46 AMPHOTERIC COLLOIDS. I 



The left branches rise higher than the right ones on account of the 



fact that on the left side of the isoelectric point we have gelatin chloride 



which as a salt has a high dissociation constant and which dissociates 



+ 

 into gelatin and CI, while on the right of the isoelectric point the 



curves are the expression of the electrolytic dissociation of common 



+ 

 gelatin which dissociates like a very weak acid into gelatin and H. 



Hence the limited rise of conductivity on the right and the considerable 



rise on the left of the isoelectric point. 



The parallelism between the curve for conductivity and that for the 

 other physical properties of the gelatin solution is so complete that the 

 idea is unavoidable that the degree of conductivity, i.e. of the ioniza- 

 tion of the gelatin, determines all the other physical properties of the 

 gelatin solution; the pH only entering to the extent as it influences 

 the possibility and character of the ionization of the gelatin. We 

 shall see presently, however, that this parallelism between the con- 

 ductivity and the curve for the other physical properties of the 

 gelatin solution does not hold throughout but that in addition to the 

 degree of electrolytic (and hydrolytic) dissociation still another 

 variable enters. The main fact for us is the typical asymmetry of the 

 two branches of the curve to the left and to the right of the iso- 

 electric point, the branch on the left (the more acid side) being steeper 

 and much higher than the branch on the right. 



A comparison of the curves of Figs. 2,3, and 4 with those of Fig. 1 

 will prove that gelatin when it ionizes will ionize on the right, less acid 

 side of the isoelectric point as an anion, on the left, more acid side of 

 the isoelectric point as a cation. 



Fig. 2 represents the effect of the same concentrations of HCl 

 as Fig. 1 , except that each acid solution contained so much Na2S04 as 

 to make them all m/16 for this salt. We will call this system of curves 

 the Na2S04-HCl curves to distinguish them from the pure HCl 

 curves represented by Fig. 1. The powdered gelatin was put for 30 

 minutes at 20° into these acid-salt mixtures and then freed from all 

 the supernatant free acid and salt by the process of washing described 

 in the writer's previous papers. If it is true that the isoelectric point 

 is also a chemical turning point and that gelatin can ionize only as an 



