Ferric arsenite, 4Fe₂O₃.As₂O₃

Ferric arsenite, 4Fe₂O₃.As₂O₃.5H₂O, may be prepared by shaking freshly precipitated ferric hydroxide with an aqueous solution of arsenious oxide, or by adding sodium arsenite (or an aqueous solution of arsenious oxide) to ferric acetate. It is brown in colour, and oxidised by the air when moist.

By the action of potassium hydrogen arsenite, K₂O.As₂O₃.H₂O, on dilute aqueous ferric chloride a yellow powder of composition Fe₂O₃.As₂O₃ or FeAsO₃ has been obtained. A substance of similar chemical composition has been found in a crystalline deposit formed during the Deacon process of making chlorine. The crystals of the pure salt are monoclinic, their crystallographic elements being: -

a: b: c = 0.9405: 1: 0.6234. β = 105° 10.5'.

The pentahydrate, FeAsO₃.5H₂O, found with the preceding salt, crystallises in the rhombic system.

On adding freshly precipitated ferric arsenite to potassium hydroxide solution until no more dissolves, and subsequently evaporating, the soluble potassium salt, 6K₂O.5Fe₂O₃.9As₂O₃.24H₂O, is obtained as a reddish brown amorphous substance, which dissolves in water, yielding an alkaline solution.