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Manganese Arsenites

The orthoarsenite, Mn3(NaO3)2, occurs in Nature as the mineral armangite, and is associated with lead in trigonite. X-ray examination of Swedish armangite showed that it contains a hexagonal cell of 9 molecules, with a = 13.44 and c = 8.72 A. The arsenite has been obtained as the trihydrate, Mn3(NaO3)2.3H2O, by adding a solution of potassium orthoarsenite, made nearly neutral with acetic acid, to a solution of manganese acetate. The pale rose-coloured precipitate is washed with aqueous alcohol and dried in hydrogen over sulphuric acid. In the air it rapidly darkens in colour. Manganese tetrarsenite, Mn3As4O9.5H2O, is precipitated when potassium tetrarsenite or ammonium arsenite is added to a solution of a manganese salt. The pure compound is white, but in contact with air it rapidly turns reddish-brown. It loses 1 molecule of water at 100° C. and the remainder at 130° C., while at a higher temperature it decomposes. By adding a slight excess of sodium metarsenite to manganese sulphate solution Reichard obtained a precipitate of empirical composition Mn5As2O8, which, if a chemical individual, may be considered a basic orthoarsenite, Mn3(NaO3)2.2MnO. It is decomposed on heating.

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