830 DONNAN EQUILIBRIUM AND VISCOSITY 



outflow for pure water was 56 seconds at 24°C. The pH of the 

 solutions was determined with the aid of the potentiometer. 



The reader will notice (Fig. l) that the relative viscosity of a 

 gelatin solution is a minimum at the isoelectric point (pH = 4.7), 

 that it rises with a rise in the hydrogen ion concentration until 

 it becomes a maximum at pH of about 2.7, and that it drops again 

 with a further rise in the hydrogen ion concentration. The question 

 is how to explain the apparent changes in the relative volume of 

 the gelatin in solution which, according to the theory, must be the 

 main cause of the variation of the viscosity with the pH. 



A change in the ratio of volume of gelatin to volume of water 

 is only possible if water is added to the gelatin. Pauli^° suggested 

 that the ionized particle of protein is surrounded by a shell of water 

 which is lacking in the non-ionized molecule. The volume of the 

 protein ions in solution is increased by this jacket of water. Since 

 the gelatin in solution is practically non-ionized at the isoelectric 

 point, the relative volume of the gelatin in solution is a minimum at 

 this point, while when we add an acid, e.g., HCl, gelatin chloride is 

 formed, which, like all salts, ionizes readily. On this basis we can 

 understand why the viscosity should increase with an increase of the 

 hydrogen ion concentration of the gelatin solution; since with this 

 increase in the hydrogen ion concentration, the concentration of 

 hydrated gelatin ions and hence the volume of the gelatin particles 

 should also increase. The work of Lorenz,i^ Born,^^ and others 

 casts, however, a doubt on the assumption of a general hydration 

 of polyatomic ions. We shall see presently that there are still other 

 facts which show that the mere ionization and consequent hydra- 

 tion of the individual protein ions cannot well be the cause of the 

 influence of the pH on the relative viscosity of gelatin solutions. 



//. The Influence of the Hydrogen Ion Concentration on the Viscosity 

 of Solutions of Amino-Acids and of Crystalline Egg Albumin. 



If hydration of the individual protein ions were the cause of the 

 variation of the viscosity of gelatin solutions, a variation of the hy- 



10 Pauli, W., Kolloid Z., 1913, xii, 222. 



11 Lorenz, R., Z. Elektrochem., 1920, xxvi, 424. 



12 Born, M., Z. Elektrochem., 1920, xxvi, 401. 



