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

Magnesium Arsenates





Magnesium Orthoarsenate

Magnesium Orthoarsenate may be obtained in various hydrated forms. When solutions containing equivalent quantities of magnesium sulphate and normal sodium orthoarsenate are mixed, an amorphous precipitate forms which, on prolonged keeping in contact with the solution, becomes crystalline, and on drying has the composition Mg3(AsO4)2.22H2O. The crystals are monoclinic and isomorphous with the corresponding phosphate. The density at 15° C. is 1.788, and the indices of refraction along the three axes α = 1.548 to 1.563, β = 1.556 to 1.571 and γ = 1.574 to 1.596. The octahydrate, Mg3(AsO4)2.8H2O, is obtained by mixing aqueous solutions of magnesium sulphate, sodium monohydrogen arsenate and sodium bicarbonate and keeping the amorphous precipitate in contact with the solution for 24 hours, when it becomes crystalline. This hydrate is found in Nature as the mineral haernesite, the crystals of which are monoclinic and probably isomorphous with vivianite. The density of the mineral is 2.474; that of the artificially prepared product is 2.609. The heptahydrate, Mg3(AsO4)2.7H2O, is obtained if sodium dihydrogen arsenate is substituted for the monohydrogen salt in the above preparation. The decahydrate, Mg3(AsO4)2.10H2O, is formed when potassium or ammonium magnesium orthoarsenate is digested with water for several hours. On dehydration, hydrates containing 6, 5, 3, 2 and 1H2O are obtained. The anhydrous salt decomposes when heated above 1100° C. in a vacuum, magnesium and arsenious oxides and oxygen being formed.

A light form of magnesium arsenate may be obtained by heating a slurry of magnesium hydroxide with the theoretical quantity of arsenic acid and 15 per cent, of the equivalent amount of sodium hydroxide in an autoclave at 180° C. until the filtrate contains less than 0.2 per cent, of arsenic pentoxide.


Magnesium Monohydrogen Orthoarsenate, MgHAsO4

Magnesium Monohydrogen Orthoarsenate, MgHAsO4.7H2O, may be prepared by adding the calculated amount of magnesium sulphate to a solution of sodium monohydrogen arsenate neutralised with acetic acid, and keeping the amorphous precipitate in contact with the solution for some days, during which it becomes crystalline. It loses water when heated. This hydrate occurs in Nature as roesslerite, a fibrous mineral consisting of monoclinic crystals with a:b:c = 0.4473:1:0.2598 and β = 94°26'. A hemihydrate, 2MgHAsO4.H2O, is obtained in the form of prismatic crystals by heating a solution of magnesium carbonate in excess of arsenic acid at 225° C. in a sealed tube.

Magnesium Dihydrogen Orthoarsenate, Mg(H2AsO4)2

Magnesium Dihydrogen Orthoarsenate, Mg(H2AsO4)2, has been described as a very deliquescent crystalline mass obtained by dissolving magnesium oxide in aqueous arsenic acid. It is soluble in water.

Magnesium Pyroarsenate, Mg2As2O7

Magnesium Pyroarsenate, Mg2As2O7, results from ignition of the monohydrogen arsenate or the double arsenate of magnesium and ammonium. It is a white powder of density 3.75 which, on ignition, melts but does not decompose. When heated above 800° C. in a vacuum it yields the normal orthoarsenate, arsenious oxide and oxygen.

Several double arsenates of magnesium and the alkali metals, ammonium and calcium are known. The most important of these is magnesium ammonium orthoarsenate, MgNH4AsO4.6H2O, which is obtained as a crystalline precipitate when ammonium orthoarsenate is added to a solution of a magnesium salt; it may be dried over sulphuric acid. It is also formed when an alkali arsenate is added to a solution of a magnesium salt in the presence of an ammonium salt and ammonia. When heated above 40° C. it loses ammonia and water and it is doubtful if any lower hydrate is formed, although such are described in the literature. On strong ignition it forms the pyroarsenate, as stated above.

The solubility in water of magnesium ammonium arsenate is as follows (expressed in grams of MgNH4AsO4.6H2O per 100 grams of solution):

Temp.,°C.0204050607080
Solubility0.033880.020660.027460.022610.021030.015640.02364


It is less soluble in the presence of ammonia, 1 part of the anhydrous salt dissolving in about 15,800 parts of water containing 2.46 per cent. NH3 (1 part of 0.96 ammonia to 3 parts H2O).

Magnesium can be estimated by precipitation as this double arsenate, which is then ignited or titrated.

Sodium Magnesium Arsenate, NaMgAsO4

Sodium Magnesium Arsenate, NaMgAsO4, has been obtained by dissolving magnesia in molten sodium dihydrogen arsenate, the alkali being removed from the residue by washing with very dilute nitric acid. By precipitation from magnesium sulphate with excess of sodium orthoarsenate, the enneahydrate, NaMgAsO4.9H2O, is obtained. This loses water at 110° C. to give the octahydrate. In the presence of water the salt yields normal magnesium orthoarsenate. The corresponding potassium salt has been prepared, and also the more complex salts Na4Mg4As6O21, K4Mg4As6O21; Mg2KH(AsO4)2.xH2O (x = 2, 4, 5 and 15); Mg2KNa(AsO4)2.14H2O.

Certain magnesium calcium arsenates, sometimes associated with other metals, are found in Nature and are enumerated in the table of minerals.
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