Ferrous orthophosphate, Fe₃(PO₄)₂

Ferrous orthophosphate occurs in nature in the octahydrated form as vivianite, Fe₃(PO₄)₂.8H₂O, in monoclinic crystals. When perfectly pure the crystals are colourless, but most specimens are tinged with a greenish blue in consequence of slight oxidation. They melt at 1114° C.

The salt may be obtained by heating diferrous orthophosphate with water to 250° C.; by heating in a sealed tube mixed solutions of sodium phosphate and ferrous sulphate in an atmosphere of carbon dioxide; or by electrolysis of sodium phosphate solution, using an iron anode.

A powder containing at least 90 per cent, of the octahydrated ferrous phosphate is obtained by mixing solutions of sodium acetate (2 parts), sodium phosphate (10 parts), and ferrous sulphate (8 parts), and allowing to stand in the absence of air for several days. The precipitate is collected on a calico filter and dried at 40° C. If the liquid and precipitate are maintained together at a temperature of 60° to 80° C. for a week, small crystals develop.

The salt is insoluble in water; it readily oxidises in air, becoming bluish in colour.

The Ferrous orthophosphate hexahydrate, Fe₃(PO₄)₂.6H₂O, has been prepared by the prolonged heating at 83° C. of finely powdered ferrous carbonate held in suspension in an aqueous solution of ammonium phosphate. The same substance results if a solution of ferrous carbonate in carbonic acid is heated with ammonium phosphate. Gautier suggests that the mineral vivianite has originated in a similar manner to this, but that the reaction has proceeded for a longer time and at a lower temperature, which would explain the higher water content of the mineral.