6 BULLETIN 144, U. S. DEPARTMENT OF AGRICULTURE. 



known just how these constituents are chemically united. It is gen- 

 erally assumed that the phosphoric acid is combined with the lime 

 in a hypothetical compound — tricalcium phosphate (known to the 

 trade as bone phosphate of lime, b. p. 1.), represented by the formula 

 Ca 3 (P0 4 ) 2 , and that this compound, when treated with sulphuric acid 

 (H 2 S0 4 ) and water (H 2 0) in the right proportions, is converted into 

 a mixture of gypsum (CaS0 4 .2H 2 0) and monocalcium phosphate 

 [Ca(H 2 P0 4 ) 2 ]. Both gypsum and monocalcium phosphate are per- 

 fectly definite, well-known compounds. The former is but slightly 

 soluble, the latter readily soluble in water. As a matter of fact, in 

 the reaction cited above, it is probable that diealcium phosphate 

 [Ca(XTP0 4 ) 2 ] is formed as well. Both these calcium phosphates are 

 decomposed by water, so that a solution of monocalcium phosphate, 

 if diluted, will precipitate diealcium phosphate and if the dilution 

 be carried further, a phosphate even more basic than the tricalcium 

 phosphate is formed. 1 Obviously, the more basic the calcium phos- 

 phate, the less soluble it is in water. It is equally obvious that when 

 incorporated in the soils, the soil water, while dissolving and dis- 

 tributing the phosphate, is at the same time decomposing it into 

 less soluble forms. Assuming now, as we may do for convenience, 

 that the reaction takes place in the mixing as outlined above, it may 

 be represented thus: 



Tricalcium phosphate or pure phosphate rock. Sulphuric acid. Water. 



Ca 3 (P0 4 ) 2 + 2H 2 S0 4 + 4H 2 = 



1 molecule, weight 310. 2 molecules, 4 molecules, 



weight 196. weight 72. 



Gypsum. Monocalcium phosphate or superphosphate. 



2(CaS0 4 2H 2 0) + CaH 4 (P0 4 ) 2 . 



2 molecules, weight 344. 1 molecule, weight 234. 



The above equation means that in order to change completely 310 

 parts of tricalcium phosphate or pure phosphate rock into acid 

 phosphate, 196 parts of pure sulphuric acid are required, or 1 ton of 

 phosphate rock requires 0.63 ton of sulphuric acid. Factory practice 

 and long experience in the manufacture of acid phosphate have 

 shown, however, that much better results are obtained by employing 

 sulphuric acid containing from 30.35 to 37.82 per cent of water 

 ("chamber acid")- A part of the water contained in this acid is 

 evaporated hj the heat of the chemical reactions taking place, and a 

 part is taken up by the calcium sulphate formed to produce gypsum, 

 as shown in the above equation. 



IMPURITIES IN PHOSPHATE ROCK. 



Besides calcium phosphate the phosphates of commerce always 

 contain varying quantities of impurities, such as organic matter, 

 silica or silicates, calcium fluoride, oxides or phosphates of iron and 



i Vide, Bui. 41, Bureau of Soils, U. S. Dept. Agr., pp. 22-25 (1907). 



