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 



in Experiment III, for pH = 4.1, (NaOH)„ = 8.0, NaOH (iso- 

 electric) = 5.5, and Bin = 2.85. 5.5 -f- 2.85 = 8.35, while (NaOH)n 

 actually found is 8.0. Table II gives a comparison of the agree- 

 ment in all the experiments. 



In Table II we call the sum of NaOH (isoelectric) -|- Brn the cal- 

 culated and (NaOH)n the observed value for a given pH. The table 

 shows that the calculated and observed values agree within the limits 

 of the degree of accuracy of the experiments. 



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



