Ferric dihydrogen orthophosphate, Fe(H₂PO₄)₃

Ferric dihydrogen orthophosphate, Fe(H₂PO₄)₃, results when ferric oxide is dissolved in phosphoric acid until a precipitate begins to appear. The solution is washed free from excess of acid with ether and dried, whereby a pink crystalline powder is obtained. When boiled with water, it yields ferric orthophosphate, FePO₄.2H₂O.

With water at the ordinary temperature it yields the anhydrous phosphate FePO₄ and free phosphoric acid. The greater the proportion of water, the more complete the decomposition.

With a large excess of cold water a precipitate of composition 6Fe₃PO₄.Fe₂(HPO₄)₃ has been obtained.