54 AMPHOTERIC COLLOIDS. I 



a cation when pH is <4.7. Hence it can dissociate as neither when 

 pH =4.7. Not being able to dissociate electrolytically, it cannot 

 react chemically with the ions of a neutral salt, as was found to be the 

 case. 



This inabihty to dissociate electrolytically and hence to react with 

 neutral salts at the isoelectric point should be a common property of 

 all amphoteric electrolytes and this seems to be the case. While 

 this note goes to press a paper has appeared by Van Slyke and 

 Baker/^ containing the following sentence. "At the isoelectric point 

 casein combines with neither acid nor base;" i.e., with neither cation 

 nor anion. 



V. 



The facts shown thus far support the idea that amphoteric electro- 

 lytes dissociate in a different way on the two sides of the isoelectric 

 point; on the less acid side of that point they can ionize only as 

 anions capable of combining with the cations of neutral salts, but not 

 with their anions; on the acid side of the isoelectric point they can ion- 

 ize only as cations capable of combining with the anion of a neutral 

 salt, but not with its cation. The question then arises: What will 

 happen when v/e bring a salt like sodium gelatinate in which gelatin is an 

 anion on the acid side of the isoelectric point? Our theory demands 

 that sodium gelatinate produced by treating gelatin with NaOH 

 should not continue to exist when pH<4.7, and that in this case gela- 

 tin should behave as if it had not been treated with NaOH. This is 

 what actually happens. The writer made this experiment sometime 

 before he had arrived at the theory published in this paper. He 

 had undertaken the experiment in the expectation that gelatin first 

 treated with alkali and then with acid should show a superposition of 

 the effects of both treatments. 1 gm. of finely powdered gelatin was 

 put for 3D minutes at 20°C. into each of a series of beakers all con- 

 taining m/32 NaOH. After this, each powder was put on a filter, the 

 alkali was allowed to drain off, and each filter was perfused four times 

 with 25 cc. of H2O to remove the last traces of NaOH held in the 

 capillary spaces (except the NaOH formed by the hydrolytic dissocia- 

 tion of the sodium gelatinate) . Then each filter was perfused three 



1" Van Slyke, L. L., and Baker, J. C, /. Biol. Chan., 1918, xxxv, 147. 



