Ferric metaphosphate, Fe₂O₃.3P₂O₅

Ferric metaphosphate, Fe₂O₃.3P₂O₅.6H₂O or Fe(PO₃)₃.3H₂O, has been prepared by dissolving ferric oxide in glacial phosphoric acid at 100° C. and maintaining the liquid at this temperature for several hours. The salt separates out in small crystals which take the form of pink rhombic plates derived from a monoclinic prism. The crystals are decomposed by water.

If the foregoing reaction is carried out at 150° to 200° C., crystals of the dihydrate, Fe(PO₃)₃.2H₂O, are obtained as pink, rectangular lamellae which are more stable towards water and are not attacked by alcohol.

At temperatures above 200° C. the anhydrous salt, Fe(PO₃)₃, is obtained, the form of which depends upon the temperature at which crystallisation is allowed to take place. This salt was not first made known to science by this method, however, but by addition of ferric chloride to an excess of diluted metaphosphoric acid, concentration of the liquid, and finally heating the residue to 315° C. (600° F.).

It may also be prepared by addition of ferric oxide, phosphate, or of anhydrous ferrous sulphate to metaphosphoric acid until no more of the salt will dissolve. The whole is then heated until the sulphuric acid has been expelled and crystallisation sets in.

As obtained in this manner, the salt is insoluble in water and in dilute acid, but soluble in concentrated sulphuric acid. Density 3.02.