Chemical elements
  Arsenic
      Occurrence
      Ubiquity
      History
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    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

Sodium Arsenates





Sodium Orthoarsenate, Na3AsO4

Sodium Orthoarsenate, Na3AsO4, is formed when excess of sodium hydroxide reacts with arsenic acid. It is usually manufactured by causing arsenious oxide, sodium hydroxide or carbonate and a reducible metallic oxide, for example antimony trioxide, to react in the fused condition, the sodium arsenate being extracted from the melt with water and crystallised from the resulting solution. The process may be performed in the presence of molten lead to alloy the antimony which is formed. Metallic arsenic may be used instead of arsenious oxide. Or, arsenious oxide vapour mixed with excess of air or oxygen is passed over the alkali at about 500° C.6 The mixture of alkali and arsenious oxide may be oxidised with nitric acid, but large quantities of nitrous vapours are evolved during the reaction. An improved method is to use sodium peroxide as the oxidising agent.

The arsenate may be prepared, however, without the presence of an oxidiser other than arsenious oxide if excess of the latter be used. Thus, sodium carbonate with excess of arsenious oxide heated at 500° to 550° C. yields the arsenate by the following reaction:

9Na2CO3 + 5As2O3 = 6Na3AsO4 + 4As + 9CO2

Sodium orthoarsenate is also obtained electrolytically. Yields up to 100 per cent, may be obtained by employing a cell with a diaphragm between iron electrodes. The anolyte should contain sodium arsenite, or sodium hydroxide and arsenious oxide (equivalent to 150 g. As2O3 per litre), and the catholyte sodium hydroxide (150 g. per litre). With a current density of 3 amps, per sq. dm. the current efficiency is 100 per cent. A solid crust of sodium arsenate forms around the anode. The process may be rendered continuous by circulating the anolyte and removing the precipitated arsenate. Iron or nickel electrodes are satisfactory; the latter yield a purer product, but iron has the advantage of hindering the formation of arsine. Electrodes of Acheson graphite may also be used with advantage. The diaphragm should be of asbestos cement.

The density of anhydrous sodium orthoarsenate is 2.835. From solution it crystallises as the dodecahydrate, Na3AsO4.12H2O, of density 1.76 and melting point 85.5° C. The crystals are hexagonal prisms, isomorphous with the corresponding phosphate. The following refractive indices have been determined:

Na3AsO4.12H2OLiNaTl
ω1.4553 1.45891.4624
ε1.46301.46691.4704


The molecular refraction is 61.4.

The salt dissolves in water yielding a strongly alkaline solution. Reliable determinations of the solubility have not been made; Graham stated that at 13.5° C. 100 parts of water dissolve 28 parts of the crystallised salt, but Schiff gave a much lower value.

The densities of solutions of various concentrations at 17° C. have been determined as follows:

Concentration. Parts per 100 Parts Solution.Density
Na3AsO4.12H2ONa3AsO4
20.9811.0167
41.9621.0215
62.9441.0325
83.9251.0435
104.9061.0547
125.8871.0659
146.8681.0773
167.8501.0887
188.8311.1003
209.8121.1120


The heat of formation of sodium orthoarsenate in aqueous solution from the elements is 381,500 calories, and that of the solid 360,800 calories. The heat of neutralisation (3NaOH.H3AsO4.aq.) is 35,920 calories, and the heat of formation from the oxides is:

3Na2O + As2O5 = 2Na3AsO4 + 202,800 calories

Other hydrates of sodium orthoarsenate have been obtained. The decahydrate, Na3AsO4. IOH2O, crystallises from a solution containing sodium monohydrogen orthoarsenate (100 g.) in 150 c.c. of 50 per cent, sodium hydroxide after keeping for some time at about 77° C.; or it may be prepared by treating a saturated solution of arsenious oxide with an excess of sodium peroxide in the cold, and concentrating on a water-bath, the crystals being deposited on cooling. The decahydrate is efflorescent in dry air. The crystals are regular, resemble those of the corresponding vanadate, and have melting point 85° C. A (2, 9)-hydrate, 2Na3AsO4.9H2O, is deposited from a solution containing 100 g. of the monohydrogen arsenate in 40 c.c. of 50 per cent, sodium hydroxide when allowed to crystallise at about 86° C.

