Chemical elements
  Arsenic
      Occurrence
      Ubiquity
      History
    Isotopes
    Energy
    Production
    Application
    Physical Properties
    Chemical Properties
      Aluminium Arsenide
      Antimony Arsenides
      Barium Arsenide
      Bismuth Arsenides
      Cadmium Arsenides
      Calcium Arsenide
      Cerium Arsenide
      Chromium Arsenides
      Cobalt Arsenides
      Copper Arsenides
      Gold Arsenides
      Iridium Arsenide
      Iron Arsenides
      Lead Arsenides
      Lithium Arsenide
      Magnesium Arsenide
      Manganese Arsenides
      Mercury Arsenides
      Molybdenum Arsenide
      Nickel Arsenides
      Niobium Arsenide
      Palladium Di-arsenide
      Platinum Arsenides
      Potassium Arsenides
      Rhodium Arsenide
      Ruthenium Arsenide
      Silver Arsenides
      Sodium Arsenide
      Strontium Arsenide
      Thallium Arsenide
      Tin Arsenides
      Tungsten Arsenide
      Uranium Arsenide
      Zinc Arsenides
      Arsenic Subhydride
      Arsenic Monohydride
      Arsenic Trihydride
      Arsenic Trifluoride
      Arsenic Pentafluoride
      Arsenic Nitrosyl Hexafluoride
      Arsenic Trichloride
      Arsenic Oxychloride
      Arsenic Pentachloride
      Arsenic Tribromide
      Arsenic Oxybromide
      Arsenic Moniodide
      Arsenic Diiodide
      Arsenic Triiodide
      Arsenic Pentiodide
      Arsenic Suboxide
      Arsenious Oxide
      Aluminium Arsenite
      Ammonium Arsenites
      Antimony Arsenite
      Barium Arsenites
      Beryllium Arsenite
      Bismuth Arsenite
      Cadmium Arsenites
      Calcium Arsenites
      Chromic Arsenite
      Cobalt Arsenites
      Copper Arsenites
      Gold Arsenites
      Iron Arsenites
      Lead Arsenites
      Lithium Arsenite
      Magnesium Arsenites
      Manganese Arsenites
      Mercury Arsenites
      Nickel Arsenites
      Palladium Pyroarsenite
      Platinum Arsenites
      Potassium Arsenites
      Arsenites of Rare Earth Metals
      Rubidium Metarsenite
      Silver Arsenites
      Sodium Arsenites
      Strontium Arsenites
      Thallous Orthoarsenite
      Tin Arsenites
      Titanyl Tetrarsenite
      Tungsto-arsenites
      Uranyl Metarsenite
      Zinc Arsenites
      Zirconium Arsenite
      Arsenic Tetroxide
      Arsenic Pentoxide
      Aluminium Arsenates
      Ammonium Arsenates
      Barium Arsenates
      Beryllium Arsenates
      Bismuth Arsenates
      Cadmium Arsenates
      Caesium Arsenate
      Calcium Arsenates
      Chromium Arsenates
      Cobalt Arsenates
      Copper Arsenates
      Hydroxylamine Orthoarsenate
      Iron Arsenates
      Lead Arsenates
      Lithium Arsenates
      Magnesium Arsenates
      Manganese Arsenates
      Mercury Arsenates
      Molybdenum Arsenates
      Nickel Arsenates
      Palladium Arsenate
      Platinic Arsenate
      Potassium Arsenates
      Rare Earth Metals Arsenates
      Rhodium Arsenate
      Rubidium Arsenates
      Silver Arsenates
      Sodium Arsenates
      Strontium Arsenates
      Thallium Arsenates
      Thorium Arsenates
      Tin Arsenates
      Titanyl Arsenate
      Tungsto-arsenic Acids
      Uranium Arsenates
      Vanado-arsenates
      Zinc Arsenates
      Zirconium Arsenates
      Perarsenates
      Arsenic and Sulphur
      Arsenic Subsulphide
      Tetrarsenic Trisulphide
      Arsenic Disulphide
      Arsenic Trisulphide
      Arsenic Pentasulphide
      Thioarsenates
      Ammonium Thioarsenates
      Antimony Thioarsenate
      Barium Thioarsenates
      Beryllium Thioarsenate
      Bismuth Thioarsenate
      Cadmium Thioarsenates
      Calcium Thioarsenates
      Cerium Thioarsenates
      Chromium Thioarsenate
      Cobalt Thioarsenate
      Copper Thioarsenates
      Gold Thioarsenates
      Iron Thioarsenates
      Lead Thioarsenates
      Lithium Thioarsenates
      Magnesium Thioarsenates
      Manganese Thioarsenates
      Mercury Thioarsenates
      Molybdenum Thioarsenates
      Nickel Thioarsenates
      Platinic Thioarsenate
      Potassium Thioarsenates
      Silver Thioarsenates
      Sodium Thioarsenates
      Strontium Thioarsenates
      Thallium Orthothioarsenate
      Tin Thioarsenates
      Uranyl Thioarsenate
      Yttrium Thioarsenate
      Zinc Thioarsenates
      Zirconium Thioarsenate
      Trioxythioarsenic Acid
      Dioxydithioarsenic Acid
      Oxytrithioarsenic Acid
      Arsenic Monosulphatotrioxide
      Arsenic Disulphatotrioxide
      Arsenic Trisulphatotrioxide
      Arsenic Tetrasulphatotrioxide
      Arsenic Hexasulphatotrioxide
      Arsenic Octasulphatotrioxide
      Complex salts of Sulphato-compounds of Arsenic
      Arsenic Nitride
      Arsenic Imide
      Arsenic Amide
      Arsenic Phosphides
      Arsenic oxyphosphides
      Arsenic Phosphate
      Arsenic Thiophosphate
      Arsenic Tricarbide
      Arsenic Pentasilicide
      Boron Arsenate
    Detection of Arsenic
    Estimation of Arsenic
    Physiological Properties
    PDB 1b92-1ihu
    PDB 1ii0-1tnd
    PDB 1tql-2hmh
    PDB 2hx2-2xnq
    PDB 2xod-3htw
    PDB 3hzf-3od5
    PDB 3ouu-9nse

Iron Arsenates





Ferrous Orthoarsenate, Fe3(AsO4)2

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

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

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.

Ferric Orthoarsenate

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

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

2FeAsO4Fe2O3 + As2O5

Ferric Monohydrogen Orthoarsenate, 2Fe2(HAsO4)3

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

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.
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