Vanadic Oxide, V₂O₅

Vanadic Oxide or vanadium pentoxide gives rise to the vanadates, the most numerous and important class of vanadium compounds. All vanadium compounds in a lower stage of oxidation have a tendency on being warmed to become converted into vanadates or derivatives of vanadates. Vanadium pentoxide hence is strongly acidic, but the basic character exhibited by the lower oxides is not completely lost, since the pentoxide dissolves in strong acids to give rise to salts, such as, for example, V₂O₅.2SO₃. These salts, however, are not very well defined or stable, and readily undergo hydrolysis to give rise to compounds which contain the trivalent radical [VO]^•••, e.g. vanadium oxytrichloride, VOCl₃, or the monovalent radical [VO₂]^•, for example, ammonium vanadium dioxyfluoride, 3NH₄F.VO₂F. The pentavalent ion V^{•••••} appears to be incapable of free existence, since no pentavalent compounds are known, excepting those with oxygen and sulphur, in which all the five valencies are saturated with negative elements or groups. As in the case of the lower oxide, VO₂, vanadium pentoxide has a strong tendency to form condensed poly-acids which give rise to salts of the type R₂O.xV₂O₅. In this respect vanadium pentoxide is analogous with other weak acids which are formed from metallic elements; compare, for instance, tungstic acid and chromic acid, several molecules of which frequently combine with one molecule of a base to form a salt. The chromates and vanadates are in fact so comparable in their general behaviour that the formula VO₃ was at one time assigned to vanadium pentoxide in harmony with CrO₃ for chromic anhydride. Vanadium pentoxide has the further well-pronounced property of combining with other acid oxides to form heteropoly-acids. The most common acid oxides are phosphorus pentoxide, arsenic pentoxide, molybdenum tri-oxide, tungsten trioxide, silica. The heteropoly-acids yield well-defined, erystallisable salts with basic oxides.