40 AMPHOTERIC COLLOIDS. I 



acid is washed away from such powdered gelatin it is affected only by 

 the anion of a neutral salt, while the cation of the salt used has appar- 

 ently no effect. 



The writer found that when powdered gelatin is treated for some 

 time with a base, e.g. NaOH, the gelatin (after the supernatant NaOH 

 is washed away) is* only capable of combining with the cations of 

 neutral salts, while the anions of neutral salts are without effect. 



It seemed necessary to find out where the turning point for these 

 two different modes of reaction lies, whether at the point of neutrality 

 of the solution or at the isoelectric point of gelatin. The fact that 

 gelatin solutions at the point of neutrality (pH = 7.0) react with 

 neutral salts as if they had previously received an alkaline treatment, 

 suggests that the turning point lies at a more acid reaction of the 

 gelatin and since the isoelectric point lies at a more acid reaction it 

 was a priori likely that the isoelectric point for gelatin might be the 

 turning point. 



Amphoteric electrolytes are defined by the fact that they can dis- 

 sociate electrolytically as acids as well as bases. When they dis- 

 sociate as acids they are supposed to send more H ions than OH ions 

 into solution and the ampholyte ion migrates in the electric field to 

 the anode. When they dissociate as bases they are supposed to send 

 more OH ions than H ions into solution, and the ampholyte migrates 

 in the electric field as a cation. At the isoelectric point the ampho- 

 lyte is supposed to send out an equal number of H and OH ions and 

 it migrates in neither direction. Gelatin is naturally a stronger acid 

 than base and hence migrates to the anode. By increasing the hydro- 

 gen ion concentration of the surrounding solution through the addi- 

 tion of some acid the dissociation of gelatin as acid can be repressed to 

 the level of its dissociation as a base. This is the isoelectric point at 

 which it migrates in neither direction and at which the total number 

 of ions it sends into a solution is a minimum. With the addition of 

 more acid it migrates to the cathode. Michaelis determined the iso- 

 electric point of gelatin solutions by the migration method and 

 found it in the neighborhood of Ch = 2.10~^ (or in Sorensen's 

 logarithmic symbol pH = 4.7).^ Since the solution of gelatin used had 

 a pH of 7.0 it was on the alkali side of the isoelectric point. We shall 



^ Michaelis, L., Die Wasserstoffionenkonzentration, Berlin, 1914. 



