XVIII. Researches on vanadium.—Part III

Abstract

Listen

Translate article

I. Metallic vanadium. In the second Part of my “Researches on Vanadium,” communicated to the Royal Society (Phil. Trans. 1869, p. 691), I stated that metallic vanadium absorbs hydrogen. This conclusion has been fully borne out by subsequent experiment; and it appears that the amount of absorbed or combined hydrogen taken up by the metal varies according to the state of division, first, of the chloride (VCl 2 ) from which the metal is prepared, and secondly, and especially, of the metallic powder itself. The metal containing absorbed hydrogen on exposure to dry air slowly takes up oxygen, water being formed and the metal undergoing oxidation to a substance which resembles the metal in its appearance, but possesses a darker grey colour, and has a less brilliant metallic lustre than vanadium itself. At this point the oxidation stops, although in moist air it proceeds still further. Thus a portion of pure dichloride was reduced in hydrogen; of the reduced substance, free from chlorine, 0·2666 grm. yielded on complete oxidation 0·4441 of V 2 O 5 , corresponding to a percentage of 93·6 of pure metal. On exposure to the air for some days this substance absorbed oxygen, losing its brilliant metallic lustre; and when burnt in a current of dry oxygen, water was given off, thus:— (1) 0·4232 grm. gave 0·0502 grm. of water and 0·6615 grm. V 2 O 5 , (2) 0·2695 „ „ 0·0315 „ „ 0·414 „ „ or (1) gives 87·8 p c. vanadium; 1·3 p. c. hydrogen; 10·9 p. c. oxygen. (2) „ 86·7 p. c. vanadium; 1·3 p. c. hydrogen; 12·0 p. c. oxygen. The difficulty of obtaining metallic vanadium perfectly free from admixture of oxide was again rendered evident. Pure tetrachloride was prepared in quantity, and from this the dichloride was made. On heating this dichloride to whiteness for forty-eight hours a substance was obtained which gained on oxidation 70·7 per cent., and therefore still contained a slight admixture of oxide. The reducing action of sodium on the solid chlorides was next examined; in this case the reduction takes place rapidly but quietly in an atmosphere of hydrogen at a red heat, and may be best conducted in strong iron tubes proved air-tight under hydraulic pressure of 200 lbs. on the square inch. Explosions occur when the tetrachloride is heated with sodium. The substance thus obtained by the action of sodium was found on lixiviation to be free from chlorine, and on washing it was found to separate into two parts—(1) a light and finely divided black powder (trioxide), soluble in hydrochloric acid, which remains in suspension, and (2) a heavier grey powder (insoluble in hydrochloric acid), which is deposited, and which by repeated washing may be entirely freed from the lighter trioxide. This bright grey powder consists of metallic vanadium mixed with more or less oxide. If the finely divided metallic powder, after drying in vacuo , be reduced at a low red heat in a current of pure hydrogen, it takes fire spontaneously, when cold, on exposure to air or oxygen, a distinct flame being seen playing on the surface whilst water is formed. In one experiment a product thus prepared contained 91·1 per cent. of metallic vanadium (0·354 substance gave 0·574 grm. V 2 O 5 ). This substance, exposed for some weeks to dry air, slowly absorbed oxygen, and on roasting gave a percentage increase of 53·75 (0·453 grm. yielded 0·6965 V 2 O 5 ), whilst 0·034 grm. or 7·5 per cent. of water was at the same time formed. This shows that the point of oxidation at which the metal containing hydrogen becomes stable in dry air nearly corresponds to the oxide V 2 O.