58 



THEORY OF COLLOIDAL BEHAVIOR 



find out the amount of any acid in combination with a given mass 

 of gelatin of a certain pH. 



With the new method we can also confirm the statement that 

 weaker dibasic or tribasic acids, like oxalic or phosphoric, com- 

 bine with gelatin in molecular proportions. Table V gives the 

 equivalents of HNO 3 , oxalic, and phosphoric acids in combination 

 with gelatin at different pH in 10 c.c. of 0.8 per cent solution of 

 originally isoelectric gelatin. 



The values found for HNO 3 in Table V are slightly less than 

 those found for HBr in Table IV and HC1, due to the fact that 

 the concentration of gelatin was slightly less in the experiments 

 recorded in Table V than in Table IV. 1 A comparison of the 

 figures for NaOH values for HNO 3 , and for the PO 4 values (Table 

 V, Rows 1 and 3), found by direct titration for PO 4 with the uranyl- 

 acetate method, shows for the two values practically the ratio of 

 1 : 3 at the same pH; i.e., three times as much H 3 P0 4 as HNO 3 is in 

 combination with the same mass of gelatin. The figures for 

 HNO 3 and oxalic acid (Rows 1 and 2, Table IV) give the ratio of 

 approximately 1:2 for pH 3.5 or below. Hence, oxalic and phos- 

 phoric acids combine in molecular proportions with gelatin. In 

 the same way it was shown that H 2 S04 combines in equivalent 

 proportions with gelatin, 



These measurements confirm the conclusions at which we 

 arrived by the other method. 



TABLE V. CUBIC CENTIMETERS OF 0.01 N ACID IN COMBINATION WITH 



GELATIN IN 10 c.c. OF AN 0.8 PER CENT GELATIN SOLUTION AT 



DIFFERENT pH 



1 In these earlier experiments 1 gm. of powdered gelatin was brought to 

 the isoelectric point. This entailed some loss, especially during the wash- 

 ing, which varied slightly in different experiments. In later experiments 

 this source of error was avoided by using a stock solution of about 8 per 

 cent isoelectric gelatin and ascertaining the concentration of isoelectric 

 gelatin by dry-weight determinations. 



