320 
PROFESSOR ROSCOE’S RESEARCHES ON VANADIUM. 
3. Vanadium Tribromide , VBr 3 , molec. wt. =29T3. — This body condenses as a 
greyish-black opake amorphous sublimate, when dry bromine vapour is passed in excess 
over vanadium nitride heated to redness. Brown vapours are given off, which soon con- 
dense in the cooler portions of the tube. The tribromide is a very unstable compound, 
losing bromine even at the ordinary temperature in dry air, and being converted into 
V 2 0 3 when gently heated. It deliquesces rapidly on exposure to moist air, giving rise 
to a brown-coloured liquid, in this respect resembling the trichloride, but on addition of 
a few drops of hydrochloric acid the brown liquid changes to the green colour charac- 
teristic of a solution of a vanadous salt (V 2 0 3 ). No free bromine is evolved when the 
tribromide dissolves in water. In order to prepare the tribromide, pure nitride of vana- 
dium, contained in a porcelain boat, was introduced into a combustion-tube, and after 
all the air had been displaced by dry carbonic acid, the part of the tube containing the 
nitride was heated to redness, the other part of the tube being kept at such a tem- 
perature as to volatilize any excess of bromine which might pass over. After all the 
nitride had burnt away, the bulb containing the bromine was sealed off, and a current of 
dry carbonic acid passed over the solid bromide to displace all traces of free bromine, 
A second method of preparing the tribromide is to pass bromine vapour over a mixture 
of vanadium trioxide and charcoal ; in this reaction the oxytribromide is first formed, 
then the oxydibromide, and lastly, the tribromide, VBr 3 ; but this plan is not to be 
recommended, as the tube soon becomes stopped up by the formation of these solid com- 
pounds. The bromide thus prepared was not analyzed, but it presented exactly the same 
appearance and properties as the tribromide obtained by the first method. 
No higher compound of bromine and vanadium than the tribromide could be obtained. 
The volatile liquid passing into the bulb in the first preparation was carefully rectified, 
and it was all found to distil over at the boiling-point of bromine, leaving only a small 
quantity of the tribromide in the bulb. Some difficulty was experienced in obtaining 
satisfactory analytical results with the tribromide, owing to the fact, already observed by 
Stas*, that bromide of silver, when boiled with excess of nitrate of silver, carries down with 
it some of this latter salt inclosed in the bromide, and that this impurity cannot be got rid 
of by washing. Owing to this admixture of nitrate of silver the bromine determinations 
usually come out about two per cent, too high, whilst the vanadium determinations 
gave constant numbers, agreeing much more nearly with the calculated results. Thus 
in four analyses of the tribromide prepared on different occasions the mean percentage 
of bromine was found to be 84T5, the calculated percentage being 82-4; whilst the 
vanadium determinations of the same portions gave 18-57 per cent, instead of 17*6 per 
cent. In order to lessen as much as possible this error, the precipitated bromide of 
silver was reduced in hydrogen until no further diminution of weight occurred, and the 
percentage of bromine calculated from this loss. 
* Stas, Recherches sur les Lois des Proportions chimiques, p. 156. 
