A part of the iron sulfate formed reacts with the acid calcium phosphate present to produce a permanent jelly-like compound, difficult to dry and handle. As much as two per cent of iron phosphate, however, may be present without serious interference with the commercial handling of the product. By using more sulfuric acid as much as four or five per cent of the iron phosphate can be held in solution. Larger quantities are very troublesome from a commercial point of view. The reaction of the ferric sulfate with monocalcium phosphate, is as follows:
3CaH(PO₄)₂ + Fe₂(SO₄)₃ + 4H₂O = 2(FePO₄·2H₃PO₄·2H₂O) + 3CaSO₄.
Pyrite and the silicates containing iron are not attacked by sulfuric acid, and these compounds are therefore left, in the final product, in a harmless state. If the pyritic iron is to be brought into solution aqua regia should be employed.
With sufficient acid the aluminum phosphate is decomposed with the formation of aluminum sulfate and free phosphoric acid:
AlPO₄ + 3H₂SO₄ = Al₂(SO₄)₃ + 2H₃PO₄.
140. Reaction with Magnesium Compounds.—The mineral phosphates, as a rule, contain but little magnesia. When present it is probably as an acid salt, MgHPO₄. Its decomposition takes place in slight deficiency or excess of sulfuric acid respectively as follows:
2MgHP₄ + H₂SO₄ + 2H₂O = [MgH₄(PO₄)₂·2H₂O] + MgSO₄
and MgHPO₄ + H₂SO₄ = H₃PO₄ + MgSO₄.
The magnesia, when in the form of oxid, is capable of producing a reversion of the monocalcium phosphate, as is shown below:
CaH₄(PO₄)₂ + MgO = CaMgH₂(PO₄)₂ + H₂O.
One part by weight of magnesia can render three and one-half parts of soluble monocalcium phosphate insoluble.