Lead Arsenates

Lead Orthoarsenate, Pb₃(AsO₄)₂, is formed when a mixture of arsenious oxide and lead monoxide is heated at 450° to 800° C. It is difficult to obtain in a pure state by precipitation, but products approximating in composition to it are obtained when solutions of lead salts are added to alkali hydrogen arsenates. It appears to be unstable and to exist only under limited conditions.

The anhydrous salt yields yellowish-white crystals of density 7.3 at 15° C. and melting point 1042° C. The specific heat is 0.0728 and the molar heat 65.4. The index of refraction is 2.14. It is insoluble in water, aqueous ammonia and in solutions of ammonium salts, and only slightly soluble in aqueous alkali or brine. It is decomposed when heated with hydrogen chloride, forming lead and arsenic chlorides, and in hot nitric acid it dissolves to form arsenic acid and lead nitrate, the latter being precipitated if the acid is sufficiently concentrated.

Lead Monohydrogen Orthoarsenate, PbHAsO₄, is deposited in the form of white crystalline needles when a boiling solution of the normal arsenate in nitric acid is diluted with water or treated with a dilute solution of ammonia. It is also obtained by the prolonged action of arsenic acid and air upon metallic lead, or by decomposing aqueous lead nitrate with arsenic acid or sodium monohydrogen arsenate. It has been found as a mineral in S.W. Africa and named schultenite; the colourless monoclinic crystals have axial ratios a:b:c = 0.8643:1:0.7181, β = 84°36', and indices of refraction along the three axes α 1.8903, β 1.9077 and γ 1.9765. When heated above 280° C. lead pyroarsenate is formed, which fuses at a bright red heat.

The monohydrogen arsenate is insoluble in cold water and only very slightly soluble on long contact with boiling water, but the prolonged action of cold water converts it to a basic arsenate of composition Pb₅(OH)(AsO₄)₃. It is attacked by halogen acids and salts and by nitric acid.

Commercial lead arsenate usually consists mainly of the monohydrogen arsenate, but may also contain the normal arsenate. It is in great demand as an insecticide. Many methods of manufacture are described in the patent literature, some of the more recent being as follows: (1) Metallic lead and arsenious oxide are added to a concentrated solution of arsenic acid containing nitric acid; lead arsenate is precipitated, the concentration of the arsenic acid remaining constant. At intervals the precipitate is removed and the arsenic acid solution again treated. (2) A solution of a soluble arsenate is treated with lead chloride to cause partial conversion to lead arsenate and the reaction completed by adding lead nitrate. (3) Steam is passed into a chamber in which a mixture of litharge and arsenious oxide is heated. (4) Lead fluosilicate is heated with a metallic arsenate, for example:

3PbSiF₆ + 2CaHAsO₄ = Pb₃(AsO₄)₂ + 2CaSiF₆ + H₂SiF₆

Lead arsenate may also be obtained electrolytically by the anodic dissolution of lead in the presence of an arsenate. An almost theoretical yield has been obtained by using a diaphragm cell with an anolyte containing 20 g. sodium arsenite and 70 g. sodium nitrate per litre and sufficient acetic acid for neutralisation, and a catholyte consisting of a 30 per cent, solution of sodium nitrate. The anode should be of lead and the cathode of iron, and the current density 5.5 to 6 amps, per sq. dm.

Lead Dihydrogen Orthoarsenate, Pb(H₂AsO₄)₂, may be obtained from the monohydrogen salt by boiling with a solution of arsenic acid of 86 per cent, concentration; the boiling temperature is about 130° C. and the crystals which form on cooling are removed by centrifuging, washed with alcohol and dried at 110° C. The product usually contains a slight excess of arsenic acid.

The crystals appear to be triclinic; the indices of refraction for sodium light at 20° C. are α = 1.74, β = 1.84 and γ = 1.82. The salt loses water when heated, the equivalent of 1 molecule being lost on prolonged heating at 150° C., when lead hydrogen pyroarsenate, PbH₂As₂O₇, is probably formed, and the whole expelled below red heat; when fusion occurs some arsenious oxide vapour is emitted.

A phase rule study of the system PbO-As₂O₅-H₂O has been made over the acid range at 25° C. and the conditions for the existence of the two acid lead arsenates defined.

Lead Pyroarsenate, Pb₂As₂O₇, is formed when the monohydrogen orthoarsenate is heated above 280° C., or by fusing together lead oxide and arsenic pentoxide or potassium metarsenate, or by precipitating lead acetate solution by addition of sodium pyroarsenate. The fused product forms colourless lamellae of density 6.85 at 15° C. and melting point 802° C. In contact with water it becomes opaque and the monohydrogen orthoarsenate is formed.

Lead Metarsenate, Pb(NaO₃)₂, is obtained by heating a mixture of arsenic pentoxide and lead monoxide, red lead or lead nitrate in suitable proportions. A transparent glass is formed which, if broken up and heated to incipient fusion, crystallises as hexagonal tablets of density 6.42. The salt is decomposed by water.

Mixed salts of composition PbNaAsO₄ and PbKAsO₄ have been obtained by dissolving litharge in molten mixtures of the alkali orthoarsenate and chloride; from fused sodium metarsenate in which 15 per cent, of litharge was dissolved, crystals of composition 4PbO.2Na₂O.3As₂O₅ or Pb₄Na₄(As₂O₇)₃ have been obtained.

The mineral mimetite, 3Pb₃(AsO₄)₂.PbCl₂, may be prepared artificially by fusing a mixture of its component salts. It occurs in yellow to brown crystal aggregates or mammillary crusts. The crystals are hexagonal pyramids, a:c = 0.73147, isomorphous with pyromorphite and the apatites. It is widely distributed and has been found in Cornwall, Cumberland and Leadhills (Lanark).