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.