leave little doubt that the high viscosity 

 of protein solutions is due to the existence of particles occluding 

 large quantities of water the amount of which is regulated by the 

 Donnan equilibrium while the isolated ions of proteins in solution 

 or the particles too small to occlude water have no share in the 

 causation of high viscosities. 



The quantities of water which can be occluded in a solid jelly of 

 gelatin are enormous. If we assume the molecular weight of gelatin 



