Ammonium orthoarsenate

Magnesium ammonium orthoarsenate can combine additively with hydrogen peroxide.3... 

E. Thllo and J. Plaetschke. Z. anorg. Chem. 260. 315 (1949). Ammonium Orthoarsenate... 

A saturated aqueous orthoarsenic acid solution is saturated with gaseous NH3. Beautiful crystals of ammonium orthoarsenate precipitate immediately. 

Ammonium Monokydrogen Orthoarsenate, (NH4)2HAs04, may be prepared by the addition of ammonia to a concentrated aqueous solution of arsenic acid, the precipitate first formed being dissolved by warming and the salt then obtained either by crystallisation 6 or by precipitation with ethyl alcohol.7 The dry salt, when gently heated, loses ammonia to form the dihydrogen arsenate, as also does the aqueous solution in the cold, however, the dry salt is stable in dry air. The crystals are monoclinic prisms,8 with a b c = 0-918 1 1-1715 and j3 =91° 13 density 1-99. 

The solubility in water of ammonium dihydrogen orthoarsenate and the densities of the saturated solutions have been determined with the results shown in the following table 1... 

Barium Arsenates.—Barium orthoarsenate, Ba3(As04)2, is formed as large colourless plates when a mixture of barium oxide, alkali chloride and alkali hydrogen orthoarsenate is fused and allowed to cool.6 It is also said to be formed7 by the action of ammonia on an aqueous solution of barium hydrogen arsenate the product, however, varies in composition, barium ammonium arsenate sometimes being formed and... 

Ammino-orthoarsenates of composition Co3(As04)2(NH3).7H20, Co3(As04)2(NH3)2.6H20 and Co3(As04)2(NH3)3.5H20 have been obtained by the interaction in solution of a cobalt salt, arsenic acid or an arsenate and free ammonia, ammonium salts being present.7 The nature of the product depends on the amount of free ammonia present. 

Colloidal cobalt arsenate has been obtained in the form of an opalescent jelly either by mixing in the cold a solution of a cobalt salt of a strong acid with potassium dihydrogen orthoarsenate,8 or by successively treating an aqueous solution of a cobaltous salt with ammonium sulphate, acetic acid and an excess of sodium orthoarsenate.9... 

Pharmaeosiderite may be obtained artificially 5 by heating ferric orthoarsenate and water in a sealed tube at 200° C. or by boiling the ferric salt with ammonium acetate solution acidified with acetic add. The product contains considerably more water than the natural mineral, but loses it when strongly heated. The mineral loses 5 molecules of water up to 233 1° C., when decomposition begins, the products being ferric and arsenic oxides.8... 

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. 

---