382 AMPHOTERIC COLLOIDS. Ill 



bination with the gelatin and the small amount of free acid found was 

 probably due to hydrolytic dissociation of gelatin bromide or to a 

 trace of HBr not removed. This quantity of free acid is the cor- 

 rection of the value for the bromine number referred to in an earlier 

 part of this paper. When, however, the free HBr is not washed ofif, 

 as was the case in the experiment represented in Fig. 1, or w^hen the 

 free acid is only incompletely removed, the value (NaOH)„ will repre- 

 sent, of course, more or less free acid and in this case the discrepancy 

 between (NaOH)n and the pH found will be correspondingly smaller 

 (see Fig. 1). This, however, manifests itself by the fact that a 

 further rise in the Br number is no longer accompanied by a corre- 

 sponding rise or is accompanied by a drop in the curves for osmotic 

 pressure, swelling, and viscosity. 



We may, therefore, consider it as proved that the bromine numbers 

 given in this paper represent practically the HBr held in chemical 

 combination by the gelatin with the exception of the small amount 

 to be deducted owing to the presence of free HBr which can be 

 calculated from the pH. 



V. Theoretical Remarks. 



Our experiments show that the influence of hydrobromic acid upon 

 the physical properties of gelatin has a purely chemical or stoichio- 

 metrical basis. Gelatin and probably all proteins and amphoteric 

 colloids behave as if they were merely amphoteric electrolytes capable 

 of adding a H or OH ion. 



Whether a protein adds an acid or a base depends on the hydrogen 

 ion concentration; when the hydrogen ion concentration exceeds a 

 critical point (which for gelatin is Ch = 2.10-^), the gelatin will add 

 acid; when it is lower it will add base. This critical hydrogen ion con- 

 centration is the isoelectric point. When gelatin is at the isoelectric 

 point, it is free from ionogenic impurities and this "pure" gelatin is 

 practically insoluble and hence can have no osmotic pressure, and, 

 moreover, all the properties which depend upon its solubihty are 

 a minimum. When it is transformed into a salt by the addition of 

 an acid (or a base) it becomes soluble, provided it is in combination 

 with a monovalent ion, like Br or Na, etc. When pure gelatin is 



