238 AMPHOTERIC COLLOmS. II 



We may apply the experience gained from aluminium hydroxide 

 to proteins which are also amphoteric electrolytes and for some of 

 which the exact location of the isoelectric point is known. For gela- 

 tin it lies at pH = 4.7. In a previous paper- it was shown that 

 when pH > 4.7, gelatin is only able to dissociate as gelatin (capable 

 of combining with metals to form metal gelatinates, e.g. sodium gelat- 

 inate); when pH < 4.7, gelatin is only capable of dissociating as 

 gelatin (capable of combining with anions to form, e.g., gelatin chlo- 

 ride). At the isoelectric point it is practically not dissociated at all. 

 There is this difference between amphoteric electrolytes of the type 

 of proteins and that of metal hydroxides, namely that in the case of 

 proteins the OH ions and the H ions are attached to different 

 groups in the molecule. 



We also have shown that a metal gelatinate must give off its metal 

 ion when the hydrogen ion concentration is raised beyond that of the 

 isoelectric point; and that a gelatin-anion compound must give off its 

 anion when the hydrogen ion concentration is lowered below that of 

 the isoelectric point. At the isoelectric point gelatin cannot be in 

 combination with either anion or cation and such gelatin free from 

 ionogenic impurities we designate as pure gelatin. It would thus 

 appear that we can free proteins and amphoteric colloids in general 

 from ionogenic impurities by bringing the ampholyte to its isoelectric 

 point. Such impurities, however, as form part of the complex gel- 

 atin ion can, of course, not be removed by this method. In this 

 and some following papers we intend to furnish additional proof 

 for the correctness of these ideas. This can be done by making the 

 gelatin combine with such ions as Ag or Br or CNS, etc., whose quan- 

 tity can be easily determined volumetrically. 



The Combination of Cation with Gelatin. 



In all experiments in which the chemical or physical influence of an 

 electrolyte upon an amphoteric colloid is to be investigated it is neces- 

 sary to remove the excess of electrolyte after it has had time to act 

 on the colloid ; and the writer has shown that the neglect of this pre- 

 caution has caused a good deal of the confusion which prevails in the 



2 Loeb, J., /. Gen. Physiol., 1918, i, 39. 



