The three elements, vanadium, niobium, and tantalum resemble those of the nitrogen group in that they chiefly form compounds belonging to the pentavalent type, and that their higher oxygen compounds have an acid character. In the elementary state they have the properties of metals; their halogen compounds are readily volatile. All three are only sparingly found in the earth's crust, although the first mentioned, vanadium, occurs in small quantities widely distributed.
Vanadium is obtained from the mixtures containing it by fusing these with soda and saltpetre. It is thereby converted into soluble sodium vanadate, and can be extracted with water. After the admixtures have been removed as far as possible, pieces of solid ammonium chloride are placed in the liquid. Ammonium vanadate is then formed, which is practically insoluble in the concentrated solution of ammonium chloride, and separates out as a crystalline powder. By heating the ammonium salt in the air vanadium pentoxide, V2O5, the anhydride of vanadic acid, is obtained as a yellow or brown powder, which dissolves in water with a red colour. The solution appears, however, to be essentially of a colloidal nature, as it is precipitated by neutral salts.
Various acids can be derived from vanadium pentoxide. Metavanadic acid, HVO3, in the form of its salts, is the best known. The above-mentioned ammonium salt is a metavanadate (NH4)VO3. Ortho- and pyro-vanadates also exist. Besides these acids, " condensed " acids, containing several combining weights of vanadium, are readily formed. Thus, for example, the salts of hexavanadic acid, H2V6O16, are known. They are formed on acidifying the simple vanadates, and are of a yellow-red to dark red colour, while the simple vanadates are white, or sometimes yellow. Little is known, however, of the conditions of formation and of mutual transformation of these various forms.
Vanadium pentoxide is capable also of uniting with strong acids to form salt-like compounds, hydroxyl being split off instead of the acid hydrogen. Such compounds, more especially with sulphuric acid, are known even in the solid state.
By reducing the pentoxide with hydrogen, or with charcoal at a high temperature, vanadium trioxide, V2O3, is obtained as a grey-black powder with metallic lustre. This was formerly regarded as metallic vanadium, since, besides having a metallic lustre, it is also a good conductor of electricity. It dissolves in acids to form dark-green salts, containing the trivalent, green trivanadion V•••, and which are also obtained by reducing acid solutions of the pentoxide with zinc.
Besides these two oxides, the compounds V2O, V2O2, V2O4, and some intermediate compounds have also been prepared. They have all a metallic appearance. The dioxide dissolves in dilute acids to form blue liquids, which evolve hydrogen, and have strong reducing properties. They contain, presumably, a divalent, violet blue divanadion V••.
The compounds with the halogens, especially with chlorine, exhibit as great variety as the oxygen compounds. Strange to say, a pentachloride, which would be expected, corresponding to the pentoxide, does not exist; the highest chloride stage is the tetrachloride, VCl4. An oxychloride, however, viz. vanadyl chloride, VOCl3(VO = vanadyl), belonging to the pentavalent type, is known. It is obtained by first passing hydrogen and then chlorine over a heated mixture of vanadium pentoxide and charcoal. It is a bright yellow liquid, boiling at 127°, which reacts with water with great rise of temperature, and fumes in the air. From this VOCl2 and VOCl are obtained by reduction with hydrogen; they are both solid, crystalline substances, the former being green, the latter brown.
If a mixture of vanadyl trichloride vapour and chlorine is passed over red-hot charcoal, the tetrachloride, VCl4, is obtained as a brown liquid, boiling at 154°. On being more strongly heated it decomposes into chlorine and vanadium trichloride, VCl3, which forms lustrous, violet-red crystals, which recall chromic chloride. They attract moisture from the air and deliquesce to a brown liquid. On heating the vapour with hydrogen the tetrachloride is converted into vanadium dichloride, VCl2. This forms apple-green, difficultly volatile crystals with a micaceous lustre, which deliquesce in the air to a violet-blue liquid.
Finally, on strongly heating the dichloride in a current of hydrogen, metallic vanadium is obtained as an unmelted, grey mass, which acquires a metallic lustre on being rubbed, and does not dissolve in dilute acid. It burns readily in a current of nitrogen, forming vanadium nitride, VN, a yellow-brown powder with a metallic lustre. On fusion with caustic potash the nitride is converted into vanadic acid with evolution of ammonia.
On passing sulphuretted hydrogen into a solution of ammonium vanadate in ammonia a precipitate is produced which, on continuing to pass the gas, dissolves, forming a fine, violet-red coloured liquid.
From this solid ammonium thiovanadate, resembling potassium permanganate in appearance, crystallises out. The salt has the composition (NH4)3VS4, and therefore belongs to the ortho series. On adding acids, sulphuretted hydrogen is evolved, and a brown precipitate is formed, which, however, does not appear to be pure vanadium penta-sulphide. The latter is obtained as a black powder by fusing the trisulphide with sulphur. The trisulphide, in its turn, is obtained by heating the pentoxide in a current of sulphuretted hydrogen, or, better, of carbon disulphide. It is grey-black in colour, and dissolves in alkali sulphides, especially in such as contain excess of sulphur, forming a red-violet solution of thio vanadate.
Vanadic acid has the property of catalytically accelerating certain oxidation processes (e.g. the oxidation of aniline to aniline black, with sodium chlorate), and is therefore employed for that purpose in the arts and manufactures. Even very small quantities of the acid are sufficient to effect a great acceleration. |