Estimation of Niobium and Tantalum

The various methods in use for opening up the natural ores, and the separation of other metals from niobium and tantalum, have already been described in dealing with the extraction of these elements; similar methods apply to quantitative processes. The quantitative determination of either niobium or tantalum is best effected by converting the niobium or tantalum compound into the pentoxide and weighing as such. In the case of the fluorides, oxyfluorides, or double fluorides with the alkali metals, and the niobates and tantalates, the conversion is effected either by digesting with concentrated sulphuric acid or by fusing with potassium hydrogen sulphate; the residue, after extraction with water, is ignited in the presence of ammonium carbonate. In technical practice the product will usually contain both niobium pentoxide and tantalum pentoxide.

Gravimetric Methods

The mixture of pentoxides of niobium and tantalum is redissolved in concentrated hydrofluoric acid and separated by Marignac s process; the potassium tantalum fluoride and potassium niobium oxyfluoride are then separately converted into the pentoxides as described above, and weighed. This method has several disadvantages: (1) The ratio of the solubilities of the two compounds on which the separation is based is only approximately 10: 1, and the process is, therefore, necessarily inaccurate; even when the recrystallisation is repeated to a tedious extent the error approaches 1 per cent. (2) The concentration of the hydrofluoric acid and of the potassium fluoride must be carefully controlled; if the acidity is too low, an oxyfluoride of tantalum is precipitated; if the acidity is too great, a normal fluoride of niobium is obtained. (3) Several platinum dishes are necessary.

An entirely different method of separation, which avoids the disadvantages of Marignac's process, is based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly acid solution. The colour of the tantalum precipitate (sulphur yellow) is much paler than that of the niobium precipitate (vermilion). The presence of titanium interferes with the precipitation. Extraction of niobium pentoxide in a mixture of equal volumes of selenium oxychloride and concentrated sulphuric acid, in which tantalum pentoxide is insoluble, also provides a convenient quantitative separation. In another recent process the niobium pentoxide is determined in the presence of tantalum by reducing it to the dioxide, NbO₂, in a stream of hydrogen, and noting the gain in weight on reoxidising it in air at a red heat. A rough method for the estimation of niobium pentoxide and tantalum pentoxide depends on the considerable difference in their densities; the ordinary specific gravity bottle is used, and the composition of the mixture ascertained by reference to a table.

The analysis of ferrotantalum alloys and of tantalum steels also involves the conversion of the tantalum present into the pentoxide. The material is dissolved in hydrofluoric acid and nitric acid, evaporated to dryness, and the residue fused with potassium hydrogen sulphate; extraction with dilute hydrochloric acid and hydrolysis yield a precipitate of hydrated tantalic pentoxide, the iron remaining in solution.

Cupferron can be employed for the estimation of niobium and tantalum together, but does not differentiate between them; any titanium present is also simultaneously precipitated.

Volumetric Methods

Pentavalent niobium compounds differ from pentavalent tantalum compounds in that they can be reduced by nascent hydrogen in hot acid solution approximately to the trivalent state, and can then be titrated back with potassium permanganate solution, the tantalum compound remaining unaltered. The final stage of the reduction has been variously reported as being equivalent to Nb₈O₁₃, Nb₁₀O₁₇, Nb₂O_3.107, and appears to depend on the quality of the reagents, the degree of dispersion of the niobic acid, and on other conditions concerning which insufficient is known at present to render the method accurate. Treadwell recommends reduction with cadmium amalgam in the presence of ammonium vanadate, ammonium molybdate, or titanium sulphate, and electrometric titration of the trivalent solution so obtained with potassium permanganate.

Colorimetric Method

A colorimetric method for the estimation of small quantities of niobium in tantalum compounds has been worked out by Meimberg. This takes advantage of the colour change that is produced when a pentavalent niobium salt in acid solution is reduced with tin; a tantalum salt remains unaffected under the same conditions.

The estimation of small quantities of tantalum in niobium compounds is more difficult, and cannot be carried out colorimetrically. The usual method is to convert the material into the potassium double fluoride, and then to take advantage of the fact that a white precipitate of potassium tantalum oxyfluoride, K₄Ta₄O₅F₁₄ , is thrown down when a solution of potassium tantalum fluoride, K₂TaF₇, is boiled. Powell and Schoeller find this test imperfect, and have modified the procedure (based on the differential hydrolytic dissociation of oxalo-niobic acid and oxalo-tantalic acid in the presence of tannin in slightly acid solution) for the detection and estimation of traces of tantalum in niobium compounds.

The determination of tantalum by ordinary methods of spectroscopy and by means of X-ray spectra also appears to be possible.