Syntheses et proprietes d'hexa niobo(V)-tungstates(VI)

Four new compounds belonging to the substituted isopolyanions family have been prepared from aqueous solutions. Their general formula is NbnW6−nO19(2+n)−; they have been separated as Na, K, Cs, or tetramethylammonium salts. The pH range of stability are shifted towards the alcaline zone when the Nb content of the anion increases. According to their properties they are likely to have the same structure as the Nb6O198− or W6O192− anions. Two of them (n = 1 and 2) are reducible in a reversible one step electron stage. The charge of the two others is too high for these compounds to be reducible. A polarographic method on stationary mercury electrode has been devised to determine niobium in presence of tungsten. This method is valid for all Nb/W ratio.