Orthovanadates, R^•₃VO₄

Orthovanadates, R^•₃VO₄ or 3R^•₂O.V₂O₅. The alkali ortho-vanadates can be prepared by fusing a mixture of vanadium pentoxide with the calculated quantity of alkali carbonate. The best known is the sodium salt Na₃VO₄.12H₂O. Ammonium orthovanadate does not appear to exist. Ditte obtained a number of vanadates by fusing a mixture of vanadium pentoxide, sodium halide, and the halide of the desired metal, and this method has furnished several orthovanadates. Copper, lead and silver orthovanadates have been prepared by double decomposition in solution, but addition of metallic salts to a soluble orthovanadate most often yields variously coloured precipitates of uncertain composition; because of their strong tendency to undergo hydrolysis the ortho-salts give rise to the corresponding pyro- and meta-vanadates, or to a mixture of them; the hydroxide of the metal may also be produced, and in the case of ferrous sulphate and other reducing agents oxidation takes place.

Silver orthovanadate reacts with alkyl halides to give esters of vanadic acid. These are yellow liquids of general formula Alk.3VO₄, in which Alk. represents the alkyl radical. Esters of orthovanadic acid are more stable than those of pyro- and meta-vanadic acids; that is, the order of stability is the reverse of that which applies to the inorganic salts. The following orthovanadates have been prepared:

Bismuth Orthovanadate, BiVO₄, is a bright yellow compound obtained by the double decomposition of bismuth nitrate with an alkali vanadate.

Cerous Orthovanadate, CeVO₄, has been prepared as dark red needles by fusing sodium metavanadate with cerous chloride.

When treated with solutions of copper salts, solutions of orthovanadates yield a greenish-yellow precipitate which consists mainly of copper orthovanadate with excess of vanadium pentoxide.

Lead Orthovanadate, Pb₃(VO₄)₂, is precipitated as a white powder when sodium orthovanadate is treated with lead acetate solution.

The yellow, anhydrous salt results on fusing a mixture of lithium carbonate and vanadium pentoxide in the requisite proportions. Crystals of the hexahydrate, Li₃VO₄.6H₂O, have been obtained by concentrating the mother-liquor left from crystallisation of lithium pyrovanadate. Two other lithium vanadates, 4Li₂O.V₂O₅.H₂O and 4Li₂O.V₂O₅.14H₂O, have also been prepared.

Nickel Orthovanadate, Ni₃(VO₄)₂, yields green, prismatic needles when a fused mixture of vanadium pentoxide, sodium bromide and a small proportion of nickel bromide is extracted with dilute nitric acid and the solution concentrated.

Potassium Orthovanadate, K₃VO₄, forms as a pale yellow, crystalline mass when vanadium pentoxide and potassium carbonate are fused together in the necessary molecular proportions. It melts at a temperature above 1000° C. A solution of vanadium pentoxide in three equivalent proportions of caustic potash yields colourless, transparent, deliquescent crystals of two hydrates, 2K₃VO₄.9H₂O and K₃VO₄.6H₂O respectively, according to the temperature of crystallisation. If a large excess of caustic potash is employed the compound obtained is 4K₂O.V₂O₅.20H₂O.

Silver Orthovanadate, Ag₃VO₄, is precipitated as a deep orange powder when a freshly prepared solution of sodium orthovanadate is treated with a carefully neutralised solution of silver nitrate. It melts between 403° and 565° C., and is soluble in nitric acid and in ammonium hydroxide; the latter solution yields yellow hexagonal crystals of the composition 3AgVO₃.2NH₃.H₂O.

12H₂O.Sodium Orthovanadate, Na₃VO₄.12H₂O, is the orthovanadate most frequently met with. It is readily obtained by adding excess of caustic soda to a solution of sodium pyrovanadate:

Na₄V₂O₇ + 2NaOH ⇔ 2Na₃VO₄ + H₂O.

