Polyvanadates

Polyvanadates or Acid Vanadates. In addition to the salts of vanadium pentoxide which have been described, there exists a large number of polyvanadates in which the molecular proportion of vanadium pentoxide to basic oxide is greater than in the case of the metavanadates. A general method for preparing the alkali polyvanadates consists in adding acetic acid to a solution of the ortho-, pyro-, or meta-vanadate, and concentrating. Usually several compounds of varying composition can be isolated. The polyvanadates of the heavy metals can sometimes be prepared by double decomposition of alkali vanadates with solutions of salts of these metals, but more usually complex salts containing both the heavy metal and the alkali (double vanadates) are precipitated. By precipitating a solution of acid barium vanadate, 2BaO.3V₂O₅.12H₂O, with salts of various heavy metals, Ephraim and Beck have recently prepared a series of acid vanadates of general formula 2R^••O.5V₂O₅.12 or 15H₂O, or, less frequently, 3R^••O.5V₂O₅.nH₂O. These lose their water of crystallisation largely or completely at 220° C., with decomposition in the case of the oxidisable heavy metals. The acid vanadates are unstable towards weak acids, and are frequently decomposed even by boiling water. They are intensely coloured, usually red. The following polyvanadates are known:

The compound (NH₄)₂O.3V₂O₅ appears to be the most stable of the acid ammonium vanadates. It is formed by addition of a 10 per cent, solution of acetic acid to a solution of ammonium metavanadate, or by boiling an aqueous solution of any other ammonium polyvanadate. It forms thin, transparent, yellow, octagonal plates, or golden, microscopic, rhombic plates which become temporarily vermilion on being heated, and which are only slightly soluble in water. It has also been obtained combined with two, five, or six molecules of water, according to the temperature of crystallisation or extent of dehydration.

2(NH₄)₂O.3V₂O₅.4H₂O can be looked upon as ammonium hexa-Vanadate9 (NH₄)₄V₆O₁₇.4H₂O. It is obtained as transparent, ruby-red, monoclinic crystals when a solution of ammonium oxalate, saturated at 30° C., is treated with vanadium pentoxide and evaporated in a vacuum. Crystals containing 6H₂O can also be obtained. It is very soluble in water.

By the action of small quantities of acetic acid of varying concentration on hot, saturated solutions of ammonium metavanadate, other complexes have been obtained: (NH₄)₂O.2V₂O₅.H₂O, 2H₂O, or 3H₂O; 3(NH₄)₂O.7V₂O₅.4H₂O; 2(NH₄)₂O.5V₂O₅.10H₂O.

Barium hexavanadate, 2BaO.3V₂O₅.12H₂O, is prepared by the addition of barium chloride to an acid solution of calcium vanadate under definite conditions of concentration and acidity. Another hydrate, 2BaO.3V₂O₅.14H₂O or Ba₂V₆O₁₇.14H₂O, is obtained as red, rhombic crystals when barium nitrate is added to a boiling solution of sodium metavanadate acidified with acetic acid. Addition of barium chloride to a solution of potassium pyrovanadate in the presence of a large excess of acetic acid yields red crystals having the composition 3BaO.5V₂O₅.19H₂O. These are soluble in about 5000 parts of water; the mother-liquor, after crystallisation of the foregoing salt, gives the compound 4BaO.5V₂O₅.24H₂O.

Beryllium Polyvanadate, 2BeO.3V₂O₅.11H₂O, forms dark red, hygroscopic crystals which are obtained by the action of beryllium sulphate on barium hexavanadate.

Two are known, 2CdO.3V₂O₅.15H₂O and CdO.3V₂O₅.2H₂O. The former is obtained by the action of cadmium sulphate on the corresponding barium salt, and the latter by the action of cadmium nitrate on ammonium metavanadate in the presence of acetic acid.

Addition of calcium chloride to a solution of potassium pyrovanadate in the presence of acetic acid has furnished 3CaO.4V₂O₅.15H₂O; CaO.2V₂O₅.6H₂O; CaO.2V₂O₅.9H₂O; 3CaO. 7V₂O₅.7H₂O; 2CaO.5V₂O₅.5H₂O; 3CaO.8V₂O₅.26H₂O, according to the concentration, temperature, and acidity of the mixture. Addition of calcium nitrate to excess of ammonium metavanadate solution in the presence of nitric acid has given the compound CaO.3V₂O₅.12H₂O, which is readily soluble in water.

