Iron Arsenates

Ferrous Orthoarsenate, Fe₃(AsO₄)₂.6H₂O, is obtained as a white precipitate when ammonium orthoarsenate or sodium monohydrogen arsenate is added to an aqueous solution of ferrous sulphate. In the latter case the reaction is stated to be:

4FeSO₄ + 4Na₂HAsO₄ = Fe₃(AsO₄)₂ + Fe(H₂AsO₄)₂ + 4Na₂SO₄

The precipitated hexahydrate gradually undergoes oxidation on exposure to moist air, yielding ferric arsenate and ferric oxide. It is sparingly soluble in aqueous ammonia, but is insoluble in the presence of ammonium salts.

Ferrous orthoarsenate occurs naturally as the mineral symplesite, which is the oetahydrate, Fe₃(AsO₄)₂.8H₂O. This is found in pale blue or green prismatic or tabular crystals, probably isomorphous with vivianite and having axial ratios a:b:c = 0.7806:1:0.6812, and β = 72°43'. The mineral is decomposed by caustic alkali with formation of ferric hydroxide.

Colloidal ferrous arsenate has been obtained in the form of an opalescent jelly by successively treating an aqueous solution of a ferrous salt with ammonium sulphate, acetic acid and an excess of sodium orthoarsenate. The jelly cannot be kept indefinitely, but crystallises after a few weeks.

Ferrous Hydrogen Orthoarsenates have not been prepared in the pure state. According to Wittstein, the dihydrogen arsenate remains in the mother liquor when the normal orthoarsenate is precipitated (see equation above); and when iron is acted upon by arsenic acid over a long period an asbestos-like deposit is formed which probably contains a ferrous hydrogen arsenate.

The dihydrate, FeAsO₄.2H₂O, may be prepared by heating to 150° C. metallic iron and arsenic acid, by heating at 80° C. in a sealed tube a mixture of anhydrous ferric arsenate and aqueous arsenic acid or by crystallisation from a solution of ferric arsenate in aqueous hydrochloric acid. It is widely distributed in Nature as the mineral scorodite, which is usually greenish-brown and transparent. The crystals are rhombic, with axial ratios a:b:c = 0.8785:1:0.9550, and the refractive indices along the three axes are a = 1.771, β = 1.805 and γ = 1.820. The mineral is easily fused, dissolves in hydrochloric acid, and is decomposed by caustic alkali with formation of ferric hydroxide. The solution in aqueous ammonia is red or yellow, but turns blue on acidifying.

When hydrogen is allowed to react with an aqueous solution of an alkali arsenate containing ferric oxide at a temperature of 300° C. and under a pressure greater than 150 atmospheres, green crystals of scorodite are formed. If the action is prolonged, elementary arsenic and ferric arsenides are produced.

A commercial method of utilising scorodite consists in roasting it and heating the residue with sulphuric acid, crystallising out the ferric sulphate and igniting the crystals to ferric oxide; the mother liquor, containing arsenious oxide and sulphuric acid, is used as a weed-killer. Weed-killers consisting of mixtures of iron and calcium arsenates are made by causing a soluble iron salt to react with a soluble arsenate in solution and then adding a basic calcium arsenate.

The monohydrate, FeAsO₄.H₂O, is formed when anhydrous ferric arsenate, arsenic acid, hydrogen peroxide and water are heated in a sealed tube for 14 days at 170° C.; a hemihydrate may be obtained in a similar manner by substituting precipitated ferric dihydrogen arsenate for the normal salt.

The monohydrate is a dull white insoluble substance which, when treated with aqueous sodium hydrogen carbonate, causes effervescence, 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.

Anhydrous Ferric Orthoarsenate, FeAsO₄, is obtained by heating the monohydrate at 100° C. or the dihydrate at 220° to 250° C. It yields black monoclinic prisms, with crystallographic elements a:b:c = 0.6155:1:0.3221, β = 77°8'. The density is 4.32, and the mean refractive index for sodium light 1.78.

When strongly heated, ferric orthoarsenate undergoes decomposition. The speed of the reaction in the presence of sodium chloride has been determined at 700° to 1000° C. and the decomposition is found to be of the second order, and may be formulated

2FeAsO₄ → Fe₂O₃ + As₂O₅

9H₂O.Ferric Monohydrogen Orthoarsenate, 2Fe₂(HAsO₄)₃.9H₂O, results when solutions of sodium monohydrogen arsenate and ferric chloride are mixed. The salt separates out as a white gelatinous precipitate, soluble in aqueous mineral acids forming yellow solutions and in ammonia forming a red solution. If the gelatinous precipitate is thoroughly washed it may be peptized by a small quantity of aqueous ammonium hydroxide to yield a stable colloid, which on long-continued dialysis forms a deep red gel. The composition of both sol and gel approximates closely to the formula, FeAsO₄.Fe₂O₃.xH₂O, the change in composition from that of the precipitated arsenate being due to removal of ammonium arsenate during dialysis.

Ferric Dihydrogen Arsenate, Fe(H₂AsO₄)₃.5H₂O, is deposited when a solution of artificial pharmacosiderite (see below) in syrupy arsenic acid is heated. It is decomposed by water, and with acids and ammonia it yields solutions similar to those of the monohydrogen salt.

A study of the viscosity of solutions of ferric oxide in aqueous arsenic acid at concentrations between 2.6 and 23.3 per cent. As₂O₅ shows that two solid phases exist within these limits, the normal salt, FeAsO₄.xH₂O (x about 3), which adsorbs arsenic acid, and an acid salt of composition Fe₂O₃.2As₂O₅.8H₂O, which has also been obtained by the action of excess of arsenic acid on a solution of ferric chloride.

On addition of sodium monohydrogen arsenate, Na₂HAsO₄, to ammonium iron alum, in the proportion of two mols. 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 gradually adding 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. Other basic salts have been described, for example 4Fe₂O₃.3As₂O₅.nH₂O (with n = 15.4, 20.5 and 33.5) and 16Fe₂O₃.As₂O₅.24H₂O. The following minerals also contain basic ferric arsenates: pharmacosiderite, Fe₄(OH)₃(AsO₄)₃.6H₂O; yukonite, (Ca₃, Fe₂)As₂O₈.2Fe(OH)₃.5H₂O; arseniosiderite, Ca₃Fe₄(OH)₉(AsO₄)₃.

Pharmacosiderite may be obtained artificially 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 acid. 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.

Mixed ortho- and pyro-arsenates with sodium and potassium of composition, respectively, M₃Fe₂(AsO₄)₃ and MFeAs₂O₇, have been described. In the presence of alkali hydroxides, arsenic pentoxide reacts with ferric hydroxide to form complex ferri-arsenates analogous with alumini-arsenates and with ferri-phosphates; the products so obtained are formulated NaH₂[Fe(AsO₄)₂].H₂O; KH₂[Fe(AsO₄)₂]; Ba₃H₆[Fe(AsO₄)₃]₂. The products obtained by Curtman by the action of potassium or ammonium monohydrogen arsenate on an acidified solution of ferric chloride and formulated KH₂AsO₄.FeAsO₄ and (NH₄)H₂AsO₄.FeAsO₄, are probably similar complexes.