278 AGRICULTURAL ANALYSIS 



monocalcium salt and the further reaction of this with the tri- 

 calcium compound, as follows: 



' CaH 4 (P0 4 ) 2 +H 2 0+Ca 8 (P0 4 ) 2 + 7 H 2 

 =2[Ca 2 H 2 (P0 4 ) 2 . 4 H 2 0]. 



This reaction represents, theoretically, the so-called reversion 

 of the phosphoric acid. When there is an excess of sulfuric acid 

 there is a complete decomposition of the calcium salts with the 

 production of free phosphoric acid and gypsum. The reaction 

 is represented by the following formula: 

 Ca 3 (P0 4 ) 2 -f 3 H 2 S0 4 -f6H 2 

 =2H 8 PO 4 +3[CaSO 4 .2H 2 O]. 



The crystallized gypsum absorbs the six molecules of water in 

 its molecular structure. While the above reactions represent the 

 theoretical conditions, there is a wide divergence from them in 

 actual manufacture. In the case of dissolved bone, especially, the 

 actual quantity of sulfuric acid used is not so great as is indi- 

 cated. The proportions of acid and raw materials are necessarily 

 changed from time to time to meet the emergencies which may 

 arise. 



238. Reactions with Fluorids. Since calcium fluorid is pres- 

 ent in nearly all mineral phosphates, the reactions of this 

 compound must be taken into consideration in a chemical study 

 of the manufacture of acid phosphates. When treated with sul- 

 furic acid the first reaction which takes place consists in the for- 

 mation of hydrofluoric acid : CaF 2 -f- H 2 SO 4 = 2HF -(- CaSO 4 . 

 Since, however, there is generally some silica in reach of the 

 nascent acid, all, or a portion of it, combines at once with this 

 silica, forming silicon tetrafluorid : 4HF-|-SiO 2 =2H 2 O-|-SiF 4 . 

 This compound, however, is decomposed at once in the presence 

 of water, forming hydrofluosilicic acid : 3SiF 4 -}-2H 2 O=SiO 2 + 

 2H 2 SiF 6 . The presence of calcium fluorid in natural phosphates 

 is extremely objectionable from a technical point of view, both 

 on account of the increased consumption of oil of vitriol which 

 it causes, and also by reason of the injurious nature of the gaseous 

 fluorin compounds produced. Each 100 pounds of calcium flu- 

 orid entails the consumption of 125.6 pounds of sulfuric acid, for 

 which no economic return is secured. 



