BAXTER AND JONES. — ATOMIC WEIGHT OF PHOSPHORUS. 143 



diphenylamine and for sodium by the spectroscope showed that all of 

 the first three substances named could be completely washed out. 



Joly 19 states that disilver phosphate is stable in the presence of 

 phosphoric acid containing 40 per cent (11.8 N) of phosphoric anhy- 

 dride, but is transformed into trisilver phosphate if the acid contains 

 38 per cent (11.0 N) or less of phosphoric anhydride. Since all the solu- 

 tions used for the preparation of silver phosphate were nearly neutral, 

 it is evident that the precipitation of disilver phosphate as a distinct 

 phase in equilibrium with the solution is not to be feared. 



It is, however, not such a simple matter to prove the absence of 

 occluded disilver hydrogen phosphate or monosilver hydrogen phos- 

 phate. Much light is thrown on this point in a recent paper by 

 Abbott and Bray 20 upon the dissociation constants of the three hydro- 

 gens of phosphoric acid, which were found to be 1.1 X 10~ 2 , 1.95 X 10 -7 

 and 3.6 X 10~ 13 respectively. Since the phosphate ion (P0 4 =) is almost 

 completely hydrolyzed to the monohydrophosphate ion (HP0 4 = ), even 

 in slightly alkaline solutions, and since in slightly acid solutions the 

 dihydrophosphate ion (H 2 P0 4 ~) acquires an appreciable concentration, 

 the possibility of occlusion must be examined with especial care. 



The concentrations in the following table are either taken directly 

 from a table given by Abbott and Bray or calculated from these num- 

 bers with the help of the values of the dissociation constants of phos- 

 phoric acid. The values are expressed in formular weights per liter, 

 the total concentration of the salt being in each case 0.05. 



It will be noted that the replacement of the remaining hydrogen in 

 sodium ammonium hydrogen phosphate by sodium decreases the concen- 



19 C. R., 1SS6, 103, 1071. 



20 Jour. Amer. Chem. Soc, 1909, 31, 755. 



21 These values are taken directly from the table of Abbott and Bray. 



22 These values are calculated from the others in the above table by the 

 aid of the following equations : 



(H+)(OH-) =0.59 X 10- 1 " 



( H+ X P0 ^ = 3-6 X 10- (H + )(HPQ 4 =) = i 95 X 10-7 



(HP0 4 =) * X (H 2 P0 4 -) -yo X 



