Lead Arsenites

An investigation of the ternary system PbO-As₂O₃-H₂O at 25° C. gives evidence of the formation of the metarsenite, Pb(AsO₂)₂, only. Several other well-defined arsenites, are known, however.

Lead Orthoarsenite, Pb₃(NaO₃)₂, is obtained as a white precipitate on adding a solution of basic lead acetate to a boiling aqueous solution of arsenious oxide, or of potassium tetrarsenite, or by the action of an alkali plumbite on an alkali arsenite. 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, but some arsenate and free arsenic are formed. All specimens of the orthoarsenite are not photo-sensitive. A dry mixture of litharge and arsenious oxide is remarkably sensitive, arsenic being liberated in the reaction which occurs; and when the arsenite is prepared from lead hydroxide by heating with arsenious oxide and dilute aqueous sodium hydroxide, the product is also light-sensitive. The commercial product and those obtained by the methods described above are not sensitive. It has been suggested that in the latter case the salt has the composition 3Pb(OH)₂.As₂O₃. No other metallic arsenite has been observed to be photo-sensitive. Lead arsenite may be used as a light-sensitive agent in photographic, photometric and actinometric compositions, the exposed negative being developed by dry heating or wet reduction; other light-sensitive substances may be admixed.

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

The Swedish mineral Jinnemanite contains lead orthoarsenite in combination with lead chloride, the composition being 3Pb₃(NaO₃)₂.PbCl₂. It occurs in dark grey or black prismatic crystals belonging to the hexagonal system and of axial ratio a:c = 1:0.6880.

Lead arsenite is readily obtained by an electrolytic process. The catholyte may be a 15 per cent, solution of sodium hydroxide or of a sodium salt, or a 30 per cent, solution of potassium hydroxide, the cathode being of nickel. The anolyte is a solution of an alkali arsenite neutralised towards phenolphthalein by means of acetic acid, and the anode is of lead. The cathode space is separated by a diaphragm of vegetable parchment. Ormont obtained a current yield of 97 per cent., and the average energy yield amounted to 1 kg. per kilowatt-hour.

The monohydrate, Pb₃(NaO₃)₂.H₂O, was described by Stavenhagen, who obtained it by drying in air the precipitate resulting from the addition of potassium orthoarsenite to a dilute aqueous solution of lead nitrate.

Lead Pyroarsenite, Pb₂As₂O₅, is a white powder formed by decomposing normal lead acetate with ammoniacal arsenious oxide solution, or with potassium tetrarsenite or pyroarsenite. According to Simon, it is also formed by the combination of arsenious oxide vapour with lead oxide; Stavenhagen, however, found the product to be merely a mixture of oxides. When heated, lead pyroarsenite fuses to form a yellow glass. It liberates ammonia from ammonium salts even in the cold.

Lead Metarsenite, Pb(AsO₂)₂, is formed from a hot saturated solution of arsenious oxide in ammonia by addition of a neutral lead salt. It forms microscopic prisms of density 5.85 at 23° C., very slightly soluble in water. When heated in an inert atmosphere it decomposes readily into lead oxide and arsenious oxide.

A tetrarsenite, Pb₃As₄O₉.3H₂O, was described by Bloxam but could not be obtained by Stavenhagen.

A colloidal solution of lead arsenite has been obtained by the following method. A solution of lead acetate is treated with a 10 per cent, solution of sodium lysalbinate, and after washing the precipitate with hot water, it is dissolved in a dilute aqueous solution of sodium hydroxide containing sodium dihydrogen arsenite. The liquid, after keeping for some hours, is filtered and dialysed until the outside liquor is free from arsenic. The solution in the dialyser is concentrated, mixed with alcohol and ether, and dried over sulphuric acid in a vacuum. A pale yellow horny mass remains, which dissolves readily in water to form a sol.