Niobium Atomic weight

An increase in the Me F ratio leads to an increase in the acidity of the initial solution, whereas the acidity of alkali metals increases according to their molecular weight, from Li to Cs. Therefore the additives of fluorides of alkali metals having higher atomic weight provide formation of complex fluorides with lower coordination number of tantalum or niobium. 

The addition of alkali metal or ammonium fluorides reduce the acidity of the system and shift the equilibrium between the two ions toward the formation ofNbOFs2 ions [60,61]. The shift depends on the alkalinity of the cation. The more alkaline the cation is (higher atomic weight), the stronger the shift toward NbOF52 ion formation. Fig. 48 shows typical Raman spectra of niobium-containing solutions before and after such additions were made. 

From 1904 to 1910 C. W. Bailee (7, 18, 55) analyzed many niobium and tantalum compounds and determined the atomic weights of both metals. In 1906 Werner von Bolton of the Siemens Halske Company... 

Jean-Charles Galissard de Marignac, 1817-1894. Swiss chemist who discovered ytterbia and gadolima and made many important contributions to the chemistry of the rare earths. Professor of chemistry at the University of Geneva. He made precise determinations of the atomic weights of many elements, and by separating tantahc and columbic (mobic) acids, proved that tantalum and columbium (niobium) are not identical. 

Hafnium had lain hidden for untold centuries, not because of its rarity but because of its dose similarity to zirconium (16), and when Professor von Hevesy examined some historic museum specimens of zirconium compounds which had been prepared by Julius Thomsen, C. F. Rammelsberg, A. E. Nordenskjold, J.-C. G. de Marignac, and other experts on the chemistry of zirconium, he found that they contained from 1 to 5 per cent of the new element (26, 27). The latter is far more abundant than silver or gold. Since the earlier chemists were unable to prepare zirconium compounds free from hafnium, the discovery of the new element necessitated a revision of the atomic weight of zirconium (24, 28). Some of the minerals were of nepheline syenitic and some of granitic origin (20). Hafnium and zirconium are so closely related chemically and so closely associated in the mineral realm that their separation is even more difficult than that of niobium (columbium) and tantalum (29). The ratio of hafnium to zirconium is not the same in all minerals. 

The pentoxides of vanadium, niobium, and tantalum react with hydrogen peroxide to produce per-acids of the general formula HR04. H20. These per-acids increase in stability with increase in atomic weight. Pertantalic acid is a white solid which can be heated to 100° C. without undergoing decomposition. The oxyfluorides of these metals also take up active oxygen to yield peroxyfluorides, which are much better defined in the case of niobium and tantalum than with vanadium. 

Marignac 3 also showed that previous methods for the separation of niobium and tantalum were far from perfect, and for the first time he succeeded in preparing pure niobium and tantalum compounds. His methods are still in use to some extent, and his analyses provided the first reliable values for the atomic weights of these elements. It should be stated, too, that Rose s earlier researches, which extended over a period of nearly twenty years, have provided a valuable source of information for the chemistry of niobium and tantalum.4 His calculations and formulae were revised by Rammelsberg 5 in the light of subsequent discoveries. 

Atomic Weight of Niobium.—The first determinations were carried out by Hermann 8 and Rose,4 but these are now only of historical interest. In 1864 Blomstrand5 analysed niobium pentachloride but obtained... 

In the following year Marignac1 published the results of about twenty very carefully conducted analyses of potassium niobium oxy-fluoride, 2KF.Nb0F3.H20 from his data the following figures for the atomic weight of niobium can be calculated 2... 

The fundamental values set out on p. viii of the General Introduction have been used. The same fundamental values have been employed in the recalculation of the subsequent values for the atomic weight of niobium mentioned in this section. 

The values for the atomic weight of niobium as determined by the various investigators since 1864 are summarised in the following table —... 

The pentavalent halides and oxyhalides, as in the case of other niobium compounds, are the most stable. It is remarkable that the pentavalency is maintained with increase in the atomic weight of the halogen. All the halogen compounds are characterised by their ready tendency to undergo hydrolysis on the addition of water or even in damp air with precipitation of niobie acid and formation of the hydrogen halide. Their preparation can, therefore, be effected only in the dry way (a) synthetically, or (b) by the action of chlorine, carbon tetrachloride, or sulphur monochloride on the oxide or sulphide. They do not possess saline properties, and cannot be prepared by the action of the halogen acids on the oxide. 

Ilmenium, by Hermann, 1846, in ilmenite and also accompanying niobium and tantalum in various minerals it was closely allied to them in its general characteristics and to it he ascribed an atomic weight of 104-6. Several years later he relinquished his claims, but brought them forward again in... 

Whilst engaged in writing his Principles of Chemistry from 1868, Mendeleeff sought for some system of classification of the elements, and that based on atomic weights seemed the most promising. He was acquainted with the previous work of Dumas, Lenssen, Pettenkofer, and Kremers, but not that of Strecker, de Chancourtois, or Newlands. He was influenced by the Karlsruhe Conference (see p. 489), which he attended, and by the recent work of Roscoe on vanadium and of Marignac on niobium. He says he had become convinced that ... 

Roscoe discovered tungsten pentachloride and pentabromide, uranium pentachloride,2 and niobium trichloride (which he found gave NbOClg and CO when heated in COg). He determined the vapour densities of PbClg and TlCl, showed that Delafontaine s supposed new rare-earth metal philippium is a mixture of yttrium and terbium, and redetermined the atomic weight of carbon (C = 12 002). Roscoe published several papers and a book on spectrum analysis. 

He made important contributions to the solution of the controversy over niobium and tantalum that will be described in Chapters 22 and 23. He also co-operated with Berzelius over atomic weight determinations and got the highest endorsement of the Swedish master for his acairate scientific work. 

The scrupulous Marignac conclusively proved that niobium and tantalum are different elements. He made the separation using the fact that potassium heptafluo-rotantalate K2TaE is much less soluble in dilute hydrofluoric acid than is pentafluo-rooxoniobate, K NbOE. After careful separation Marignac could determine the atomic weights of the two metals. 

---