Ferrous metaphosphate, Fe(PO₃)₂

Ferrous metaphosphate, Fe(PO₃)₂, is prepared by the action of fused metaphosphoric acid on ferrous phosphate, oxalate, or chloride, or even metallic iron, at red heat in a current of carbon dioxide. The salt is obtained as a white insoluble powder, unaffected by hydrochloric or nitric acid, but attacked by hot concentrated sulphuric acid.