Uranium Arsenates

Uranous Monohydrogen Orthoarsenate, U(HAsO₄)₂.2H₂O, may be prepared by digesting freshly precipitated uranous hydroxide with cold dilute arsenic acid; or by adding arsenic acid to a solution of the hydroxide in sulphuric acid. It yields pale green needle-shaped crystals, insoluble in water but soluble in excess of arsenic acid. A tetrahydrate, U(HAsO₄)₂.4H₂O, is formed as a green precipitate when an alkali arsenate is added to a solution of a uranous salt.

Uranyl Orthoarsenate, (UO₂)₃(AsO₄)₂.12H₂O, occurs in Nature as trogerite. It is yellow in colour, but on heating turns reddish-brown, with loss of water; on cooling, the product again becomes yellow. It breaks down when heated with water, yielding micaceous leaflets. The compound has been produced artificially.

Uranyl Monohydrogen Orthoarsenate, UO₂HAsO₄.4H₂O, is obtained as a pale yellow powder by the addition of uranyl acetate to a solution of arsenic acid or sodium arsenate. It is insoluble in water and acetic acid.

Uranyl Dihydrogen Orthoarsenate, UO₂(H₂AsO₄)₂.3H₂O, separates in small yellow crystals when a solution of uranyl acetate or nitrate containing an excess of arsenic acid is concentrated. The crystals lose their water of crystallisation at 150° C.

Uranyl Pyroarsenate, (UO₂)₂As₂O₇, remains as a yellow powder when the arsenate, UO₂HAsO₄.4H₂O, or ammonium uranyl arsenate (see below), is heated.

Several double arsenates occur in Nature, for example: zeunerite, Cu(UO₂)₂(AsO₄)₂.8H₂O; uranospinite, Ca(UO₂)₂(AsO₄)₂.8H₂O, corresponding to the phosphates chalcolite and autunite; and walpurgite, a basic bismuth uranyl arsenate. Compounds having the same composition as zeunerite and uranospinite may be formed artificially by mixing a solution of precipitated copper carbonate, or of lime, in excess of arsenic acid with a solution of uranyl acetate or nitrate; on allowing to crystallise, rectangular laminae are obtained. By similar means the ammonium salt, NH₄UO₂AsO₄, containing 3 and 5H₂O, has been prepared. The potassium salt is not easily obtained, since the product resulting from the reaction between potassium arsenate and a uranyl salt varies in composition with the concentration of the solutions. The salt, KUO₂AsO₄.3.5H₂O, has been obtained by the prolonged action of arsenic acid on potassium uranate as a yellow powder, stable towards water.