Ferric orthophosphate, FePO₄

The Ferric orthophosphate dihydrate, FePO₄.2H₂O, occurs in nature as strengite, and may be obtained in the laboratory in crystalline form by heating a concentrated solution of ferric chloride with phosphoric acid in a sealed tube at 180° to 190° C. for several hours, when it separates out as small rose-coloured monoclinic crystals, of density 2.74 at 15 ° C. The crystals closely resemble those of strengite, but the latter are rhombic. They are insoluble in nitric acid, but dissolve in hydrochloric acid.

The amorphous dihydrate results as a yellowish-white precipitate on mixing solutions of sodium phosphate and ferric chloride or sulphate. It may also be formed by dissolving iron in excess of phosphoric acid and exposing to the air; by saturating a 48 per cent, solution of phosphoric acid on the water-bath with iron powder; by boiling solutions of ferric metaphosphate, ferric dihydrogen orthophosphate, or of acid ferric phosphate; or by evaporating a mixed solution of ferric chloride (4 molecules) and disodium hydrogen phosphate (1 molecule) with an excess of nitric acid. As the liquid becomes syrupy, a copious separation of the phosphate occurs.

For the preparation of the pure salt, Caven recommends addition of ferric chloride solution to one of orthophosphoric acid - made by boiling glacial metaphosphoric acid with water until it ceases to give any precipitate with barium chloride in such proportions that rather more than twice as much acid, calculated as H₃PO₄, is present as is required to precipitate the iron. The precipitate thus obtained appears, when suspended in water, perfectly white; when strained on calico, it possesses a slightly bluish tint. Since hot water hydrolyses the salt, the washing should be conducted in the cold.

The resulting salt, whilst readily soluble in dilute mineral acids, is insoluble in cold acetic acid, phosphoric acid, and sodium phosphate. It is slightly soluble in citric and tartaric acid solutions, and readily dissolves in neutral aqueous ammonium citrate, yielding a green solution with a brownish tint. The salt is insoluble in water, but hot water hydrolyses it, and boiling with excess of ammonia solution converts it into a mixture of ferric hydroxide and ferric phosphate, or if the ammonia is present in great excess the ferric phosphate may be entirely decomposed. Thus: -

FePO₄ + 3NH₄OH ⇔ Fe(HO)₃ + (NH₄)₃PO₄.

The reaction is reversible, and on boiling a suspension of freshly precipitated ferric hydroxide in an aqueous solution of ammonium phosphate, ammonia is liberated and ferric phosphate remains. In consequence of this reversibility, and owing to the volatility of ammonia and the active mass of the ammonium phosphate produced, it is not easy to carry the first-named reaction to completion, namely, in the direction of from left to right in the foregoing equation.

These facts have an important bearing on the separation and estimation of iron by means of ferric phosphate.

The trihydrate, FePO₄.3H₂O, occurs in nature as the mineral Koninckite.

By heating molecular proportions of ferric chloride or acetate with phosphoric acid on the water-bath for two days, the pentahydrated salt, 2FePO₄.5H₂O, is obtained as a pink powder, sparingly soluble in water and dilute acids. It may be regarded as the ferric salt of hydrogen ferri-diorthophosphate, H₃[Fe(PO₄)₂].5H₂O, and therefore written as Fe[Fe(PO₄)₂].5H₂O.

Colloidal ferric phosphate is obtained by dissolving ferric phosphate in ammoniacal di-ammonium hydrogen phosphate and dialysing the solution until all electrolytes have been washed away. The colloidal solution thus obtained is tasteless and without action on litmus. Addition of electrolytes, such as alkali chlorides, effects its gelatinisation.

Both potassium ferrocyanide and thiocyanate cause gelatinisation also, but induce no colour change, showing the absence of free ions from the solution.