Silver Arsenates

Silver nitrate reacts with solutions of arsenic acid or arsenates, giving a chocolate-coloured precipitate of silver orthoarsenate, Ag₃AsO₄, containing a small proportion of silver nitrate, possibly in solid solution. The adsorption of silver nitrate is prevented by the presence of ammonium nitrate. The precipitation is incomplete in acid solution. The orthoarsenate cannot be completely dried except by fusion.

When precipitated from a boiling solution of silver nitrate by concentrated aqueous arsenic acid, the precipitate is deep purple in colour. As usually prepared it forms cubic crystals, which may be tetra-, hexa- or dodeca-hedral. The unit cell contains 2 molecules and has a side 6.12 A. The crystals have density 6.657 at 25° C. The solubility in water at 20° C. is 0.0085 g. per litre.

When heated, the salt first melts and at a higher temperature it decomposes to form silver; reduction to the metal occurs more readily when heated with carbon. Partial reduction also occurs when the orthoarsenate is treated with formaldehyde or with ferrous sulphate solution; in the former case silver is formed, but in the latter silver suboxide.

Silver orthoarsenate dissolves in hydrochloric acid, forming arsenic trichloride, and in nitric acid with partial conversion to nitrato-arsenate, which may be separated by fractional crystallisation, silver orthoarsenate crystallising out first and then the nitrato-arsenate; the latter, in contact with water, reverts to silver nitrate and silver orthoarsenate. The latter is also soluble in acetic acid, and in aqueous ammonia or ammonium carbonate. By crystallising the solution in ammonia over a mixture of quicklime and sal-ammoniac, colourless needles of silver tetrammino-orthoarsenate, Ag₃AsO₄.4NH₃, are obtained; these lose ammonia in the air and turn reddish-brown.

When silver nitrate solution is dropped on a gelatin gel containing an alkali arsenate, periodic precipitation of silver orthoarsenate occurs only in darkness; if the gels are exposed to the light a continuous precipitate is formed. Colloidal silver orthoarsenate has been prepared by mixing solutions of silver nitrate and sodium protalbinate and dissolving the precipitate formed in dilute aqueous sodium hydroxide containing sodium monohydrogen orthoarsenate. After dialysis in the dark, the solution was evaporated in vacuo over sulphuric acid; the powder obtained gave a hydrosol which was dark green by reflected light and red by transmitted light.

Silver Monohydrogen Orthoarsenate, Ag₂HAsO₄, has not been obtained in the pure condition.

Silver Dihydrogen Orthoarsenate, AgH₂AsO₄, is readily obtained as white monoclinic crystals from a solution of arsenic acid (of composition H₃AsO₄.H₂O) saturated with silver orthoarsenate. If the solution is saturated with the silver orthoarsenate at a temperature a little below 100° C., it deposits orange-red hexagonal prisms similar to those of silver monohydrogen phosphate; these are probably the monohydrogen arsenate, but they decompose rapidly in the presence of water. Also when the dihydrogen arsenate is heated to 100° C. it first turns red, probably forming the monohydrogen salt, but rapidly loses water to form silver metarsenate, AgNaO₃, as a white powder.

When a syrupy solution of silver orthoarsenate in arsenic acid is heated above 100° C. it yields a white granular powder of composition Ag₂O.2As₂O₅, which is decomposed by water into silver orthoarsenate and arsenic acid.