In cold aqueous solution, sodium orthoarsenate reacts with sodium hydrosulphite to form sodium arsenohydrosulphite, Na3As(S2O4)3, a creamy white granular powder. In the presence of sodium sulphite this compound decomposes, forming sodium arsenothiosulphate, an unstable intermediate product, and finally arsenious sulphide. If the reduction by sodium hydrosulphite takes place in the presence of hydrochloric acid, some arsenic subsulphide, As3S, is also precipitated, and this product is also obtained when the orthoarsenate in aqueous solution is treated with phosphorus trichloride and the mixture saturated with sulphur dioxide.


Sodium Monohydrogen Orthoarsenate, Na2HAsO4

Sodium Monohydrogen Orthoarsenate, Na2HAsO4, is produced when aqueous arsenic acid is mixed with a large excess of sodium carbonate, or when the normal orthoarsenate is treated in solution with a dilute acid or chlorine. It is prepared on a large scale by dissolving arsenious oxide in aqueous sodium hydroxide and adding sodium nitrate. The mixture is concentrated by boiling and finally heated in a furnace until dry and the residue calcined; this residue is extracted with dilute alkali and allowed to crystallise. The product is used in calico-printing as a substitute for cow-dung, which was formerly used for clearing the cloth after mordanting with either iron or aluminium acetate (the use of dung originated in India and its action has not been satisfactorily explained).

The anhydrous salt is obtained by heating the crystals to 120° C. If crystallisation takes place at the ordinary temperature, the dodecahydrate, Na2HAsO4.12H2O, is obtained; while if the crystals are formed above 36° C. the heptahydrate, Na2HAsO4.7H2O, is produced. The transition point determined from the solubility curve of sodium monohydrogen arsenate in water is at 22° C.

The dodecahydrate is efflorescent at room temperature in air. At 22° C. it becomes damp and the dampness increases as the temperature rises until finally the salt becomes completely liquid. A definite melting point is not exhibited. The liquid becomes clear at 56.2° C. and is then a true solution of the heptahydrate. When this is supercooled to about 40° C. and seeded with a crystal of the salt, crystals of the heptahydrate are deposited and the temperature rises to 56.2° C. A similar evolution of heat, but less marked, occurs at 22° C. The heptahydrate does not effloresce appreciably at room temperature in air.

The dissociation pressures of the two hydrates have been determined by a dynamical method with the following results:

For the reaction Na2HAsO4.12H2ONa2HAsO4.7H2O + 5H2O, at 14.9° C., 5.24 mm.; and at 20° C., 7.36 mm.

For the reaction Na2HAsO4.7H2ONa2HAsO4 + 7H2O, at 24.92° C., 9.98 mm.; at 30° C., 14.39 mm.; and at 35° C., 20.73 mm.

The dissociation pressure-temperature curves confirm the transition point as very near 22° C.

The specific heats of the hydrates over the range +16° to -12° C. have been found to be approximately: Na2HAsO4.12H2O, 0.414 calorie per gram; Na2HAsO4.7H2O, 0.350 calorie per gram; the respective molecular heats being 166.3 and 109.3 calories.

The solubility expressed in grams of anhydrous salt per 100 g. of solution has been determined up to 34° C. as follows:

Temperature, ° C.SolubilitySolid Phase.
05.46Na2HAsO4.12H2O
8.69.66
11.812.53
16.017.30
21.025.75
22.526.75Na2HAsO4.7H2O
28.030.48
30.132.34
34.035.09


Both hydrates crystallise in the monoclinic system, the crystal elements being

Na2HAsO4.12H2O, a:b:c = 1.7499:1:1.4121; β = 121°49'
Na2HAsO4. 7H2O, a:b:c = 1.2294:1:1.3526; β = 97°14'

and the densities respectively 1.67 and 1.88.

