Magnesium Arsenites

An investigation of the ternary system MgO-As₂O₃-H₂O at 25° C. gave no evidence of chemical combination, but considerable adsorption of arsenious oxide by the magnesium hydroxide occurred. Combination of the two oxides occurs on heating; at the melting point of arsenious oxide some metarsenite is formed, and at a higher temperature a basic arsenite of composition Mg₃(NaO₃)₂.3MgO is produced.

Magnesium Orthoarsenite, Mg₃(NaO₃)₂, is obtained as a white precipitate, easily soluble in water and in dilute acids, by adding to a solution of magnesium chloride in 50 per cent, alcohol a solution of potassium orthoarsenite made just neutral with acetic acid. It is also formed when a solution of potassium tetrarsenite is added slowly to an aqueous solution of a magnesium salt. When heated in an inert atmosphere the salt decomposes forming arsenious and magnesium oxides.

4H₂O.Magnesium Pyroarsenite, Mg₂As₂O₅.4H₂O, is produced by double decomposition between barium pyroarsenite and magnesium sulphate. It is a white, granular, hygroscopic, amorphous powder, soluble in water and dilute acids; on heating it undergoes decomposition and darkens in colour. Bloxam obtained a product which he described as the pyroarsenite by precipitating magnesium sulphate with ammonium arsenite and heating the precipitate; but Stavenhagen could not obtain the salt by this method.

Magnesium Tetrarsenite, Mg₃As₄O₉.nH₂O, is formed when a solution of magnesium sulphate or magnesia mixture is added to an aqueous solution of arsenious oxide.