384 AMPHOTERIC COLLOIDS. Ill 



semi-permeable membranes — , which, when dealing with real solutions, can only 

 be used in exceptional cases and under especial circumstances."" 



SUMMARY. 



1. The method of removing the excess of hydrobromic acid after 

 it has had a chance to react chemically with gelatin has permitted 

 us to measure the amount of Br in combination with the gelatin. 

 It is shown that the curves representing the amount of bromine bound 

 by the gelatin are approximately parallel with the curves for the 

 osmotic pressure, the viscosity, and swelling of the gelatin solution.' 

 This proves that the curves for osmotic pressure are an unequivocal 

 function of the number of gelatin bromide molecules formed under 

 the influence of the acid. The cc. of 0.01 n Br in combination with 

 0.25 gm. of gelatin we call the bromine number. 



2. The explanation of this influence of the acid on the physical prop- 

 erties of gelatin is based on the fact that gelatin is an amphoteric 

 electrolyte, which at its isoelectric point is but sparingly soluble in 

 water, while its transformation into a salt with a univalent anion 

 like gelatin Br makes it soluble. The curve for the bromine number 

 thus becomes at the same time the numerical expression for the num- 

 ber of gelatin molecules rendered soluble, and hence the curve for 

 osmotic pressure must of necessity be parallel to the curve for the 

 bromine number. 



3. Volumetric analysis shows that gelatin treated previously with 

 HBr is free from Br at the isoelectric point as well as on the more 

 alkaline side from the isoelectric point (pH > 4.7) of gelatin. This 

 is in harmony with the fact that gelatin (like any other amphoteric 

 electrolyte) can dissociate on the alkaline side of its isoelectric point 

 only as an anion. On the more acid side from the isoelectric point 

 gelatin is found to be in combination with Br and the Br number 

 rises with the pH. 



4. When we titrate gelatin, treated previously with HBr but pos- 

 sessing a pH = 4.7, with NaOH we find that 25 cc. of a 1 per 

 cent solution of isoelectric gelatin require about 5.25 to 5.5 cc. of 0.01 

 N NaOH for neutralization (with phenolphthalein as an indicator). 



" Sorensen, Compt. rend. trav. Lab. Carlsberg, 1917, xii, 5-6. 



