102 



OSMOTIC PRESSURE AND VISCOSITY 



sary to let the suspensions stand for at least 1 hour to allow the particles 

 of powdered gelatin to swell before the viscosity measurements were 

 made. During this time the liquid gelatin at, or near, the isoelect- 

 ric point increases rapidly in viscosity while this increase in viscosity 

 is suppressed where the hydrogen ion concentration is higher. This 

 is proved by Fig. 3 which gives the viscosity of the supernatant 

 solutions of gelatin (without the suspended particles) which had 

 been standing for 1 hour. Inside the range of pH 4.4 and 4.6 the 

 viscosity had risen more rapidly on standing than at the lower pH ; 



pH 14 1.6 liS 20 2.2 2.4 2.6 2.6 3.0 32 34 3.6 38 40 42 44 4.6 



Fig. 3. Increase of viscosity of gelatin solution at the isoelectric point on 

 standing due to the formation of submicroscopic particles of gel. 



which means that at, or near, the isoelectric point new submicroscopic 

 particles of solid jelly are constantly formed from the molecules 

 while this process is the slower the higher the hydrogen ion concentra- 

 tion. While thus the addition of acid to a solution of isoelectric 

 gelatin retards the rate of formation of new submicroscopic particles 

 of jelly, it increases the swelling of those already present when the 

 acid is added. On the other hand, powdered particles of isoelectric 

 gelatin in water of pH 4.7 do not increase their volume on standing. 

 The fact that the addition of acid to a solution of isoelectric gelatin 

 inhibits or retards the formation of new solid particles on standing 



