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Uranium Arsenates

Uranous Monohydrogen Orthoarsenate, U(HAsO4)2

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

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

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

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

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

Double arsenates

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.

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