THE MANUFACTURE OF ACID PHOSPHATE. 9 
of an indefinite composition. The exact reactions that take place 
when this substance is treated with sulphuric acid are not known. 
Unquestionably, however, the iron is distributed between the two 
acids. A mixture* of " sticky," disagreeable physical properties 
results, the composition of the solid part of the mixture changing 
with the composition of the liquid part which is formed at the same 
time. Both the solid and the liquid contain all three constituents — 
iron, sulphuric acid, and phosphoric acid. Dilution of this liquid 
mass by the addition of water causes a precipitation of more jellylike 
material containing relatively more iron and phosphoric acid than 
sulphuric acid. The general course of the reactions are sufficiently 
well known to justify the assumption that they go mainly according 
to the following equations: 
2FeP0 4 + 3H 2 S0 4 t;Fe 2 (S0 4 ) 3 + 2H 3 P0 4 
But a part of the iron sulphate produced reacts with the phosphoric 
acid or monocalcium phosphate in the mass forming hydrated phos- 
phate of iron, the gelatinous precipitate almost insoluble in water, 
and when present in any quantity causing the acid phosphate to be 
sticky and difficult to handle. The reactions may be represented 
thus : 
Iron sulphate. Phosphoric acid. Water or steam. Hydrated iron Sulphuric acid. 
phosphate 
Fe 2 (S0 4 ) 3 + 2H 3 P0 4 + 4H 2 <=± 2FeP0 4 .2H 2 + 3H 2 S04 
According to Fritsch, 1 however, two per cent of iron oxide in the 
raw material is not objectionable, because the quantity of iron 
sulphate produced therefrom remains unaltered in the superphos- 
phate. It is true that in properly made acid phosphate nearly all of 
the phosphoric acid is soluble in water even though there is sufficient 
iron present to cause part of it to revert, but Fritsch is probably in 
error in attributing this to the fact that the iron is all in the form of 
sulphate. Schneider 2 has shown experimentally that solutions of 
sulphate of iron increase the solubility of iron phosphate and Cameron 
and Bell 3 have demonstrated that gypsum, lime, and phosphoric 
acid also increase the solubility of this substance. 
Hydrated iron phosphate may be converted into the anhydrous 
and less soluble condition by reacting with anhydrous calcium sul- 
phate; the last-named compound being converted into gypsum, thus: 
Hydrated Anhydrous Anhydrous iron 
iron phosphate, calcium sulphate. Gypsum. phosphate. 
FeP0 4 2H 2 + CaS0 4 <=± =CaS0 4 .2H 2 0+ FeP0 4 . 
i Manufacture of Chemical Manures, pp. 78-79 (1911). 
2Zeit. anorg. Chem., 5, 84; 7, 386(1894). 
s Bui. 41, Bureau of Soils, U. S. Dept. Agr. (1907). 
58869°— 11 2 
