Ferrous hydrogen orthophosphate, FeHPO₄

Ferrous hydrogen orthophosphate, FeHPO₄.H₂O, has been obtained by dissolving iron in boiling phosphoric acid solution. The salt crystallises out in colourless needles which become bluish in air; it is insoluble in water, but readily dissolves in dilute acids and in ammonia. When heated with water to 250° C., it is converted into vivianite.

On adding ammonium phosphate solution to ferrous chloride in alcohol saturated with nitric oxide at 0° C., a viscid oily liquid is precipitated, which crystallises when placed in a freezing mixture. Upon careful purification at low temperatures, brown flaky crystals are obtained, which melt at 16° C., and have the composition represented by the formula FeHPO₄.NO. Upon exposure to air it oxidises slowly, yielding white ferric phosphate.