240 



AMPHOTERIC COLLOIDS. II 



out in a dark room. Then the pH is determined, and 25 cc. of the 

 solution are used to determine the quantity of silver in combination 

 with the gelatin, according to Volhard's method. 



Fig. 1 contains two curves the abscissae of which are the logarithms 

 of the concentrations of HNO3 with which the gelatin was originally 



Region of 

 Gelatin -NO3 



HNO3 

 cone, used 



Isoelectric Region of 



TDOint "*" ~ 



Pure^gelatin^ Ag-Gelatinate 



Remain clear in light Turn blacK in light 



M MM_I13M_M3M.M__M_M_M_M_0 

 16 3Z 64 128 512 256 1024 512 1024 2048 4096 8192 



H 3.6 3.8 3.8 3.9 4.1 



4.3 4.6 4.7 5.0 5.3 5,7 6.1 6.1 

 ji AgN03 



Fig. 1. Gelatin treated with different concentrations of HNO3, from m/8 to 

 m/8192, washed, and then treated with the same concentration of AgNOs (m/16), 

 and then washed again. Abscissae show concentrations of acid used. The final 

 pH of the gelatin solution is found under the figure for the concentration of 

 acid used. 



The ordinates of the lower curve give the values for the silver found in com- 

 bination with the gelatin. The curve shows that at the isoelectric point (pH = 

 4.7) and on the acid side of the isoelectric point, the gelatin was practically free 

 from silver. On the more alkaline side the amount of silver found in combination 

 with the gelatin increased with the pH. This proves that gelatin can combine 

 with a cation only on the alkaline side from the isoelectric point, and this is corrob- 

 orated by the fact that on the alkaline side from the isoelectric point only was the 

 gelatin darkened by light. The ordinates of the upper curve are the values for the 

 swelling of the same gelatin. On the alkaUne side from the isoelectric point, 

 where the gelatin had combined with silver, the curve for swelling runs parallel 

 to the curve for silver gelatinate formed. It was, therefore, the relative mass of 

 silver gelatinate formed which determined the ph}'sical properties of gelatin. 



