Hexavanadates

The principal vanadium-bearing ores are generally cmshed, ground, screened, and mixed with a sodium salt, eg, NaCl or Na2C02- This mixture is roasted at ca 850°C and the oxides are converted to water-soluble sodium metavanadate, NaVO. The vanadium is extracted by leaching with water and precipitates at pH 2—3 as sodium hexavanadate, Na V O, a red cake, by the addition of sulfuric acid. This is then fused at 700°C to yield a dense black product which is sold as technical-grade vanadium pentoxide. This product contains a minimum of 86 wt % V20 and a maximum of 6—10 wt % Na20. 

A proprietary tetrachromate bath has been used in Germany under the name of the D process. By the use of additions of magnesium oxide and sodium tungstate it is claimed that the current efficiency of the bath can be raised to as high as 35-40%. Other additives such as indium sulphate, sodium selenate or sodium hexavanadate enable bright deposits to be obtained. 

The kinetics of the contributory rate processes could be described [995] by the contracting volume equation [eqn. (7), n = 3], sometimes preceded by an approximately linear region and values of E for isothermal reactions in air were 175, 133 and 143 kJ mole-1. It was concluded [995] that the rate-limiting step for decomposition in inert atmospheres is NH3 evolution while in oxidizing atmospheres it is the release of H20. A detailed discussion of the reaction mechanisms has been given [995]. Thermal analyses for the decomposition in air [991,996] revealed only the hexavanadate intermediate and values of E for the two steps detected were 180 and 163 kJ mole-1. 

Sodium hexahydroxoantimonate(V), 3 60 Sodium hexametaphosphate, 3 416 Sodium hexavanadate, 25 517 Sodium hyaluronate... 

Fig. 11. Synthesis of an open-framework material, 6 (structure of 6-Gd given in Figs. 12 and 13). Condensation of a bis(triester)hexavanadate unit with the pendant triester groups terminating in carboxylic acids, 5, with lanthanide ions form the open-framework catalytic materials.

The typical side products in decavanadate preparations are the metavanadate and/or hexavanadate, as well as unreacted V205. These impurities are generally less soluble than the decavanadate salt and can be removed by filtration. X-Ray powder diffraction is felt to be the best criterion of phase purity unless macroscopic crystals are obtained. 

The polymeric metavanadate anions have been described above. Upon warming or ageing solutions that contain the decavanadate ion, it is possible to precipitate salts of the dark red trivanadate anion, MV3Os (M = NH4, K, Rb, Cs). The formula is sometimes written as M2V60i6, and the salts called hexavanadates in the literature. Potassium and cesium trivanadate contain infinite buckled layers of distorted VOs and VOe polyhedra.18 Another layer structure is found in yellow K3VsOi4 which also is deposited from aqueous solution. The [V5O14]3 - anion is composed of layers of comer-shared V04 tetrahedra and VO5 square pyramids.19... 

The reaction with magnesium oxide proceeds differently and gives magnesium hexavanadate, Mg2Ve017.19H20.1... 

General.—It seems certain that the free acids corresponding to these salts do not exist in the solid state, and that, with the possible exception of hexavanadic acid, mentioned below, they are also incapable of existing in solution, although salts of all the acids are known. The most stable class of salts is the metavanadates, the next in order of stability being the pyrovanadates, while the orthovanadates are few in number and undergo rapid hydrolysis even in the cold, to give the pyro-salts ... 

Beryllium Polyvanadate, 2Be0.3V205.IlH20, forms dark red, hygroscopic crystals which are obtained by the action of beryllium sulphate on barium hexavanadate.10... 

Copper Polyvanadate, 3Cu0.5Va08.22H20, is prepared by the action of copper sulphate on barium hexavanadate.7 It forms thin, iridescent spangles. 

The metavanadates crystallize from aqueous solution as MV3O9 4H2O. If, however, solutions of ammonium metavanadate and rare earth nitrate are mixed with an equal volume of alcohol at a pH value depending on the ion (shown below) hexavanadates of the composition M VbOitJs 48H2O are obtained. 

These hexavanadates, like the ortho and meta vanadates, are sparingly soluble in water. 

Kera Y. Infrared study of alkali tri- and hexavanadates as formed from their melts. J. Sohd State Chem. 1984 51 205-11. 

P. Y. Zavalij, F. Zhang, and M.S. Whittingham, Crystal structure of layered i/s (ethylenediamine)nickel hexavanadate as a new representative of the VsOm series. Acta Cryst. B55, 953 (1999). 

Other condensed vanadates, e.g. trivanadate, hexavanadate, dodecavanadate and pentadecavanadate, are known, but play minor roles in the aqueous systems (tri-and hexavanadate) or are thermodynamically unstable with respect to decavanadate (polyoxovanadates with more than 10 vanadium centres). An interesting case from the structural point of view is the tridecavanadate [Hj2Vi304q], included in Table 2.3 I 1... 

As pointed out in the previous section, phosphate derivatives related to H3VO4 are of interest when it comes to the phosphate-vanadate antagonism. Similarly, vanadate esters are analogues of phosphate esters (Section 5.2.1). Esters of orthovanadic acid have been known since IQIS.I They are obtained by, e. g., condensation of vanadates and alcohols (ROH) in aqueous media [Equation (2.11)], alcoholysis of VOCI3 [Equation (2.12)] or the reaction of vanadium pentoxide and alcohols in the presence of water scavengers [Equation (2.13)]. The triesters are sensitive to moisture. The primary product of hydrolysis is the hexavanadic acid derivative V50i3(0R)4, giving rise to a yellow colouration as the colourless triesters come into contact with moist air. 

Hexaniobates, 1028 Hexatamalates, 1029 Hexatungstates, 1034 Hexavanadates, 1027 Holmium complexes phenanthroline, 1069 Homopolymolybdates, 1377 Hydrates, 4 amides, 6 ammines, 4 solvates,4... 

Ageing or warming Vio02s with NH4 or large Alk can precipitate dadt-ied MV3O8, sometimes called hexavanadates . 

The precipitated polymeric rare earth vanadates are amorphous or microcrystalline. Except for the decavanadates the crystal structures are uncertain. The existence of VjO, VO3, and V2O17 radicals has been proposed and orthorhombic and monoclinic structures have been suggested for the hexavanadates. 

The rare earth vanadites are paramagnetic at room temperature and except for GdVOj become antiferromagnetic at low temperatures (Sakai et al., 1976). The magnetic susceptibiUties of rare earth hexavanadates have been reported to be higher than those of ortho vanadates (Arbit and Serebrennikov, 1968b). 

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