246 



AMPHOTERIC COLLOIDS. II 



When we reverse the order of treatment, i.e. when we treat gelatin 

 first with m/8 NaBr at pH about 7,0, only sodium gelatinate should 

 be formed according to our theory, and no Br should combine with the 

 gelatin. This was found to be correct. When we treat gelatin first 



■Region of Gelatin -Br 

 and Gelatin- NO3 



Isoelectric 

 point 

 Eeldktin 



T?egion oF_ 

 Na-Gelatinate 







HNO3 M 



cone, used 8 



M_]^ _M M_3MM_3MM M_ _M_ M_ M .0 

 16 32 64 126 512 256 1024 512 1024 2048 4196 8192 



pH 37 3.8 3.8 3.9 4.1 42 4.6 4.7 5.1 5.8 6.4- 68 7.0 7.1 



Fig. 4. Gelatin treated with different concentrations of HNO3, washed, treated 

 with m/8 NaBr, and washed again. The ordinates of the lower curve are the 

 values of Br in combination with gelatin, showing that on the right (alkaline) 

 side from the isoelectric point and at the isoelectric point gelatin contains no Br, 

 while on the left, more acid side from the isoelectric point, the amount of the Br 

 found increases with the pH. The ordinates of the upper curve represent the 

 swelUng of the same gelatin. The two curves on the left side are almost parallel, 

 showing that the degree of swelling is determined by the relative mass of the 

 gelatin bromide (or nitrate) formed. 



with m/8 NaBr, then with varying concentrations of HNO3, no Br 

 should enter into ionogenic combination with the gelatin. This is 

 confirmed by the volumetric analysis which shows that such gelatin 

 is free from ionic Br. Only sodium gelatinate is formed under these 

 conditions. ^f 



