DAVID I. HITCHCOCK 599 



ascertained by means of the hydrogen electrode, using a salt bridge 

 of saturated potassium chloride and a saturated potassium chloride 

 calomel cell. The measurements were made at 33° ± 0.5°, and were 

 referred to 0.1 M HCl as a standard, its pH being taken as 1.036. 



The titration curve obtained with 0.45 per cent solutions of edestin 

 in phosphoric acid is given in Fig. 1 , Curve I. As a means of determin- 

 ing how much of the acid was combined with the edestin. Curve II 

 was obtained by measuring the pH of mixtures of phosphoric acid 

 and water alone. Curve III, which gives the amount of phosphoric 

 acid apparently combined with the edestin, was obtained by sub- 

 tracting the ordinates of Curve II from those of Curve I at corres- 

 ponding pH values. Since phosphoric acid was known to be a weak 

 acid, it was suspected that some of the acid indicated by Curve III 

 to be combined with the edestin might really be present as un-ionized 

 molecules, due to the common ion effect of the ionized edestin phos- 

 phate. Accordingly a calculation was made of the first ionization 

 constant of phosphoric acid at 33°, from the data used in plotting 

 Curve II. The results are as follows: 



( 



These values may be compared with that found by Abbott and Bray 

 at 18°, 0.011.' Their constancy indicates that over this range of pH 

 the acid is really monobasic. Accordingly this ionization constant, 

 roughly 0.01, was used to calculate the amount of edestin phosphate 

 actually present, assuming complete ionization of this salt. This was 

 done as follows: 



Let c = total concentration of H1PO4 (ionized, un-ionized, and present as 

 edestin phosphate), 

 X = concentration of HiPO* from edestin phosphate (assumed to be 



completely ionized), 

 h = concentration of H+ = concentration of HjPO* from HjPO*, 

 k = primary ionization constant of H3PO4, 



'Abbott, G. A., and Bray, W. C. /. Am. Chem. Soc, 1909, x.xxi, 760. 



