Silver orthoarsenate

Arsenates are oxidizing agents and are reduced by concentrated hydrochloric acid or sulfur dioxide. Treatment of a solution of orthoarsenate with silver nitrate in neutral solution results in the formation of a chocolate-brown precipitate of silver orthoarsenate, Ag AsO, which may be used as a test to distinguish arsenates from phosphates. With hydrofluoric acid, orthoarsenate solutions yield hexafluoroarsenates, eg, potassium hexafluoroarsenate [17029-22-0] (KAsFg)2 H2O. Arsenates of calcium or lead are used as insecticides sodium arsenate is used in printing inks and as a mordant. 

Arsenic acid reacts with metal salts forming their orthoarsenates, e.g., calcium orthoarsenate. Reaction with silver nitrate in neutral solution produces a chocolate-brown precipitate of silver orthoarsenate. It forms pyroarsenic acid (or pyroarsenate) on heating over 100°C. It is reduced to arsenous acid (or arsenites) when treated with reducing agents. 

Silver Arsenates.—Silver nitrate reacts with solutions of arsenic acid or arsenates, giving a chocolate-coloured precipitate of silver orthoarsenate, Ag3As04, containing a small proportion of silver nitrate, possibly in solid solution.1 The adsorption of silver nitrate is prevented by the presence of ammonium nitrate. The precipitation is incomplete in acid solution.2 The orthoarsenate cannot be completely dried except by fusion. 

Silver orthoarsenate dissolves in hydrochloric acid, forming arsenic trichloride,11 and in nitric acid with partial conversion to nitrato-arsenate,12 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, Ag3As04.4NTI3, are obtained 13 these lose ammonia in the air and turn reddish-brown. 

When a syrupy solution of silver orthoarsenate in arsenic acid is heated above 100° C. it yields a white granular powder of composition Ag80.2As206, which is decomposed by water into silver orthoarsenate and arsenic acid.2... 

Obtained by dissolving silver orthoarsenate in sulphuric acid.3... 

Therefore, a solution of alkali hydroxide should be added continuously to the solution to keep it neutral in the process of the precipitation of the silver orthoarsenate. During the precipitation of the brown silver orthoarsenate, it may be observed sometimes that yellowish-white silver arsenate-phosphate precipitates. However, this precipitate of silver arsenate-phosphate has never been isolated, because it decomposes rapidly into silver orthoarsenate and soluble phosphates. 

After the silver orthoarsenate is removed by filtration, an excess of silver nitrate is added to the filtrate to precipitate silver polyphosphates. The pH of the solution should be kept lower than 7.5 during the precipitation of the silver polyphosphates. Otherwise, silver oxide might be precipitated. Since the resulting precipitate of the silver polyphosphates still contains alkali ions, the precipitate is at once converted into a solution of sodium polyphosphates by adding the precipitate to a neutral solution of sodium chloride. By adding the resulting solution of sodium polyphosphates to a concentrated solution of silver nitrate, a pure precipitate of silver polyphosphates is formed. 

For the glasses of sodium meta-arsenate-phosphates, a pure pre- cipitate of silver polyphosphates is obtained directly by treating the filtrate of the silver orthoarsenate precipitation with a concentrated solution of silver nitrate The silver polyphosphates thus obtained are washed with acetone and dried. 

Hydroxylamine Orthoarsenate, (NH2OH)3.H3As04, is obtained13 when aqueous arsenic acid is made alkaline with an excess of sodium carbonate and then treated with hydroxylamine hydrochloride until faintly acid. Microscopic rhombic prisms slowly form, which yield an acid solution in hot water, from which they may be recrystallised. The salt readily reduces ammoniacal silver nitrate and Fehling s solution. 

When heated, the salt first melts and at a higher temperature it decomposes to form silver 7 reduction to the metal occurs more readily when heated with carbon.8 Partial reduction also occurs when the orthoarsenate is treated with formaldehyde 9 or with ferrous sulphate solution 10 in the former ease silver is formed, but in the latter silver suboxide. 

Silver Monohydrogen Orthoarsenate, Ag2HAs04, has not been obtained in the pure condition. 

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