Hexavanadic acid

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 ... 

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. 

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. 

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.

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