58 AGRICULTURAL ANALYSIS 



make a volume of two liters. Instead of the solution of 22 grams 

 of calcined magnesia, 1 10 grams of crystallized magnesium chlorid 

 (MgCl 2 .6H 2 O) may be used. 



Dilute Ammonia for Washing. This solution is prepared so 

 as to contain 2.5 per cent. NH 3 . 



Magnesium Nitrate Solution. Dissolve 320 grams of cal- 

 cined magnesia in nitric acid, avoiding an excess of the latter; 

 then add a little calcined magnesia in excess ; boil ; filter from the 

 excess of magnesia, ferric oxid, etc., and dilute with water to two 

 liters. 



Formulas for the Reactions. The reactions which take place 

 when a mineral acid, for instance, nitric, dissolves tricalcium 

 phosphate, may be represented as follows: Ca 3 (PO 4 ) 2 -f-4HNO 3 

 =:Ca(H 2 PO 4 ) 2 -{-2Ca(NO 3 ) 2 . In the case of a large excess of 

 acid, free phosphoric acid may be formed thus : 



Ca 3 (P0 4 ) 2 +6HN0 3 = 3 Ca(N0 3 ) 2 +2H 8 P0 4 . 

 Assuming that the phosphoric acid is in a soluble state in the 

 solutions prepared with the strong hot acids, the reactions which 

 take place in the process of separating it are as follows : 



1. (For free phosphoric acid) : 



2H 3 PO 4 +24 ( NH 4 ) 2 MoO 4 +42HNO 3 =2 ( NH 4 ) S PO 4 . i2MoO, 

 +42NH 4 NO 3 -f24H 2 O. 



2. (For monocalcium phosphate) : 

 Ca(H 2 PO 4 ) 2 +24(NH 4 ) 2 MoO 4 +44HNO 3 =2(NH 4 ) 3 PO 4 . 



i2MoO 3 +Ca(NH 3 ) 2 +42NH 4 NO 3 +24H 2 O. 

 The yellow precipitate 2(NH 4 ) 3 PO 4 .i2MoO 3 is dissolved in 

 ammonia with regeneration of ammonium molybdate as follows : 



3. 2(NH 4 ) 3 PO 4 .i2MoO 3 +48NH 4 OH=2(NH 4 ) s PCVr- 



24 ( NH 4 ) 2 MoO 4 +24H 2 O. 



4. Precipitation of the phosphoric acid with magnesia salts: 



(NH 4 ) 8 PO 4 +MgCl 2 =NH 4 MgP0 4 -fNH 4 Cl. 



5. Conversion of the ammonia-magnesium phosphate into mag- 

 nesium pyrophosphate by heat: 



2NH 4 MgP0 4 =Mg 1 P 1 T +2NH,+H f O. 

 The factors for calculating the phosphorus pentoxid and tri- 



