A REVISION OF THE ATOMIC WEIGHT OF PHOSPHORUS. 1 73 



essentially unchanged, while the concentration of the phosphate ion is increased 

 seven hundred times. Disodium phosphate doubtless takes a position inter- 

 mediate between the other two solutions in this regard, since it is more alkaline 

 than microcosmic salt and less so than disodium ammonium phosphate. The 

 numbers given above refer to solutions which are five times as strong as those 

 used in this research, but the conditions in the more dilute solutions must be 

 very similar. Furthermore, the exact values have no great importance, as the 

 concentrations of the various ions change continuously during precipitation. 

 It is evident from the figures given above and from the value of the dissociation 

 constant of the second hydrogen of phosphoric acid that if the concentration of 

 hydrogen ion increases above its value in a microcosmic salt solution, the con- 

 centration of the dihydrophosphate ion must increase greatly at the expense of 

 the monohydrophosphate ion. If there is any tendency for the occlusion of 

 disilver hydrogen phosphate or m.onosiiver hydrogen phosphate, the amounts 

 of these salts occluded would be expected to depend on the concentration of the 

 undissociated molecules of these salts in the solution, and therefore on the 

 concentration of the silver ion and on the concentration of the monohydrophos- 

 phate or dihydrophosphate ion respectively. 



The exact concentrations of the ions during the precipitation can not be 

 calculated, since the solubility of silver phosphate in slightly acid solutions and 

 the solubility-product of silver phosphate are not known. It is, however, easy 

 to understand from a study of the conditions under which the various samples 

 of silver phosphate were precipitated, that these concentrations must have 

 varied greatly in the preparation of the different samples and therefore con- 

 stancy of composition gives a strong presumption that there is very little or no 

 tendency for the occlusion of the undesired acid salts. 



Samples N and O. — A 0.03 normal solution of silver nitrate was slowly 

 poured into a 0.03 normal solution of disodium hydrogen phosphate with fre- 

 quent shaking. This reaction may be roughly considered to take place in two 

 stages represented by the equations 



3AgN03 + 2Na2HP04 = AgsPOi + NaH2P04 + 3NaN03 

 3AgN03 + NaH2P04 = Ag3P04 + NaNOs + 2HNO3 



At the beginning of the precipitation the solution is very sHghtly alkaline and 

 remains very nearly neutral during the addition of the first half of the silver 

 nitrate. The concentration of the silver ion is kept very low by the excess of 

 phosphate and, therefore, Kttle occlusion of the acid salts is to be expected in 

 spite of the fact that the solution contains appreciable concentrations of the 

 monohydrophosphate and dihydrophosphate ions. The precipitate during tliis 

 stage is very finely divided and does not settle well and, therefore, no attempt 

 was made to collect it separately. 



During the addition of the second half of the silver nitrate the solution be- 

 comes slightly acid and the solubility of the silver phosphate increases rapidly. 







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