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Ferric ortho-arsenate, FeAsO4

Ferric ortho-arsenate occurs in nature as the mineral scorodite, FeAsO4.2H2O, and may be produced artificially by heating iron to 150° C. with a solution of arsenic acid, or by heating ferric arsenate in a similar manner with arsenic acid.

The monohydrate, FeAsO4.H2O, is precipitated from solution. The salt, when dried in air, is a dull white insoluble substance, containing one molecule of water. When heated at 100° C. it becomes anhydrous. On treatment with sodium hydrogen carbonate solution, both the monohydrate and the anhydrous salt cause effervescence to take place, a soluble double arsenate being produced. It would appear, therefore, that the hydrated salt is acidic, and determinations of its basicity indicate that its formula is FeO.AsO2(OH)2.

Colloidal ferric arsenate is prepared by the action of ammonium hydroxide on the insoluble salt.

The anhydrous salt has been found as a deposit in the Deacon process for the preparation of chlorine, in the form of black prismatic crystals, belonging to the monoclinic system. Its crystallographic elements are: -

a: b: c = 0.6155: 1: 0.3221. β = 77° 8'.

Density 4.32.

A hydrated acid salt, 2Fe2(HAsO4)3.9H2O, results when solutions of disodium hydrogen arsenate and ferric chloride are mixed. The salt separates out as a white precipitate, soluble in aqueous hydrogen chloride.

On addition of disodium arsenate, Na2HAsO4, to iron ammonium alum, in the proportion of two molecules of the former to one of the latter, a precipitate of composition, Fe2O3.As2O5, is obtained, which varies in tint according to circumstances. Thus, on adding a small quantity of the arsenate to the alum solution a white precipitate is obtained; but, on reversing the procedure, the precipitate is brownish. The white precipitate turns yellow, and finally brown, however, when washed with water. In the presence of large excess of either constituent the basic salt, 3Fe2O3.2As2O5, is obtained. The following substances have also been prepared: 4Fe2O3.3As2O5, Fe2O3.As2O5.17H2O, 2Fe2O3.3As2O5.22.5H2O, and Fe2O3.3As2O5.16-7H2O.

Several basic arsenates occur as minerals, namely, iron sinter or pharmaco-siderite, 3FeAsO4.Fe(HO)3.6H2O; Yukonite, (Ca3, Fe2)As2O8.2Fe(HO)3.5H2O.

The double arsenates, ammonium ferric ortho-arsenate, (NH4)H2AsO4.FeAsO4, and potassium ferric arsenate, possibly represented by the formula KH2AsO4.FeAsO4, have been obtained.

An arsenate containing copper - namely, CuFe4As4O17 or CuO.2Fe2O3.2As2O5 - has been found in the form of rhombic crystals as a deposit during the manufacture of chlorine by the Deacon process.

Other double ferric arsenates are: K2O.Fe2O3.2As2O5, Na2O.Fe2O3.2As2O5, 3K2O.2Fe2O3.3As2O5, and 3Na2O.2Fe2O3.3As2O5.

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