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 !^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 4- 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). 



2 Zeit. anorg. Chem., 5, 84; 7, 386 (1894). 



3 Bui. 41, Bureau of Soils, IT. S. Dept. Agr. (1907). 



58869°— 11 2 



