48 AMPHOTERIC COLLOIDS. I 



anion on the right side of this point, and as a cation on the left side, 

 we should be able to predict the difference between the two systems 

 of curves, namely the HCl curves and the Na2S04-HCl curves 

 (Figs. 1 and 2), as far as osmotic pressure, swelling, and alcohol number 

 are concerned. On the right, less acid side of the isoelectric point 

 gelatin can dissociate only as a negative ion and hence, reacting with 

 Na2S04, it should be transformed into sodium gelatinate. On the 

 left side, where gelatin can only exist as cation, in a mixture of HCl 

 and Na2S04 gelatin chloride and gelatin sulfate should be formed. 

 In the Na2S04-HCl curves (Fig. 2) we should, therefore, expect to 

 find on the right side of the isoelectric point (pH>4.7) sodium gelat- 

 inate, showing a high osmotic pressure, a high alcohol number, 

 etc. On the left side of the isoelectric point where gelatin exists as 

 gelatin chloride and gelatin sulfate, we should find lower values for 

 osmotic pressure, alcohol number, etc., since we have shown in former 

 experiments that the addition of SO4 to CI represses the increase in 

 osmotic pressure, viscosity, swelling, caused by CI. Hence we should 

 expect an asymmetry in this system of curves of exactly the reverse 

 kind of that found in the pure HCl experiment. A glance at Fig. 2 

 shows that this is the case, for osmotic pressure, alcohol number, but 

 not for conductivity and pH, which are the same for the same 

 HCl concentrations as in the pure HCl experiment (Fig. 1). We 

 shall return later to this difference which is of great theoretical 

 importance. 



Hence we reach the conclusion that the isoelectric point of the 

 gelatin is also the turning point for the chemical reaction between 

 gelatin and neutral salt, inasmuch as the gelatin on the less acid, 

 right side of the isoelectric point can exist in an ionized state only as a 

 negative gelatin ion capable of combining with cations but not with 

 anions, while on the left (more acid) side of the isoelectric point it 

 can exist in an ionized state only as a positive gelatin ion capable of 

 combining with the anions of a neutral salt. 



This conclusion is confirmed by the experiments of the Ca(N03)2- 

 HCl series. In this series, represented by the curves of Fig. 3, the 

 same concentrations of HCl from m/8 to m/4096 were used and each 

 HCl solution contained so much Ca(N03)2 as to make it m 16 in 

 regard to Ca(N03)2. If our theory is correct, the asymmetry of the 



