380 



AMPHOTERIC COLLOIDS. Ill 



NaOH number and the Br number for different values of pH are 

 given. It is easy to show that the equation holds within the limits 

 of accuracy of our experiments. Thus the NaOH number for the iso- 

 electric point is practically a constant in all experiments, namely be- 

 tween 5.25 and 5.5; and this value represents the binding capacity of 

 "pure" gelatin for NaOH; or in other words, the binding capacity of 

 0.25 gm. of gelatin freed from ionogenic impurities is between 5.25 and 

 5.5 cc. 0.01 N NaOH, with phenolphthalein as indicator. Thus 



TABLE I. 

 Experiment I. No washings, but made up in H2O and dialyzed against H2O. 



Experiment II. One washing. 



pH 



Br number. 

 NaOH " 



3.3 



8.3 



12.5 



3.4 



6.4 



12.0 



3.7 



5.8 



11.0 



4.0 



4.2 

 10.0 



4.2 



2.75 



9.50 



4.3 

 1.05 



7.5 



4.4 

 0.6 

 7.0 



4.7 

 0.1 



5.5 



4.9 

 



4.5 



5.1 







3.0 



5.7 







2.0 



5.7 







1.5 



5.8 







1.5 



6.3 







1.5 



Experunent III. Four washings. 



pH 



Br number. 

 NaOH " . 



5.9 







1.5 



Experiment IV. Eight washings. 



4 1 I 4 1 I 4 ? I4 7. I 4.4I4.5 I 4. 51-/. 7 I 4.915.2 15.4 I 5.515.6 



We have now to make sure that the HBr measured in these titrations 



