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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

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.

Magnesium Pyroarsenite, Mg2As2O5

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

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

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