Note 50 — Chapter XIX · PHOSPHORUS AND THE OTHER ELEMENTS OF THE FIFTH GROUP

Strong acids and alkalis dissolve vanadic anhydride in considerable quantities, forming yellow solutions. When it is ignited, especially in a current of hydrogen, it evolves oxygen and forms the lower oxides; V2O4 (acid solutions of a green colour, like the salts of chromic oxide), V2O3, and the lowest oxide, VO. The latter is the metallic powder which is obtained when the vanadium oxychloride is heated in an excess of hydrogen, and was formerly mistaken for metallic vanadium. When a solution of vanadic acid is treated with metallic zinc it forms a blue solution, which seems to contain this oxide. It acts as a reducing agent (and forms a close analogue to chromous oxide, CrO). Metallic vanadium can only be obtained from vanadium chloride which is quite free from oxygen. Moissan (1893) obtained it by reducing the oxide with carbon in the electric furnace, and considered it to be most infusible of the metals in the series Pt, Cr, Mo, U, W, and V (he also obtained a compound of vanadium and carbon). The specific gravity of this metal is 5·5. It is of a grey-white colour, is not decomposed by water, and is not oxidised in air, but burns when strongly heated, and can be fused in a current of hydrogen (forming perhaps a compound with hydrogen). It is insoluble in hydrochloric acid, but easily dissolves in nitric acid, and when fused with caustic soda it forms sodium vanadate.

As regards the salts of vanadic acid, three different classes are known; the first correspond with metavanadic acid, VMO₃ = M₂OV₂O₅, the second correspond with the dichromates—that is, have the composition V₄M₂O₁₁, which is equal to M₂O + 2V₂O₅—and the third correspond with orthovanadic acid, VM₃O₄ or 3M₂O + V₂O₅. The latter are formed when vanadic anhydride is fused with an excess of an alkaline carbonate.

Vanadic acid gives the so-called ‘complex’ acids (which are considered more fully in Chapter XXI. in speaking of Mo and W)—i.e. acids formed of two acids assimilated into one. Thus Friedheim (1890) obtained phosphor-vanadic acid, and Schmitz-Dumont (1890) a similar arseno-vanadic acid. The former is obtained by heating V₂O₅ with sirupy phosphoric acid. The resultant golden-yellow tabular crystals have the composition H₂OV₂O₅P₂O₅9H₂O, and there are corresponding salts—for example, (NH₄)₂V₂O₅P₂O₅ with 3 and 7H₂O, &c. These salts cannot be separated by crystallisation, so that there are ‘complexes’ of these acids in a whole series of salts (and also in nature). It may be supposed (Friedheim) that V₂O₅ here, as it were, plays the part of a base, or that those acids may be looked upon as double salts. Among the true double salts of vanadium (Nb and Ta) very many are known among the fluorides, such as VF₃2NH₄F, VOF₂2NH₄F, VO₂F,3NH₄F, &c. (Pettersson, Piccini, and Georgi, 1890–92).

Vanadium was discovered at the beginning of this century by Del-Rio, and afterwards investigated by Sefström, but it was only in 1868 that Roscoe established the above formulæ of the vanadic compounds.