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.