172 



RESEARCHES UPON ATOMIC WEIGHTS. 



for nitrate with diphenylaniine and for sodium by the spectroscope showed that 

 all of the first three substances named could be completely washed out. 



Joly ^ states that disilver phosphate is stable in the presence of phosphoric 

 acid containing 40 per cent (11,8 N) of phosphoric anhydride, but is trans- 

 formed into trisilver phosphate if the acid contains 38 per cent (11. o N) or 

 less of phosphoric anhydride. Since all the solutions 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 phosphate. Much light is 

 thrown on this point in a recent paper by Abbott and Bray ^ upon the dissocia- 

 tion constants of the three hydrogens of phosphoric acid, which were found to be 

 I.I X 10*^, 1.95 X 10'^ and 3.6 X 10"^^ respectively. Since the phosphate 

 ion (P04=) is almost completely hydrolyzed to the monohydrophosphate ion 

 (HP04=), even in slightly alkaline solutions, and since in slightly acid solutions 

 the dihydrophosphate ion (H2PO4 — ) 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 numbers with the 

 help of the values of the dissociation constants of phosphoric 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 concentration of the 

 hydrogen ion to 0.16 per cent of its value in the microcosmic salt solution and 

 decreases the concentration of the dihydrophosphate ion to 0.2 per cent of its 

 former value. The concentration of the monohydrophosphate ion remains 



1 C. R., 103, 1071 (1886). 2 Jour. Amcr. Chem. Soc, 31, 755 (1909). 



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

 * These values are calculated from the others in the above table by the aid of the following 

 equations : 



(H+)(OH-) = 0.59 X 10-14 



(H+ )(P04S) 

 (HP04=) 



3 • 6 X 10-" 



(H+)(HP04=) ^ _. 



