828 



DONNAN EQUILIBRIUM AND VISCOSITY 



on the osmotic pressure and swelling. In the literature, however, 

 it is usually stated that the influence of acid on viscosity resembles 

 that of acids on osmotic pressure and swelling, and the question arises 

 whether or not the theory of the Donnan equilibrium can be applied 

 to the explanation of this type of viscosity curves found when freshly 

 prepared gelatin solutions are used (Fig. 1). 



76 3.5 



3.0 



03 



2.5 



2.0 





CO 

 o 



CO ^-^ 



> 



1.0 



pH 1.4 1.6 Ifi ^0 21 2.4 Z6 2.8 3.0 3.2 34 3.6 3.8 4D 4.2 44 4.6 



Fig. 1. Influence of pH on viscosity of freshly prepared gelatin chloride 

 solutions. 



• Several formulae exist for the calculation of the influence of a so- 

 lute on the viscosity of a solvent. The first one (1) was derived 

 by Einstein* 



r, = r,,{\ + 2.5 <p) (1) 



where rjo is the viscosity of the pure solvent at the temperature of 

 the experiment, t? the viscosity of the solution, and tp the fraction 

 of the volume occupied by the solute in the solution. This formula 

 holds only when ^ is very small and when the particles of the solute 

 are spherical and large compared with the molecules of the solvent. 



-* Einstein, A., Ann. Physik, 1906, xix, 289; 1911, xxxiv, 591. 



