Chemical elements
    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
      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
      Zinc Arsenates
      Zirconium Arsenates
      Arsenic and Sulphur
      Arsenic Subsulphide
      Tetrarsenic Trisulphide
      Arsenic Disulphide
      Arsenic Trisulphide
      Arsenic Pentasulphide
      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

Molybdenum Arsenates

Molybdous Arsenate, Mo(HAsO4)2

Molybdous Arsenate, Mo(HAsO4)2.nH2O(?), is said to be formed as a grey precipitate when molybdous chloride is treated with sodium monohydrogen arsenate; the precipitate first redissolves, but afterwards becomes permanent. Molybdic arsenate, obtained in a similar manner from molybdic chloride, has been described by Berzelius, who also considered that an acid salt was produced on dissolving the hydrate of molybdenum dioxide in excess of arsenic acid, since the solution turned blue on standing.

Molybdic acid forms with arsenic acid a series of heteropoly-acids analogous to the molybdophosphoric acids. When a mixture of molybdic acid, arsenic acid and an ammonium salt is boiled for some time, a yellow crystalline precipitate is obtained of an ammonium molybdo-arsenate. To this compound Debray gave the formula 3(NH4)2O.As2O5.20MoO3, and considered that on boiling with aqua regia and evaporating the solution, the residue contained two acids - one yellow, in which the ratio of As2O5 to MoO3 was 1:20, and the other white, with As2O5:MoO3 = 1:16. This was contested by Seyberth, who considered that both the precipitate and the acid obtained from it contained As2O5:MoO3 = 1:7. Various other acids and salts have been described in which the ratio of As2O5 to MoO3 differs considerably, those compounds rich in molybdic acid generally being yellow in colour, while those containing less molybdic acid are white. A satisfactory formulation of these compounds is due to the application of a modification of Werner's co-ordination theory suggested by Miolati and extended by Rosenheim.

According to this theory the complex molybdo-arsenic compounds may be considered as derived either from an acid containing a septa-valent complex anion in which arsenic is the central atom with a coordination number of 6, or from an acid containing a tervalent anion in which arsenic has a co-ordination number of 4. The hypothetical parent compounds correspond therefore to H7[AsO7] and H3[AsO4].


The free acid corresponding to ordinary phosphomolybdic acid has not been prepared, but the two acid salts, R3'H4[As(Mo2O7)6].4H2O, where R' = NH4 or K, have been obtained in the form of deep yellow crystalline precipitates by adding arsenic acid in small quantities to solutions of the molybdates previously made acid with nitric acid and containing the alkali nitrates. The ammonium salt serves for the analytical detection of arsenic acid, and the precipitation is quantitative providing the concentration of hydrogen ion present is greater than 0.030 per cent., and that ammonium nitrate is present. In less acid or neutral solutions the precipitate is white, and contains less molybdenum than the yellow salt. The corresponding guanidinium salt has not been obtained, but when hydrochloric acid is added to a solution containing 12 gram-molecules of guanidinium molybdate and 1 gram-molecule of arsenic acid, deep yellow leaflets separate consisting of the 10-molybdo-arsenate, (CH5N3)3H7[AsO(Mo2O7)5].5H2O. By treating 1 gram-molecule of this salt with 2 gram-molecules of guanidinium carbonate, the heptabasic salt, (CH5N3)7H7[AsO(Mo2O7)5].6H2O, separates in the form of light yellow crystals which decompose when recrystallised. The conductivity corresponds with its formation as a normal salt. The free acid has not been prepared.

A series of compounds, free acids and ammonium salts of the types


in which x + y + z = 6 and in which x or y may equal 0, has been prepared. They are all more or less of an intense red colour, depending on the value of The crystals belong to the tetragonal system and are uniaxially birefringent. The individual salts are isomorphous and form mixed crystals, as they also do with the corresponding phospho-compounds.


In this series the complex anion is more stable than in the two preceding series. The salts are derived from the acid,

which is formed by agitation of a solution of normal sodium arsenate, saturated with molybdic acid, with hydrochloric acid and ether. It is very soluble in water and yields two hydrates: H12[As2O2(Mo2O7)9].24H2O, red crystals, stable at ordinary temperatures; and H12[As2O2(Mo2O7)9].34H2O, yellow crystals, stable below 8° C. On treating the solution with alkali hydroxide or carbonate, the alkali salts are obtained. The sodium salt, Na6H6[As2O2(Mo2O7)9].20H2O, is yellow, and is an acid salt, indicating that the basicity of the acid is higher than 6. On standing, the crystals of this salt crumble and change into a colourless 25-hydrate. The corresponding yellow potassium salt, containing 11H2O, changes similarly into a colourless 25-hydrate. Normal salts, of composition R12'[As2O2(Mo2O7)9].xH2O (R' = Cs, Ag, Tl, CH6N3), have been obtained by adding the metallic or organic chloride to a solution of the yellow acid sodium salt, thus indicating that the basicity of the acid is 12.

Arsenic compounds corresponding to the 17-molybdo-2-phosphates have not been prepared, but by mixing solutions of paramolybdates and alkali dihydrogen arsenates, white precipitates of composition 5R2O.As2O5.16MoO3.xH2O (R' = NH4 or K) have been obtained, which may be regarded as 8-molybdo-arsenates, R5'As(OH)2(Mo2O7)4].(x/2 - 1)H2O.


The complex anion of this series is the most stable of those derived from [XO4]'''. The free acid, H3[AsO(MoO4)3], is tribasic, stronger than arsenic acid, and extremely stable, and is obtained on adding fuming nitric acid to a concentrated solution of arsenic acid saturated with molybdic acid. It readily dissolves in water, from which it may be crystallised in the form of white prisms. The trisodium salt crystallises from water with 11.5H2O, and the triguanidinium salt forms pale yellow anhydrous needles. Other salts have been described.

Salts of acids containing less molybdic acid - for example, 2- and 1-molybdo-arsenates of the types R2H[AsO2(MoO4)2].xH2O and R'H2[NaO3(MoO4)].xH2O - have been described, as also have sulphur-containing compounds of the types R4[As2S5(MoS4)2].xH2O and R'[AsS2(MoS4)].xH2O.

A compound of composition [4MoO3.MoO2]2.H3AsO4.4H2O has been obtained in the form of sapphire-blue hexagonal platelets by the addition of sodium monohydrogen arsenate to a solution of sodium molybdate acidified with sulphuric acid; the mixture was reduced by boiling under reflux with aluminium filings and, after cooling, extracted with ether. The extract was shaken with water and the blue compound passed into the water layer, from which it was obtained by evaporation in an atmosphere of carbon dioxide.
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