Atomic Weight of Niobium

The first determinations were carried out by Hermann and Rose, but these are now only of historical interest. In 1864 Blomstrand analysed niobium pentachloride but obtained discordant figures. They were all higher than the atomic weight of molybdenum, the next higher element in the periodic system, and it seems certain that Blomstrand's material was not free from tantalum.

In the following year Marignac published the results of about twenty very carefully conducted analyses of potassium niobium oxyfluoride, 2KF.NbOF₃.H₂O; from his data the following figures for the atomic weight of niobium can be calculated:

1. = 100/44.36; hence Nb=93.37.
2. = 100/57.82; hence Nb=94.10.
3. = 100/57.82; hence Nb = 93.70.

Marignac's material contained traces of titanium, and he therefore assumed that the higher limit would most probably be the more correct, and accordingly suggested an atomic weight of 94. This figure was confirmed by Marignac's analyses of niobium pentachloride and was accepted until 1908, when Balke and Smith redetermined the ratio

2NbCl₅/Nb₂O₅,

using material of more reliable purity. The pentachloride was decomposed by water with the aid of a small quantity of nitric acid, and the oxide so produced was ignited and weighed. The mean of nine experiments gave the ratio

2NbCl₅/Nb₂O₅ = 1/49.305

This gives an atomic weight of 93.52, which is in harmony with the vapour-density determinations of niobium pentachloride and niobium oxychloride carried out previously by Deville and Troost. In 1915 Smith and Van Haagen criticised the foregoing method on the following grounds: (a) The pentachloride used may have contained traces of oxychloride; (b) the residual oxide may have retained traces of chlorine; (c) slight loss of niobium may have occurred, because niobium pentoxide is volatile in hydrogen chloride. These investigators obtained an appreciably lower figure, using a totally different method. Pure sodium metaniobate was decomposed by sulphur monochloride vapour and the residue of sodium chloride was weighed, the niobium being expelled either as chloride or oxychloride. Seven experiments gave the mean ratio

NaNbO₃/Na = 2.80759; whence Nb = 93.12.

The value 93.1 was adopted in 1916 by the International Committee on Atomic Weights; this was altered to 93.3 in 1929.