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Rubidium Arsenates

Rubidium Orthoarsenate, Rb3AsO4

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

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

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.

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