Lead orthoarsenite

Lead Orthoarsenite, Pb3(As03)2, is obtained as a white precipitate on adding a solution of basic lead acetate to a boiling aqueous solution of arsenious oxide,9 or of potassium tetrarsenite,10 or by the action of an alkali plumbite on an alkali arsenite.11 When dried in a dark air oven the arsenite remains white, but when exposed to light some specimens turn brown, a change which has been attributed to reduction of the lead to suboxide,12 but some arsenate and free arsenic are formed. All specimens... 

When a mixture of litharge and arsenious oxide is heated, formation of lead orthoarsenite begins at 250° C., but oxidation occurs,3 and with rising temperature increasing amounts of the orthoarsenate are formed, the latter being the sole product above 450° C. 

The Swedish mineral finnemanite 4 contains lead orthoarsenite in combination with lead chloride, the composition being 3Pb3(As03)2. PbC l2 (see p. 15). It occurs in dark grey or black prismatic crystals belonging to the hexagonal system and of axial ratio a c = 1 0-6880. 

The monohydrate, Pb3(As03)2.H20, was described by Stavenhagen,6 who obtained it by drying in air the precipitate resulting from the addition of potassium orthoarsenite to a dilute aqueous solution of lead nitrate. 

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