Complex Uranyl Arsenates

Several double arsenates occur in nature: 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 double decomposition 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.