Metavanadates, R^•VO₃

Metavanadates, R^•VO₃ or R^•₂O.V₂O₅, are more stable than either the ortho- or pyro-vanadates. Solutions of the latter yield metavanadates on being evaporated or by treatment with carbon dioxide. The alkali metavanadates are prepared directly by dissolving vanadium pentoxide in the calculated quantity of alkali hydroxide. The metavanadates of other metals are prepared by fusing vanadium pentoxide with the oxide or carbonate of the metal in calculated quantity, or by the action of a soluble salt of the metal on a neutral solution of alkali metavanadate. The alkali and alkaline earth metavanadates are white or pale yellow; metavanadates of the heavy metals are deep yellow, brown, or red. Most of the alkali metavanadates are soluble in water; vanadates of the heavy metals are almost insoluble in water or dilute acetic acid. According to Bleecker, all the vanadates of the metals seem to be soluble in water to some extent, but become insoluble in the presence of small quantities of the precipitating agent. They show a tendency to precipitate in the colloidal state; especially is this the case with the vanadates of iron, copper, zinc and aluminium. The vanadates of mercury, lead, copper and iron fuse at about 600° C., but the vanadates of aluminium, calcium, zinc and tungsten do not fuse at much higher temperatures. The fused vanadates of iron and copper are extremely hard and are good conductors of electricity. On being treated with mineral acid, vanadates decompose with the formation of red colloidal vanadium pentoxide.

The following metavanadates have been prepared:

Aluminium Metavanadate, Al(VO₃)₃, may be prepared electrolytically or by the addition of an aluminium salt in solution to an alkali vanadate.

Ammonium Metavanadate, NH₄VO₃, is one of the commonest compounds of vanadium, and forms the starting material for the preparation of a large number of vanadium salts. It is obtained in the laboratory by dissolving vanadium pentoxide in excess of ammonium hydroxide and concentrating, or, instead of concentrating, alcohol may be added, in which the salt is insoluble. Recrystallisation from dilute ammonium hydroxide gives a pure product. Ammonium metavanadate is also insoluble in a saturated solution of ammonium chloride, and it is quantitatively precipitated by the addition of an excess of solid ammonium chloride to a neutral solution of sodium metavanadate or pyrovanadate. It is a white powder which can be obtained in colourless, granular crystals, isomorphous with potassium metavanadate. Its density is 2.326. One hundred parts of water dissolve 5.18 parts at 15° C. and 10.4 parts at 32° C. On being gently heated in air ammonium metavanadate becomes yellow, red, and then brown, with loss of ammonia:

2NH₄VO₃ (Solid) = V₂O₅ (solid) + 2NH₃ (gas) + H₂O (liquid) - 43,600 calories.

At temperatures above 210° C. the salt undergoes reduction and leaves a residue of the lower oxides of vanadium, and may also give some nitride.

Barium Metavanadate, Ba(VO₃)₂.H₂O, is obtained in white or yellow microscopic crystals by the action of barium chloride on potassium metavanadate. It undergoes dehydration between 190° and 200° C. The anhydrous salt has also been prepared by the action of barium peroxide on vanadium pentoxide.

Addition of solutions of beryllium salts to alkali vanadates gives rise to basic vanadates of indefinite composition. The pure salt is obtained by boiling beryllium hydroxide and vanadium pentoxide in water in the required proportions. The solution is filtered, concentrated to a syrup and poured into alcohol, whereupon isometric cubes, modified by an octahedron, are obtained. The larger crystals polarise light. Density, 2.273. One gram dissolves in a litre of water at 25° C.

Cadmium Metavanadate, Cd(VO₃)₂, is obtained as brilliant, transparent, slender, yellowish needles by fusing a mixture of vanadium pentoxide, sodium bromide and cadmium bromide.

Cesium Metavanadate, CsVO₃, has been obtained by boiling vanadium pentoxide with caesium carbonate solution.

