280 AGRICULTURAL ANALYSIS 



phosphate, however, may be present without serious interference 

 with the commercial handling of the 'product. By using more 

 sulfuric acid, as much as four or five per cent, of the iron phos- 

 phate can be held in solution. Larger quantities are very trouble- 

 some from a commercial point of view. The reaction of the 

 ferric sulfate with monocalcium phosphate is as follows : 

 3 CaH ( P0 4 ) 2 +Fe 2 ( SO 4 ) 3 .+ 4 H 2 O=2 ( FePO 4 .2H 3 PO 4 . 2 H 2 O ) + 

 3CaSO 4 . 



Pyrite and the silicates containing iron are not attacked by 

 sulfuric acid and these compounds are therefore left, in the final 

 product, in a harmless state. If the pyritic iron is to be brought 

 into solution aqua regia should be employed. 



With sufficient acid the aluminum phosphate is decomposed 

 with the formation of aluminum sulfate and free phosphoric acid : 

 A1P0 4 + 3H 2 S0 4 = A1 2 ( S0 4 ) 3 +2H 3 P0 4 . 



241. Reaction with Magnesium Compounds. The mineial phos- 

 phates, as a rule, contain but little magnesia. When pres- 

 ent it is probably as an acid salt, MgHPO 4 . Its decomposition 

 takes place in slight deficiency or excess of sulfuric acid, respect- 

 ively, as follows: 



2MgHPO 4 +H,SO 4 +2H 2 O:= [ MgH 4 ( PO 4 ) 2 .2H 2 O ] + MgSO 4 

 and MgHPO 4 +H 2 SO 4 =H 3 PO 4 +MgSO 4 . 



The magnesia, when in the form of oxid, is capable of pro- 

 ducing a reversion of the monocalcium phosphate, as is shown 

 below : 



CaH 4 ( PO 4 ) 2 -f MgO=CaMgH 2 ( PO 4 ) 2 + H 2 O. 

 One part by weight of magnesia can render three and one-half 

 parts of soluble monocalcium phosphate insoluble. 



242. Determination of Quantity of Sulfuric Acid Necessary for 

 Solution of a Mineral Phosphate. The theoretical quantity of 

 .sulfuric acid required for the proper treatment of any phosphate 

 may be calculated from its chemical analysis and by the formulas 

 and reactions already given. For the experimental determination 

 the method of Riimpler may be followed. 38 



Twenty grams of the fine phosphate are placed in a liter flask 

 38 Kaufliche Diingestoffe und ihre Anwendung, 4th Edition, 1897 : Si. 



