144 PROCEEDINGS OF THE AMERICAN ACADEMY. 



tration of the hydrogen ion to 0.16 percent of its value in the inicrocosmic 

 salt solution and decreases the concentration of the dihydrophosphate 

 ion to 0.2 percent of its former value. The concentration of the mono- 

 hydrophosphate ion remains essentially unchanged, while the concen- 

 tration of the phosphate ion is increased seven hundred times. 

 Disodium phosphate doubtless takes a position intermediate 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 concentra- 

 tion 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 monosilver hydrogen phos- 

 phate, 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 monohydrophosphate or dihydrophosphate 

 ion respectively. 



The exact concentrations of the ions during the precipitation cannot 

 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 constancy 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 0. A 0.03 normal solution of silver nitrate was 

 slowly poured into a 0.03 normal solution of disodium hydrogen phos- 

 phate with frequent shaking. This reaction may be roughly consid- 

 ered to take place in two stages represented by the equations 



3 AgN0 3 + 2 Na 2 HP0 4 = Ag 3 P0 4 + NaH 2 P0 4 + 3 NaN0 3 

 3 AgNOs + NaH 2 P0 4 = Ag 3 P0 4 + NaN0 3 + HN0 8 

 At the beginning of the precipitation the solution is very slightly 

 alkaline and remains very nearly neutral during the addition of the 



