Vanadium exhibits five stages of valency, each stage giving rise to characteristic colours. An acid solution of vanadium pent oxide, is yellow to orange ; mild reducing agents convert to the V204 stage which is characterised by a deep blue colour. Further reduction of leads to a green solution, now con taining hydrated trichloride, corresponding with the stage. Drastic reduction gives rise to a lavender solution of VCI2 corresponding with the VO stage. A lower oxide, is de scribed but is unimportant.
Vanadium pentoxide, made by igniting ammonium metavanadate, crystallises in yellowish-red prisms spar ingly soluble in water but dissolving in aqueous alkalis to vana dates and in acids to vanadic salts such as Meta vanadic acid, is obtained as a brown powder by boiling copper vanadate with nitric acid, and when produced from this vanadate by digestion with ammonium chloride it separates in gold spangles known as "vanadium bronze" and employed as a pigment. Vanadic acid and its oxide enter into many complex combinations :—polyvanadates, phosphovanadates, arsenovana dates, molybdovanadates and tungstovanadates. Vanadyl chloride, a yellow liquid boiling at 127° C, is prepared by passing chlorine over heated vanadic oxide, but when carbon or sulphur is present vanadium tetrachloride, is also obtained as a reddish brown liquid boiling at 554° C. Vanadium tetroxide, is a blue
infusible substance obtained by mild reduction of the pentoxide ; it dissolves in acids to yield blue vanadyl salts such as and and in aqueous caustic alkalis to brown solutions of the alkali vanadates (also termed hypovanadates) ; the potassium salt is Vanadium trioxide, a black infusible powder produced by reducing the heated pentoxide in hydrogen, dissolves slowly in hydrochloric acid to yield vanadium trichloride, the green solu tion of which is more readily prepared by reducing vanadic oxide with magnesium and hydrochloric acid. Anhydrous vanadium trichloride, prepared by heating vanadium pentoxide in chlorine and sulphur chloride, forms bluish-red crystals resem bling anhydrous chromic chloride.
Vanadium trisulphate, V2(SO4)3, a crystalline yellow powder, combines with sulphates of ammonium, potassium, rubidium and caesium • to form vanadium alums. (See ALUMS.) Vanadous oxide, VO, a grey powder having metallic lustre and metallic conductivity, is obtained by reducing the higher oxides with potassium. It was erroneously regarded as a metal by Berzelius and shown to be an oxide by Roscoe. The corresponding vanadous chloride, VC12, produced by reducing the tetrachloride with hy drogen, forms light green crystals dissolving readily to a lavender solution.
Vanadium acetylacetone, V{ CH(CO.CH3)2}3, obtained by the interaction of vanadium trichloride, acetylacetone and aqueous sodium carbonate, crystallises from chloroform in dark brown tabular prisms melting at 185-190° C, and on exposure to moist air undergoes oxidation to bluish-green vanadyl acetylacetone, VO{ also prepared by dissolving vanadyl hy droxide, VO(OH)2, in alcoholic acetylacetone (G. T. Morgan and H. W. Moss, 1913).
Vanadium forms three sulphides, V2S5, V2S3 and VS, the most stable of which is the trisulphide. The nitride, VN, produced by direct combination of its constituents, forms a metallic powder formerly confused with the metal itself. The carbide, VC, an electric furnace product, forms hard silvery white crystals melt ing at 2750° C.