Mercury Arsenates

Mercurous Orthoarsenate, Hg₃AsO₄, is formed as an orange-yellow precipitate when a cold solution of mercurous nitrate is added to a large excess of aqueous sodium monohydrogen orthoarsenate. If the mercury salt is in excess, the basic double salt, 3Hg₃AsO₄.2(HgNO₃.Hg₂O), separates which, when freshly precipitated and washed, is converted to the normal arsenate by treatment with aqueous sodium monohydrogen orthoarsenate. The double salt is white and soluble in hydrochloric acid. The normal arsenate may also be obtained by prolonged heating at 230° C. of a mixture of mercury and arsenic acid solution (containing 12.5 per cent. As₂O₅) in a sealed tube.

Mercurous orthoarsenate yields orange rhombic prisms which are insoluble in water. Hydrochloric acid converts it into calomel and arsenic acid.

Mercurous Monohydrogen Orthoarsenate, Hg₂HAsO₄.½H₂O, is formed when a concentrated solution of arsenic acid is treated with a solution of mercurous nitrate until the precipitate no longer redissolves. The product is white, but becomes red when dried. On further heating it loses water of crystallisation and at a higher temperature yields mercury and mercuric arsenate, the latter also decomposing at a still higher temperature to yield arsenious oxide, oxygen and mercury. The salt dissolves in cold nitric acid and may be reprecipitated by ammonia. Boiling nitric acid and cold concentrated hydrochloric acid decompose it.

Orange-red crystals of dimercurous silver arsenate, Hg₂AgAsO₄, have been obtained by adding hydrogen peroxide solution, free from chlorides, to a solution of silver nitrate, mercurous nitrate and potassium arsenite in dilute nitric acid. The crystals are blackened by ammonium hydroxide solution owing to liberation of metallic mercury.

Mercurous Metarsenate, HgNaO₃, has been obtained by prolonged heating in a sealed tube at 150° C. of a mixture of mercury and aqueous arsenic acid, the latter of 50 to 75 per cent, concentration, and by boiling a mixture of mercuric oxide and aqueous arsenic acid until dry, thoroughly washing the residue with water and drying at 100° C. It yields pale yellow hexagonal plates with axial ratio a:c = 1:1.5096. The crystals blacken superficially in light. The salt decomposes on heating. It is insoluble in water, alcohol and acetic acid. It dissolves in dilute nitric acid and the addition of ammonia to the solution precipitates mercurous monohydrogen orthoarsenate.

Mercuric Orthoarsenate, Hg₃(AsO₄)₂, is obtained by precipitating a solution of mercuric nitrate with a solution of sodium mono- or dihydrogen arsenate, or by dropping aqueous arsenic acid into excess of mercuric nitrate solution. It is a heavy citron-yellow powder. Hot water dissolves it slightly without decomposition, and shining crystals may separate from the cooled solution. Hydrochloric acid dissolves it freely, nitric acid less readily, and arsenic acid not at all. Brine solution converts it into red-brown mercury oxychloride. Potassium bromide solution colours it brown, and a yellow residue is ultimately left. Potassium iodide forms mercuric iodide.

Sodium dihydrogen arsenate or arsenic acid does not precipitate mercuric chloride solution, but the latter reacts with normal silver arsenate to form normal mercuric arsenate and silver chloride. Yellow precipitates containing normal mercuric arsenate, mercuric chloride and mercuric oxide are thrown down from mercuric chloride solution by a solution of sodium monohydrogen arsenate.

Basic mercuric sulphate is converted into mercuric orthoarsenate and mercuric oxide by warming with sodium monohydrogen arsenate solution. Hot acetic acid will extract the oxide.

When mercuric oxide is treated with aqueous arsenic acid, the solid phase separates into two layers, one of which is mercuric oxide and the other a basic mercuric arsenate of composition 3HgO.Hg₃(AsO₄)₂. When ammonium arsenate solution acts on mercuric oxide, a product of composition Hg₂NH₂AsO₄ is obtained.

Complex pyroarsenates of composition HgSrAs₂O₇ and HgBaAs₂O₇.H₂O have been prepared.

A double salt of composition Hg₃AsO₄.2HgNO₃.2H₂O has been described as separating in warty masses or needle-shaped crystals when a solution of mercurous monohydrogen orthoarsenate in concentrated nitric acid is diluted with its own volume of water and then treated with aqueous ammonia and allowed to stand for a time.

Colloidal mercuric orthoarsenate has been prepared by adding mercuric chloride to a 10 per cent, solution of sodium protalbinate, dissolving the precipitate formed in aqueous sodium hydroxide and treating the latter solution with sodium monohydrogen arsenate. After dialysis, the solution is evaporated to dryness under reduced pressure, when an olive-green residue is obtained. This dissolves readily in water to give a colloidal solution which is olive-green in transmitted light and grey in reflected light.