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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:

Ammonium Polyvanadates

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 Polyvanadates

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

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

Cadmium Polyvanadates

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.

Calcium Polyvanadates

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

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

Cobalt Polyvanadate, 2CoO,3V2O5,15H2O

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

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

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

Lead Polyvanadate, PbO,V2O5

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

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.

Magnesium Polyvanadates

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.

Manganese Polyvanadates

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.

Nickel Polyvanadates

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.

Potassium Polyvanadates

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

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.

Samarium Polyvanadates

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 Polyvanadates

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.

Strontium Polyvanadates

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.

Thallium Polyvanadate

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

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

Ytterbium Polyvanadates

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

Zinc Polyvanadate

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

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