The refractive indices for light of various wavelengths along the three crystal axes are as follows:

LiNaTl
Na2HAsO4.12H2Oα1.44201.44531.4482
β1.44621.44961.4513
γ1.44801.45131.4545
Na2HAsO4.7H2Oα1.45871.46221.4655
β1.46231.46581.4689
γ1.47461.47821.4812


The equivalent conductivity is as follows (for solutions containing ½Na2HAsO4 in v litres):

V32641282565121024
Λ79.084.788.89294.495.9103.8


An investigation of the temperature-concentration curves of the system Na2HAsO4-H2O over the range < 0° to 120° C. reveals transition points which indicate the existence of the anhydrous salt and hydrated forms containing 0.5, 5, 7 and 12H2O. The normal boiling point of the saturated solution is 116° C. The existence of a hemihydraie has not been confirmed, but Menzel and Hagen state that a monohydrate exists, and have determined the following transition points: anhyd. ⇔ 1H2O, 99.5° C.; anhyd. (metastable) ⇔ 5H2O, 68 0° C.; 1H2O ⇔ 5H2O, 67.4° C.; 5H2O ⇔ 7H2O, 56.3° C.; 7H2O ⇔ 12H2O, 20.5° C. The cryohydric point is -1.138° C.

Sodium Dihydrogen Orthoarsenate, NaH2AsO4

Sodium Dihydrogen Orthoarsenate, NaH2AsO4, is formed when an aqueous solution of sodium carbonate is treated with arsenic acid until the solution gives no precipitate with barium chloride; after concentration and cooling thoroughly, crystals of the monohydrate, NaH2AsO4. H2O, slowly separate. The crystals may also be obtained by fusing together equivalent quantities of arsenious oxide and sodium nitrate, dissolving the residue in water and allowing to crystallise. Rhombic crystals, isomorphous with those of the corresponding phosphate, are obtained, the axial ratios being a:b:c = 0.9177:1:1.6039. The salt is dimorphous, however, and crystallisation from warm solutions yields monoclinic crystals with a:b:c = 1.087:1:1.1588 and β = 92°22'. These crystals are unstable at the ordinary temperature, rapidly becoming turbid and passing to the rhombic form without change in weight. The indices of refraction of the latter for sodium light are α = 1.5382, β = 1.5535 and γ = 1.5607. The density is 2.67. The aqueous solution, when concentrated until the density is 1.7, deposits rhombic octahedra of the dihydrate, NaH2AsO4.2H2O. These are isomorphous with those of the corresponding phosphate and have axial ratios a:b:c = 0.9177:1:1.6039 and density 2.309. The indices of refraction for sodium light are α = 1.4794, β = 1.5021 and γ = 1.5265. The dihydrate is efflorescent in air.

The monohydrate loses water of crystallisation at 130° C. and on further heating yields a product which has been described as sodium metarsenate, NaNaO3, but which dissolves in water to form sodium dihydrogen orthoarsenate. Similarly, when sodium monohydrogen orthoarsenate is strongly heated (above 250° C.) the product appears to be sodium pyroarsenate, Na4As2O7, and this re-forms the orthoarsenate on dissolution in water. The existence of the meta- and pyro-arsenates is not established. Both products when heated in hydrogen or carbon monoxide yield volatile arsenic.

Complex salts

Complex salts of composition Na3H3(AsO4)2.3H2O, Na3K3H6(AsO4)4.9H2O and Na3(NH4)3H6(AsO4)4.6H2O have been described, as also have NaKHAsO4.nH2O (n = 7-9), Na(NH4)HAsO4.4H2O and Na(NH4)2AsO4.4H2O.

In the presence of sodium hydroxide, arsenic pentoxide reacts with the hydroxides of aluminium, chromium and ferric iron to form complex salts of composition NaH2[Al(AsO4)2].0.5H2O, NaH2[Cr(AsO4)2].H2O and NaH2[Fe(AsO4)2].H2O, respectively. Corresponding salts of bismuth, cobalt, thallium or lanthanum are not formed under the same conditions.
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