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

Calcium Arsenates






In a basic form calcium arsenate is an important commercial product and is used in the form of a spray or dust as an insecticide for plants. The commercial arsenates are usually mixtures and exhibit a wide variation in chemical and physical characteristics. The system CaO-As2O5-H2O has been investigated at various temperatures, and equilibrium data, microscopic and X-ray examination point to the existence of the following compounds: CaHAsO4, Ca5H2(AsO4)4.5H2O, Ca3(AsO4)2.2H2O and 4CaO.As2O5.xH2O.

The normal salt, calcium orthoarsenate, Ca3(AsO4)2, may be obtained by the action of lime-water or milk of lime on arsenic acid, or by addition of aqueous calcium chloride to aqueous sodium monohydrogen arsenate; in the latter case if the solutions are alkaline a product of a high degree of purity is obtained. Drying at 100° C. gives the dihydrate, Ca3(AsO4)2. 2H2O, which loses its water of crystallisation at 175° C. The anhydrous salt melts at 1455° C. It is reduced by hydrogen at 400° to 450° C. to calcium oxide, arsenic and water. The density of the dihydrate is 3.23, and of the anhydrous salt 3.62. The solubility of the latter at 25° C. is 0.0133 grm. per 100 grm. of water.

The commercial product is usually obtained either by adding arsenic acid to partly slaked lime containing sufficient quicklime to combine with the free water, thus yielding a dry product, or by heating a mixture of lime and white arsenic in the presence of an oxidising medium, such as oxygen, chlorine or nitrates. It is essential that the product should contain a minimum of soluble arsenic and that the presence of the more soluble acid arsenates should be avoided; the commercial arsenate is generally slightly basic. The results of analyses of 16 samples showed that the total As2O5 ranged only from 40.3 to 44.4 per cent. [As2O5 in Ca3(AsO4)2 = 57.7 per cent.], and the average molecular quotient CaO/As2O5 was 3.4 The average commercial product contains 80 to 85 per cent, of what is probably a mixture of the normal arsenate and a basic arsenate, probably 4CaO.As2O5, together with about 6.5 per cent, each of calcium hydroxide and calcium carbonate, and small quantities of other impurities. If varying quantities of arsenic acid are added to a saturated solution of lime, the ratio of CaO:As2O5 in the precipitate shows a continuous variation according to the quantity of arsenic acid added and under these conditions there is no evidence of the real existence of calcium orthophosphate, Ca3(AsO4)2.

An electrolytic method for the manufacture of calcium arsenate is as follows. A solution of arsenious oxide in caustic soda (As2O3: NaOH = 198:250) is electrolysed between iron electrodes. Hydrogen and a small quantity of metallic arsenic are liberated at the cathode and very little oxygen at the anode, the basic arsenite in solution being oxidised to arsenate. When this oxidation is complete, any arsenic is filtered off and the solution treated with milk of lime. A basic arsenate of extremely low solubility is precipitated and, after removal, is washed and dried.3 The reactions involved are the following:

As2O3 + 2NaOH = 2NaAsO2 + H2O
2NaAsO2 + 4NaOH = 2Na3AsO4 + 2H2
2Na3AsO4 + 4Ca(OH)2 = Ca4As2O9 + 6NaOH + H2O

Less basic arsenates may be obtained by precipitation with calcium chloride or nitrate.


Calcium Monohydrogen Orthoarsenate, CaHAsO4

Calcium Monohydrogen Orthoarsenate, CaHAsO4, may be prepared by treating calcium carbonate with arsenic acid, or by precipitation from solutions of calcium salts with sodium monohydrogen arsenate; both solutions should be acidified. When dried at 100° C. the monohydrate, CaHAsO4.H2O, is obtained, but the water of crystallisation is expelled before the temperature of 175° C. is reached. By placing a porous vessel containing sodium monohydrogen arsenate solution inside a vessel containing aqueous calcium nitrate, so that the two salts were able to mix by diffusion, well-defined monoclinic prisms of the dihydrate, CaHAsO4.2H2O, have been obtained. These two hydrates occur in Nature, the monohydrate as haidingerite and the dihydrate as pharmacolite, and calcium arsenate is a constituent of many other minerals. A third hydrate, 2CaHAsO4.3H2O, has been described, but its existence has not been confirmed.

A saturated solution of calcium monohydrogen arsenate contains at 25° C. 0.3108 grm. of the anhydrous salt per 100 grms. of solution. The dihydrogen arsenate is sparingly soluble in cold water and is decomposed by hot water, yielding the monohydrogen salt and arsenic acid.

The heat developed by the saturation of a normal solution of arsenic acid with calcium hydroxide in aqueous solution is as follows:

1st equivalent CaO, 14,500 calories
2nd equivalent CaO, 12,500 calories
3rd equivalent CaO, 2,520 calories
4th equivalent CaO, 280 calories
5th equivalent CaO, 250 calories

With the addition of three equivalents the precipitate is variable in composition but approximates to the normal salt; with 4 and 5 equivalents the precipitated arsenate is somewhat basic.

Calcium Dihydrogen Orthoarsenate

Calcium Dihydrogen Orthoarsenate has been obtained in the form of a monohydrate by adding nitric acid to a mixture containing equal proportions of calcium carbonate and arsenious oxide and allowing the solution to crystallise. It may also be prepared by adding excess of arsenic acid to calcium carbonate or to the normal or monohydrogen orthoarsenate. It forms colourless plates which lose their water at 180° C. and at 360° C. yield calcium metarsenate, Ca(NaO3)2, as a crystalline mass, insoluble in hydrochloric acid.

Calcium Pyroarsenate, Ca2As2O7

Calcium Pyroarsenate, Ca2As2O7, in orthorhombic prisms, may be obtained by fusing calcium oxide with potassium monohydrogen arsenate, or with a mixture of the latter with less than 40 per cent, of calcium chloride. When treated with cold water the prisms break up into minute crystals of the hexahydrate, Ca2As2O7.6H2O. By leaving in contact with water the salt is gradually transformed into calcium monohydrogen orthoarsenate, CaHAsO4.2H2O. The pyroarsenate undergoes reduction when heated in hydrogen to above 400° C.

Several double orthoarsenates with the alkali metals are known: CaKAsO4, CaNaAsO4 and Ca3Na6(AsO4)4, Ca(NH4)AsO4.7H2O, Ca(NH4)AsO4.6H2O and Ca3(NH4)H2(AsO4)3.3H2O. Lefevre also isolated a calcium sodium pyroarsenate, Ca4Na4(As2O7)3. Arsenoapatites similar to those of barium have been obtained.
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