Cerous Polyvanadate, Ce₂O₃.5V₂O₅.27H₂O, has been prepared by evaporating a solution containing ammonium metavanadate and cerous sulphate.

Cobalt Polyvanadate, 2CoO.3V₂O₅.15H₂O, forms large, six-sided brown leaves which effloresce readily in air. It is obtained by the action of cobalt sulphate on the corresponding barium salt.

Copper Polyvanadate, 3CuO.5V₂O₅.22H₂O, is prepared by the action of copper sulphate on barium hexavanadate. It forms thin, iridescent spangles.

Gadolinium Polyvanadate, Gd₂O₃.5V₂O₅.26H₂O, and Lanthanum Vanadate are known.

Lead Polyvanadate, PbO.2V₂O₅, can be obtained as yellow, transparent needles by fusing a mixture of vanadium pentoxide, sodium iodide, and lead iodide.

Lithium Polyvanadate, 5Li₂O.6V₂O₅.30H₂O, has been prepared as red, transparent prisms by the addition of acetic acid to the mother- liquor left from the crystallisation of lithium metavanadate. If the acetic acid is added directly to a solution of lithium metavanadate or orthovanadate, the compound obtained has the composition 3Li₂O.4V₂O₅.12H₂O. The same process has also yieldedv 2Li₂O.3V₂O₅.15H₂O or 16H₂O, Li₂O.2V₂O₅.8H₂O, 9H₂O or 12H₂O, according to the acidity, temperature, and concentration. Evaporation of the mother-liquor from the crystallisation of the compound 3Li₂O.4V₂O₅.12H₂O has given the sparingly soluble compound 3Li₂O.5V₂O₅.14H₂O or 12H₂O.

Addition of a slight excess of magnesium sulphate to a hot solution of potassium pyrovanadate gives red crystals which have the composition 4MgO.6V₂O₅.19H₂O. They are only slightly soluble in water and undergo dehydration at 200° C. When magnesium oxide is boiled with vanadium pentoxide in solution and excess of acetic acid is added, a dimorphous, soluble polyvanadate having the composition 3MgO.5V₂O₅.28H₂O is formed. Decomposition of a hot, saturated solution of ammonium metavanadate with excess of magnesium chloride and acetic acid gives another compound, MgO.2V₂O₅.9H₂O or 8H₂O; and a hexavanadate, 2MgO.3V₂O₅.19H₂O, has been prepared by the action of magnesium oxide on vanadium oxytrichloride.

The action of manganese sulphate on acid barium vanadate has given the salt 2MgO.3V₂O₅.11H₂O. A compound 5MgO₂.V₂O₅.10H₂O has also been prepared. This can be looked upon as a vanadic manganite.

NiO.2V₂O₅.3H₂O is obtained as greenish-brown crystals from the mother-liquor left after crystallisation of nickel metavanadate. The compound 3NiO.5V₂O₅.24H₂O has been prepared in two crystalline forms.

By the action of acetic acid on solutions of potassium metavanadate at different temperatures and concentrations, soluble acid vanadates of the following compositions have been prepared: 3K₂O.5V₂O₅.10H₂O; K₂O.2V₂O₅.4H₂O; 2K₂O.4V₂O₅.7H₂O; 2K₂O. 5V₂O₅.12H₂O. By dissolving excess of vanadium pentoxide in potassium carbonate solution and then adding acetic acid, Ditte also isolated the compounds K₂O.2V₂O₅.3H₂O or 8H₂O or IOH₂O, and K₂O.3V₂O₅.H₂O or 5H₂O or 6H₂O. The anhydrous compound K₂O.3V₂O₅ is almost insoluble in water.

