Ferrous ortho-arsenate, Fe₃(AsO₄)₂

Ferrous ortho-arsenate, Fe₃(AsO₄)₂.6H₂O, occurs in nature as the mineral simplesite.

It may be prepared by the action of disodium hydrogen arsenate upon ferrous sulphate solution: -

4FeSO₄ + 4Na₂HAsO₄ = Fe₃( AsO₄)₂ + Fe(H₂AsO₄)₂ + 4Na₂SO₄,

and gradually undergoes oxidation upon exposure to moist air, yielding ferric arsenate and ferric oxide.