6 BULLETIN 144, IT. 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 Hme 
in a hypothetical compound — trie ale ium 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 monoealcium phosphate 
[Ca(H 2 P0 4 ) 2 ]. Both gypsum and monoealcium 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 dicalcium phosphate 
[Ca(HP0 4 ) 2 ] is formed as well. Both these calcium phosphates are 
decomposed b} r water, so that a solution of monoealcium phosphate, 
if diluted, will precipitate dicalcium 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. Monoealcium 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 by 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 vaiying 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). 
