94 DONNAN EQUILIBRIUM AND VISCOSITY 



1 gm. of powdered casein was put into 100 cc. of H2O containing 

 12.5 cc. of 0.1 HCl and NaCl in concentrations varying from to 

 M4. The mixture was shaken occasionally and kept for 16 hours 

 at 20°. Then the viscosity, volume of sediment (after settling for 

 24 hours), dry weight of sediment (after deduction of the free NaCl 

 contained in the sediment) were determined. When the corrected 



volume and the values for log — are plotted as ordinates over the 



concentration of abscissae it is found that the two curves agree 

 fairly well (Fig. 12) except where no or little salt was added and 

 where therefore some casein particles had been completely dissolved. 

 In this solution the calculated volume was too high and our curves 

 express the fact. From these experiments we may conclude that the 

 influence of electrolytes on the viscosity of casein solutions or suspen- 

 sions is due to the swelling of particles of casein suspended in the 

 solution of casein and that the volume of these particles is regulated 

 by the Donnan equihbrium. 



SUMMARY. 



1. The proof is completed that the influence of electrolytes on the 

 viscosity of suspensions of powdered particles of gelatin in water is 

 similar to the influence of electrolytes on the viscosity of solutions 

 of gelatin in water. 



2. It has been suggested that the high viscosity of proteins is due 

 to the existence of a different type of viscosity from that existing 

 in crystalloids. It is shown that such an assumption is unnecessary 

 and that the high viscosity of solutions of isoelectric gelatin can be 

 accounted for quantitatively on the assumption that the relative 

 volume of the gelatin in solution is comparatively high. 



3. Since isoelectric gelatin is not ionized, the large volume cannot 

 be due to a hydration of gelatin ions. It is suggested that this high 

 volume of gelatin solutions is caused by the existence in the gelatin 

 solution of submicroscopic pieces of solid gelatin occluding water, 

 the relative quantity of which is regulated by the Donnan equilibrium. 

 This would also explain why the influence of electrolytes on the 

 viscosity of gelatin solutions is similar to the influence of electrolytes 

 on the viscosity of suspensions of particles of gelatin. 