Calcium Metavanadate, Ca(VO₃)₂.3H₂O, gives rise to bright yellow needles when a solution of ammonium metavanadate is boiled with calcium chloride solution and precipitated with alcohol. The tetrahydrate, Ca(VO₃)₂.4H₂O, is prepared by allowing a mixture of potassium metavanadate and calcium chloride to evaporate for several days. Anhydrous calcium metavanadate is a white, porous substance, which is unaffected by strong heating, but is readily decomposed by acids to yield vanadium pentoxide. It is much more soluble in water than strontium metavanadate.

Cobalt Metavanadate, Co(VO₃)₂.3H₂O, separates out on boiling a solution of ammonium vanadate with excess of cobalt nitrate which has been feebly acidified with nitric acid. It is easily soluble in water.

Addition of copper sulphate solution to sodium metavanadate throws down a precipitate which consists mainly of copper metavanadate, which is light yellow. The precipitate may, however, be green or blue, because its composition varies considerably. Copper metavanadate can also be produced electrolytically. On being fused at a high temperature in a graphite crucible it forms copper and vanadium carbide.

2H₂O.Indium Metavanadate, In(VO₃)₃.2H₂O, is a yellow substance produced by the action of sodium metavanadate on indium chloride solution.

Iron Vanadate is, metallurgically, the most important vanadate. Precipitation of a solution of a vanadate with ferrous sulphate gives rise to a precipitate of indefinite composition, ortho-, pyro-, meta-, and perhaps a poly-vanadate being present, as well as ferric or ferrous oxide. Reduction of the vanadate to a vanadyl salt may also ensue. The precipitate is usually colloidal and carries down with it some sodium vanadate. The dried powder may be either green, yellow, brown, or red; the more nearly the precipitate approximates to a red colour the lower is its vanadium content. An iron vanadate has also been prepared by electrolysis of a solution of sodium vanadate between iron poles.

Acid solutions of vanadates on being treated with lead salts give rise to yellow basic vanadates the composition of which varies with the conditions. The precipitation of normal lead vanadate is, therefore, difficult. It has been accomplished by the addition of lead acetate to ammonium metavanadate solution in the presence of acetic acid. The lead precipitates contain all the vanadic acid originally present in solution, and precipitation of vanadates with lead salts has, therefore, been employed for the quantitative estimation of vanadium. The mineral deschenite consists chiefly of lead metavanadate; a portion of the lead is, however, frequently replaced by zinc.

Lithium Metavanadate, LiVO₃.2H₂O, forms brilliant, silky needles when lithium carbonate (1 mol.) and vanadium pentoxide (1 mol.) are boiled together in water and the product concentrated in a vacuum. It melts at 618° C. and is readily soluble in water.

Magnesium Metavanadate, Mg(VO₃)₂.6H₂O, is obtained as transparent crystals by boiling an excess of basic magnesium carbonate with vanadium pentoxide and concentrating the filtered solution in a vacuum. It is easily soluble in water.

Manganese Metavanadate, Mn(VO₃)₂.4H₂O, is a sparingly soluble, dark red powder, obtained by interaction between manganese sulphate and ammonium metavanadate in solution. When the powder is boiled in the precipitating solution, reddish-brown six-sided plates of the anhydrous metavanadate, Mn(VO₃)₂, are produced.

Mercurous Metavanadate, HgVO₃, is thrown down as an orange precipitate when mercurous nitrate is added to a solution of a vanadate. If the solutions are carefully neutralised the precipitation is complete, and it is used for the gravimetric estimation of vanadium, the mercurous vanadate being ignited and the residue weighed as vanadium pentoxide. In the presence of a slight excess of ammonia, a grey or black precipitate of complex composition is produced. Addition of mercuric chloride to a neutral solution of a vanadate produces a white precipitate, soluble in acids; in the presence of ammonia a yellow compound is thrown down.

Nickel Metavanadate, Ni(VO₃)₂, separates as greenish-yellow, transparent prisms when a solution of ammonium metavanadate is boiled with excess of nickel nitrate solution feebly acidified with nitric acid.

Potassium Metavanadate, KVO₃, is obtained as white or colourless crystals by dissolving vanadium pentoxide in hot, strong caustic potash solution. By varying the concentrations several hydrates have also been prepared: 2KVO₃.3H₂O; KVO₃.2H₂O; 2KVO₃.5H₂O; KVO₃.3H₂O. On being heated, all these hydrates lose their water and leave a white, nacreous mass of the anhydrous salt, which melts at 495° C. Two other hydrates were prepared by Rammelsberg: KVO₃.H₂O and KVO₃.7H₂O.

