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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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