Potassium Hexavanadate, 2K₂O.3V₂O₅.6H₂O or K₄V₆O₁₇.6H₂O, is obtained as orange plates by concentration of the mother-liquor from the salt K₂O.2V₂O₅. The heptahydrate, K₄V₆O₁₇.7H₂O, has also been prepared. When vanadium pentoxide is fused with potassium chloride and the product extracted with water, the following are obtained: 22K₂O.24V₂O₅.7H₂O; K₂O.2V₂O₅.4H₂O or 6H₂O; and the insoluble residue is found to contain the compound 2K₂O.8V₂O₅.3H₂O. Substitution of potassium fluoride for the chloride in this reaction gives a vanadate which contains the highest proportion of vanadium pentoxide in the series, namely, 2K₂O.9V₂O₅.

Addition of a neutral samarium salt to a solution of a metavanadate throws down a yellow, amorphous precipitate which consists mainly of samarium orthovanadate. Two acid samarium vanadates, namely, Sm₂O₃.5V₂O₅.28H₂O and Sm₂O₃.5V₂O₅. 24H₂O, have been isolated from the mother-liquors.

Sodium hexavanadate, Na₄V₆O₁₇ or 2Na₂O. 3V₂O₅, is prepared by acidifying a solution of sodium metavanadate. It has been obtained combined with 18H₂O, 16H₂O, 10H₂O, and 9H₂O. The anhydrous salt is almost insoluble in water. From electrical conductivity data under different conditions, Dullberg was able to show the existence in solution of hexavanadic acid, H₄V₆O₁₇, which gives rise also to the following acid salts: Na₃HV₆O₁₇.13H₂O and Na₂H₂V₆O₁₇.2H₂O. It is by no means certain, however, that these acid salts are derivatives of hexavanadic acid; they can be alternatively formulated as Na₂O.2V₂O₅.9H₂O and Na₂O.3V₂O₅.3H₂O respectively. The former has been prepared by the action of acetic acid on sodium metavanadate; its pentahydrate, Na₂O.2V₂O₅.5H₂O, and decahydrate, Na₂O.2V₂O₅.10H₂O, are also known, both of which undergo dehydration at about 200° C.; the residue melts at 581° C. The compound Na₂O.3V₂O₅.3H₂O has been made by the action of caustic soda on excess of vanadium pentoxide. Its pentahydrate, Na₂O.3V₂O₅.5H₂O, and nonahydrate, Na₂O.3V₂O₅.9H₂O, are also known. All these sodium salts are red, easily crystallisable substances.

Slight variations in the method of preparation have yielded several further polyvanadates of sodium: 5Na₂O.8V₂O₅.39H₂O; 4Na₂O. 7V₂O₅.33H₂O; 3Na₂O.5V₂O₅.22H₂O or 23H₂O; 4Na₂O.7V₂O₅.35H₂O; 3Na₂O.7V₂O₅.33H₂O; 2Na₂O.5V₂O₅.28H₂O; 4Na₂O. 10V₂O₅c7H₂O; 2Na₂O.8V₂O₅.15H₂O or 17H₂O.

By the action of strontium chloride on potassium metavanadate solution in the presence of acetic acid, the following compounds have been isolated: 3SrO.4V₂O₅.14H₂O; 4SrO.7V₂O₅.30H₂O; SrO.2V₂O₅.9H₂O. The last-mentioned salt has also been obtained from the mother-liquors left after the separation of strontium pervanadate. Using the compound K₂O.2V₂O₅ in place of the metavanadate yields the more acid salt, 2SrO.6V₂O₅.27H₂O. A polyvanadate containing four molecules of vanadium pentoxide, viz. SrO.4V₂O₅.11H₂O, has also been prepared.

By treating ammonium metavanadate with thallium sulphate, a red acid salt of composition 6Tl₂O.7V₂O₅ has been obtained. It melts at 408° C. A Uexavanadate, Tl₂O.3V₂O₅, has also been prepared.

Thorium Polyvanadate, ThO₂.6V₂O₅.8H₂O, is known.

Ytterbium Polyvanadates, 3Yb₂O₃.5V₂O₅.3H₂O and Yb₂O₃.15V₂O₅, are known.

Zinc Polyvanadate, 2ZnO.3V₂O₅.15H₂O, is obtained as well-defined crystals by the action of zinc sulphate on acid barium vanadate.