Silver Metavanadate, AgVO₃, is a yellow, gelatinous substance obtained by interaction between silver nitrate and a metavanadate in neutral solution. It is soluble in nitric acid and in ammonia.

Sodium Metavanadate, NaVO₃, is manufactured industrially by decomposing commercial iron vanadate with sodium carbonate or sodium hydroxide, or by dissolving vanadium pentoxide in solutions of these sodium compounds. It was prepared in the pure state by McAdam, to determine the atomic weight of vanadium. Its electrolytic production has been studied. Ditte reported the existence of several hydrates: NaVO₃.2H₂O; NaVO₃.2½H₂O; NaVO₃. 3H₂O or 4H₂O; but solubility experiments indicate the existence in solution of only the dihydrate, NaVO₃.2H₂O. Cryoscopic measurements show that the anhydrous salt associates in solution to give Na₃V₃O₉. One hundred grams of water dissolve 21.1 grams of anhydrous sodium metavanadate at 25° C. and 38-8 grams at 70° C. The salt possesses such powerful colouring properties that 1 part of it imparts a yellow tint to 200,000 parts of water. The hydrated salt is efflorescent, and melts at 562° C. to a dark red, amorphous mass. The m.pt. of the anhydrous salt is also given as 630° C. It is decomposed by mineral acids in the cold to give colloidal vanadium pentoxide; when a current of hydrogen chloride gas is passed over sodium metavanadate at 440° C., the whole of the vanadic acid is volatilised and sodium chloride is left. The volatile product condenses as a semi-opaque, reddish-brown, oily liquid, which is probably 2VO₂.4HCl.3H₂O.

Strontium Metavanadate, Sr(VO₃)₂.4H₂O, is obtained as colourless, monoclinic prisms by the action of strontium chloride on potassium metavanadate. It is only sparingly soluble in water and undergoes dehydration at 280° C.

Thallium Metavanadate, TlVO₃, is prepared by fusing thallium carbonate with vanadium pentoxide in molecular proportions. It forms dark, laminated crystals, which are almost insoluble in water. Its sp. gr. Is 6.019 at 17° C.; m.pt. 424° or 391° C.

Zinc Metavanadate, Zn(VO₃)₂.2H₂O, forms brilliant, pale yellow, cubic or rhombic crystals when zinc nitrate is added to a neutral solution of ammonium metavanadate and the mixture is concentrated. It has also been obtained electrolytically.

Metavanadic acid yields addition compounds with hydroxylamine and ammonia. When hydroxylamine hydrochloride and ammonium metavanadate are added to a cold saturated solution of ammonia and the whole kept at 0° C. until precipitation takes place, rosettes of lemon-yellow crystals having the composition HVO₃.2NH₂OH.2NH₃ or VO₅N₄H₁₃, are formed. These are stable in the presence of ammonia, but are decomposed by water, dilute caustic soda, air, and carbon dioxide. On addition of hydrochloric or sulphuric acid, evolution of nitrous oxide takes place. By decreasing the proportion of ammonium metavanadate added to the saturated ammonia solution, yellow crystals of composition HVO₃.3NH₂OH.2NH₃ or VO₆N₅H₁₆ separate out. An extremely unstable compound, HVO₃.3NH₂OH or VO₆N₃H₁₀, has also been prepared. Addition compounds with hydroxylamine are also given by arsenic acid and phosphoric acid, and by tungstates, uranates, and molybdates.

A reaction which is very comparable to that which takes place with hydroxylamine consists in the formation of amine vanadates by the action of vanadium pentoxide on the alkylamines. The simplest of those that have been prepared are:

Methylamine vanadate, CH₃NH₂.HVO₃.
Dimethylamine vanadate, (CH₃)₂NH.HVO₃.
Ethylamine vanadate, (C₂H₅)NH₂.HVO₃.
Tetraethylamine vanadate, (C₂H₅)₄N.VO₃.