Rubidium Arsenates

Rubidium Orthoarsenate, Rb₃AsO₄, is prepared by adding a solution of rubidium hydroxide to aqueous arsenic acid until the former is in excess. Very hygroscopic white lamellae of the dihydrate, Rb₃AsO₄.2H₂O, are deposited on evaporation. The salt absorbs carbon dioxide from the air and its solution is alkaline in reaction. When heated, the water of crystallisation is lost at 100° C.

Rubidium Monohydrogen Orthoarsenate, Rb₂HAsO₄, is obtained by mixing aqueous solutions of rubidium hydroxide and rubidium dihydrogen orthoarsenate in molecular proportions; it forms white, hygroscopic lamellae of the monohydrate, Rb₂HAsO₄.H₂O. It absorbs carbon dioxide slowly from the air, or more rapidly in aqueous solution. The crystals become anhydrous when heated and, above 150° C., rubidium pyroarsenate, Rb₄As₂O₇, is formed, which at a dull red heat decomposes to form a milky-white crystalline mass of rubidium metarsenate, RbNaO₃, which itself decomposes at a bright red heat.

Rubidium Dihydrogen Orthoarsenate, RbH₂AsO₄, is obtained in the anhydrous form by fusing together equal parts of arsenious oxide and rubidium nitrate, or by neutralising rubidium carbonate solution with arsenic acid using methyl orange as indicator; in the former method it yields tabular crystals and in the latter silky needles. At a dull red heat it forms the metarsenate.