240 



THEORY OF COLLOIDAL BEHAVIOR 



rapidly in viscosity while this increase in viscosity is suppressed 

 where the hydrogen ion concentration is higher. This is proved 

 by Fig. 72 which gives the viscosity of the supernatant solutions 

 of gelatin (without the suspended particles) which had been stand- 

 ing 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 ; 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 

 concentration. While thus the addition of acid to a solution of 

 isoelectric gelatin retards the rate of formation of new submicro- 



0.8' 



5% 



0.62 5 2 



V1CO 



pH 1.4 16 Ifl 20 2.2 2.4 2.6 26 3.0 32 3.4 3.6 38 40 42 4.4 4.6 



FIG. 72. Viscosity of gelatin solutions after standing for 1 hour at 20C. 

 Notice minimum at pH 4.4, indicating that the viscosity has risen more near the 

 isoelectric point on account of the formation of submicroscopic particles of gel. 



scopic 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 was discussed in the preceding chapter. 



If we now return to .the discussion of the curves in Fig. 71 we 

 may say that the results in that part of the curves which belongs 

 to the abscissae of pH above 3.8 is the expression of the fact that 

 that part of the viscosity which is due to the gelatin in solution 



