Chemical elements
  Arsenic
      Occurrence
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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

Ammonium Arsenates





Ammonium Orthoarsenate, (NH4)3AsO4

Ammonium Orthoarsenate, (NH4)3AsO4.3H2O, is precipitated on saturating an aqueous solution of arsenic acid, or of either ammonium mono- or di-hydrogen orthoarsenate, with ammonia; or it may be obtained by cooling a warm concentrated solution of arsenic acid in aqueous ammonia. The deposit consists of rhombic crystals and may be recrystallised from hot aqueous ammonia. The salt rapidly loses ammonia both in air and in solution, the dihydrogen arsenate ultimately being formed. The anhydrous salt, (NH4)3AsO4, is formed when the dry mono- or di-hydrogen salt absorbs ammonia under a pressure of 8 atmospheres; the absorption should be allowed to continue for 12 hours at about 50° C. This product also is unstable and rapidly loses ammonia to form the monohydrogen arsenate.


Ammonium Monohydrogen Orthoarsenate, (NH4)2HAsO4

Ammonium Monohydrogen Orthoarsenate, (NH4)2HAsO4, may be prepared by the addition of ammonia to a concentrated aqueous solution of arsenic acid, the precipitate first formed being dissolved by warming and the salt then obtained either by crystallisation or by precipitation with ethyl alcohol. The dry salt, when gently heated, loses ammonia to form the dihydrogen arsenate, as also does the aqueous solution; in the cold, however, the dry salt is stable in dry air. The crystals are monoclinic prisms, with a:b:c = 0.918:1:1.1715 and β = 91°13'; density 1.99.

Ammonium Dihydrogen Orthoarsenate, NH4H2AsO4

Ammonium Dihydrogen Orthoarsenate, NH4H2AsO4, may be prepared by mixing aqueous ammonia and arsenic pentoxide in theoretical proportions. It has also been obtained by heating arsenious oxide with ammonium nitrate and by gently heating ammonium mono- hydrogen orthoarsenate. It crystallises in long non-deliquescent prisms belonging to the tetragonal system, the axial ratio a:c being 1:1.0035. The salt is isomorphous with the corresponding potassium salt and with the corresponding ammonium and potassium phosphates. The density at 20° C. is 2.340. When heated above 300° C. it loses ammonia and an almost insoluble acid metarsenate, NH4H(NaO3)2, is obtained -

2NH4H2AsO4 = NH3 + 2H2O + NH4H(NaO3)2

which is stable up to 425° C., no trace of arsenious oxide or arsenic being formed at this temperature. When boiled with water the metarsenate reverts to the orthoarsenate.

The solubility in water of ammonium dihydrogen orthoarsenate and the densities of the saturated solutions have been determined with the results shown in the following table:

Solubility of NH4H2AsO4 in water

Temp., ° C.g. per 100 g. H2ODensity
033.741.1814
2048.671.2280
4063.831.2821
6083.051.3464
80107.251.4200
90122.401.4623

The following values for the heats of neutralisation at 15° C. of arsenic acid by aqueous ammonia, and for the heats of dissolution of the ammonium arsenates, have been obtained. The solutions of arsenic acid for the determination of the former contained 1/6 mole of H3AsO4 per litre, and those of ammonia were of such concentrations that 1 volume of the acid was neutralised by an equal volume of the base.

Heats of Neutralisation

H3AsO4.330H2O + NH3.330H2O = NH4H2AsO4.660H2O + 13,750 calories
H3AsO4.330H2O + 2NH3.330H2O = (NH4)2HAsO4.660H2O + 24,300 calories
H3AsO4.330H2O + 3NH3.330H2O = (NH4)3AsO4.660H2O + 25,100 calories

Heats of Dissolution

NH4H2AsO4 solid + 660H2O = NH4H2AsO4 dissolved - 4250 calories
(NH4)2HAsO4solid + 660H2O = (NH4)2HAsO4 dissolved - 3150 calories
(NH4)3AsO4solid + 1500H2O = (NH4)3AsO4dissolved - 8400 calories
(NH4)3AsO4.3H2Osolid + 1200H2O = (NH4)3AsO4.3H2Odissolved -13,700 calories

From the above data, and assuming the heat of dissolution of arsenic pentoxide to be +6000 calories, the following values for the heats of formation have been calculated:

Heats of Formation

½(As2O5solid + 3H2Oliquid) + NH3gas = NH4H2AsO4solid + 29,480 calories
½(As2O5solid + 3H2Oliquid) + 2NH3gas = (NH4)2HAsO4solid + 47,410 calories
½(As2O5solid + 3H2Oliquid) + 3NH3gas = (NH4)3AsO4solid + 61,940 calories
½(As2O5solid + 3H2Oliquid) + 3NH3gas + 3H2Oliquid = (NH4)3AsO4.3H2Osolid + 67,240 calories

The values of Q0 from Nernst's formula



for the di- and tri-ammonium salts are respectively 15,500 and 13,070 calories; these values represent the heats liberated in fixing the second and third molecules of ammonia. A study of the dissociation of the triammonium arsenate suggests that the successive withdrawal of ammonia gives rise to equilibria which indicate the existence of the following compounds; Q0 represents the heat changes of the equilibria.

(NH4)6As2O8 ⇔ (NH4)5HAs2O8 + NH3, Q0 = 13,000 cal.
(NH4)5HAs2O8 ⇔ (NH4)4H2As2O8 + NH3, Q0 = 14,000 cal
(NH4)4H2As2O8 ⇔ (NH4)3H3As2O8 + NH3, Q0 = 15,600 cal
(NH4)3H3As2O8 ⇔ (NH4)2H4As2O8 + NH3, Q0 = 16,200 cal
(NH4)2H4As2O8 ⇔ (NH4)H5As2O8 + NH3, Q0 = 16,700 cal

Mixed orthoarsenates of ammonium and sodium have been obtained, namely, diammonium sodium orthoarsenate, (NH4)2NaAsO4.4H2O, and ammonium sodium hydrogen orthoarsenate, (NH4)NaHAsO4.4H2O. The latter forms monoclinic crystals when a solution containing equivalent quantities of ammonium and sodium monohydrogen arsenates is allowed to crystallise, or when an ammoniacal solution containing ammonium chloride and sodium dihydrogen arsenate (1:6) is concentrated. When ignited, water and ammonia are expelled and sodium metarsenate remains. The diammonium sodium salt may be obtained by allowing to crystallise a solution of the ammonium sodium hydrogen salt in concentrated aqueous ammonia. The tabular crystals, when confined over dilute sulphuric acid, lose ammonia and revert to ammonium sodium hydrogen orthoarsenate.
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