Uranyl Arsenates

The orthoar senate, (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 hydrogen arsenate, 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 arsenate, 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.