It can be conveniently crystallised from caustic soda solutions, in which it is less soluble than in water. It forms hexagonal prisms isomorphous with the corresponding phosphate and arsenate, Na₃PO₄.12H₂O and Na₃AsO₄.12H₂O. The decahydrates, Na₃VO₄.10H₂O, Na₃AsO₄.10H₂O, and Na₃PO₄.10H₂O, are also isomorphous. In consequence of the reversible nature of the above reaction, solutions of sodium orthovanadate are strongly alkaline; their electrical conductivity has been studied by Dullberg. The anhydrous salt, Na₃VO₄, melts at 866° or 850° C. It can be prepared by fusing sodium carbonate and vanadium pentoxide in the required molecular proportions. Extraction of the product with water and precipitation with alcohol gives colourless needle-shaped crystals of a hydrate containing sixteen molecules of water, Na₃VO₄.16H₂O. The hepta- and octa-hydrates, Na₃VO₄.7H₂O and Na₃VO₄.8H₂O, have also been prepared.

By dissolving vanadium pentoxide in a large excess of caustic soda, Ditte obtained two crystalline vanadates which contained a larger proportion of the basic oxide than is present in the orthovanadate, and to which he gave the formulae 4Na₂O.V₂O₅.30H₂O and 4Na₂O.V₂O₅.26H₂O.

Strontium Orthovanadate, Sr₃(VO₄)₂, is obtained in transparent, pale yellow leaves by heating together a mixture of vanadium pentoxide, sodium iodide, and strontium iodide.

Thallium Orthovanadate, Tl₃VO₄, is a light brown substance obtained by carefully fusing three molecular proportions of thallium carbonate with one molecular proportion of vanadium pentoxide. Its density is 8.6. One gram dissolves in a litre of water at 15° C. M.pt. 566° or 555° C.

Although the orthovanadates of divalent metals are not very stable compounds, they form very stable double salts with certain metallic halides. These double salts are of interest in that isomorphous analogues are frequently given by orthophosphates and orthoarsenates. The following naturally occurring double salts are isomorphous:

3Pb₃(VO₄)₂.PbCl₂ (vanadinite).
3Pb₃(PO₄)₂.PbCl₂ (pyromorphite).
3Pb₃(AsO₄)₂.PbCl₂ (mimetesite).

They are isomorphous also with the calcium compounds Ca₃(VO₄)₂.CaCl₂ and Ca₃(PO₄)₂.CaCl₂. The discovery of isomorphism among these double salts led Roscoe to transfer vanadium from the chromium family, in which it had been previously placed, to Group V.

The general method of preparation of these double salts consists in fusing vanadium pentoxide with excess of the halide of the metal. Except in the case of the sodium compound, the residue is simply washed with water, in which the double salts are insoluble. The following have been described:

2Na3VO4.NaF.19H2O	Limpid octahedra, isomorphous with the corresponding phosphorus and arsenic compounds.
3Ba3(VO4)2.BaBr2	Hexagonal, transparent plates
3Ba3(VO4)2.BaI2	Hexagonal, brown, transparent prisms
3Cd3(VO4)2.CdCl2.	Hexagonal prisms; density 5.264
3Cd3(VO4)2.CdBr2	Hexagonal prisms; density 5.456

(The corresponding cadmium iodide has not been isolated.)

Ca3(VO4)2.CaCl2	Glistening, rhombic crystals
Ca3(VO4)2.CaBr2	Glistening, thin leaves
3Ca3(VO4)2.CaBr2	White, glistening crystals
3Ca3(VO4)2.CaI2	Hexagonal,colourless, transparent crystals
3Pb3(VO4)2.PbCl2	Reddish-brown, transparent, hexagonal prisms.

(This lead salt is the artificial form of the important natural ore vanadinite.)

3Sr3(VO4)2.SrBr2	Hexagonal plates
3Sr3(VO4)2.SrI2