DAVID I. HITCHCOCK 601 



Then 



[H+] [H^por ] ^ h{h + x) 

 [H3PO4] c-h-x 



kc 



A + Jfe 



-A 



The values of x obtained in this way, expressed in cc. of 0.1 N 

 H3PO4 per 100 cc, are plotted in Curve IV, which indicates the 

 amount of H3PO4 really combined with the edestin. It was found 

 that above pH 3.4 the values so calculated did not differ much from 

 those plotted in Curve III, although it is not strictly justifiable to 

 consider H3PO4 as a monobasic acid in this range of pH, since the 

 values obtained for k were not constant above pH 3. 



Curves representing the amounts of hydrochloric, sulfuric, and 

 oxalic acids combined with 0.45 gm. of edestin in 100 cc. are given in 

 Fig. 2, along with the curve for H3PO4 as given in Fig. 1, Curve IV. 

 In the case of these stronger acids the amount combined was obtained 

 in the same way as Curve III, Fig. 1, by subtracting the ordinates of 

 the acid- water curve from those of the acid-edestin curve. The curves 

 for HCl and H2SO4 are nearly identical, indicating that each com- 

 bines in equivalent proportions with edestin; i.e., H2SO4 acts as a 

 dibasic acid. The curve for H2C2O4 above pH 4 is identical with 

 with that for H2SO4, indicating that here oxalic acid is also dibasic. 

 The curves soon diverge, however, and at the maximum that for 

 oxalic acid is almost exactly twice as high as that for hydrochloric, 

 indicating that here oxalic acid combines not in equivalent but in 

 molecular proportions; i.e., it is here a monobasic acid. The curve for 

 H3PO4 seems to reach a maximum at a height a little over three times 

 that of the curve for HCl and H2SO4, indicating that H3PO4 is com- 

 bined with the edestin in molecular, not in equivalent proportions, 

 or that H3PO4 combines with edestin as a monobasic acid. 



In order to show how closely these proportions hold, and in what 

 range of pH, the curves of Fig. 2 were plotted on a large scale and the 

 values given in Table I were read off. 



The values in Table I show that the ratios are most nearly as 1:2:3 

 in the region where the curves become horizontal, indicating that the 

 edestin is all combined. 



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