Atomic Weight of Vanadium

The early investigations into the atomic weight of vanadium provide a very interesting example of the application of Mitscherlich's Law of Isomorphism. Berzelius in 1831 obtained the value 68.5 by (a) reducing vanadic acid in hydrogen at a red heat, (b) reconverting the reduced oxide to vanadic acid. Berzelius assumed in his calculations that the formula for vanadic acid was VO₃. In 1868 Roscoe pointed out that the following minerals had been shown by Rammelsberg to be isomorphous, but were not at that time represented by analogous formulae:

1. Vanadinite, 3Pb₃(VO₃)₂.PbCl₂.
2. Mimetesite, 3Pb₃(AsO₄)₂.PbCl₂.
3. Pyromorphite, 3Pb₃(PO₄)₂.PbCl₂.

If the Law of Isomorphism held good in this instance, the formula for vanadinite should be similar to those written down for the other minerals. The formula for vanadic anhydride should thus be V₂O₅, corresponding to P₂O₅ and As₂O₅ for phosphoric and arsenic anhydride respectively, and not VO₃ as was supposed by Berzelius. Roscoe was able to show that the anhydride was correctly represented as V₂O₅, and that the substance regarded by Berzelius as metallic vanadium was in fact an oxide, VO. The formula for vanadinite thus becomes 3Pb₃(VO₄)₂.PbCl₂, and the atomic weight of vanadium as originally determined becomes 68.5 – 16.0 or 52.5. Even this figure is incorrect, because the materials employed were not pure.

Roscoe carried out the first reliable determinations of the atomic weight of vanadium. He used three methods:

1. Reduction of vanadium pentoxide to vanadium trioxide by means of hydrogen. From the ratio V₂O₅: V₂O₃, the atomic weight of vanadium was found to be 51.382.
2. Vanadium oxytrichloride was treated with silver nitrate solution volumetrically until all the chlorine was removed. From the ratio VOCl₃:3Ag, the atomic weight of vanadium was 51.055.
3. Vanadium oxytrichloride was treated with silver nitrate, and the precipitated silver chloride was collected, dried, and weighed. From the ratio VOCl₃:3AgCl, the atomic weight of vanadium was 51.259.

No further investigations were carried out until 1909, over forty years later, when Prandtl and Bleyer repeated Methods (a) and (c) of Roscoe, but took care to avoid several sources of error. Method (a) gave the value 51.356, but the investigators found that the vanadium trioxide obtained on reduction took up oxygen again so readily that its weight could not be obtained with certainty. The value obtained was therefore too high. Method (c) gave the value 51.074.

In 1914 Briscoe and Little again analysed vanadium oxytrichloride, using Methods (b) and (c). The ratio VOCl₃:3Ag was studied in detail and gave an atomic weight of 50.950; two measurements of the ratio VOCl₃:3AgCl gave an atomic weight of 50.952. The investigators state, in reviewing the possible sources of error, that the figures are likely to prove a trifle low. Hence the atomic weight of vanadium lies between 50.95 and 50.96, and the higher number is most probably the more correct. This value is in good agreement with a determination carried out in 1910 by McAdam, who employed a totally different reaction. A known weight of anhydrous sodium metavanadate was heated in a stream of hydrogen chloride gas and chlorine; the residual sodium chloride was fused and weighed. From the ratio NaVO₃:NaCl an atomic weight of 50.960 was obtained.

The International Committee on Atomic Weights adopted the value 51.0 in 1911. This was altered to 50.96 in 1925 and to 50.95 in 1929.

The atomic number of vanadium is 23.