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Detection of Niobium and Tantalum

The detection of niobium and tantalum depends almost entirely on the reactions given by niobic acid and tantalic acid. All the common niobium and tantalum compounds are hydrolysed on being boiled in acid solutions, and yield precipitates of the respective acids. Natural minerals are previously fused with potassium hydrogen sulphate, and the aqueous extract of the melt usually precipitates the mixed acids spontaneously.

Wet Reactions

  1. Both niobic acid and tantalic acid dissolve readily in hydrofluoric acid, but only very slightly in concentrated hydrochloric acid and in hot concentrated sulphuric acid. The residue from the hydrochloric acid solution of niobic acid, however, readily forms a hydrosol on being triturated with water. The sulphuric acid solution of niobic acid remains clear on being diluted with water, whereas the sulphuric acid solution of tantalic acid becomes turbid on being diluted, and reprecipitates the acid.
  2. The hydrofluoric acid solution of niobic acid does not yield a precipitate on the addition of potassium fluoride (potassium niobium fluoride, K2NbF7, which is formed, being soluble in about 12.5 parts of water), whereas the hydrofluoric acid solution of tantalic acid yields colourless, rhombic needles of potassium tantalum fluoride, K2TaF7 (which is soluble in about 150 parts of water under the same conditions), when treated with a saturated solution of potassium fluoride, carefully evaporated and cooled slowly. After removal of the tantalum, and with further concentration, any niobium present separates in plates of potassium niobium oxyfluoride, K2NbOF5.H2O, if the hydrofluoric acid is not in excess, and in needles of potassium niobium fluoride, K2NbF7, if the hydrofluoric acid is in excess.

    The potassium tantalum fluoride first precipitated is a fusible substance. Its aqueous solutions on being boiled precipitate a very insoluble potassium tantalum oxyfluoride, 4KF.Ta2O5.2TaF5 or K4Ta4O5F14, as a white powder. This reaction is stated to constitute a sensitive test for tantalum.
  3. Hydrochloric acid solutions of pentavalent niobium compounds which are free from hydrofluoric acid, on being reduced with zinc, first become blue, and with further action of the reducing agent, olive-green or dark brown, according to the concentration of the acid and other conditions. The blue stage is not difficult to attain; the brown stage is best attained by passing the solution through a zinc reductor several times. The reduced solution precipitates white mercurous chloride from solutions of mercuric chloride. This reaction is given by 1 mgm. of niobium.

    Tantalum compounds in solution do not give a colour change on being reduced with zinc, and this test also serves to establish niobium in the presence of titanium, which produces a violet coloration. Vanadium, molybdenum, and tungsten solutions, however, behave similarly to niobium, and these metals must, therefore, be previously removed.
  4. When a solution of niobic acid in concentrated hydrochloric acid (2:1) is boiled with tin for some time, a deep sapphire-blue coloration is obtained, which fades on standing and is regenerated by boiling.
  5. Addition of excess of potassium thiocyanate to an alkaline solution of a niobate, followed by zinc and concentrated hydrochloric acid, produces a golden-brown colour which may be almost red in the presence of larger quantities of niobium. It is stated that neither tantalum nor titanium gives any coloration under the same conditions. Addition of potassium thiocyanate to a hydrochloric acid solution of tantalic acid or niobic acid gives a colourless solution.
  6. Potassium ferrocyanide yields a yellow or reddish-brown precipitate with a hot solution of tantalic acid in hydrochloric acid; niobic acid gives a reddish-brown or greyish-green precipitate.
  7. Tannin produces an orange-red or chocolate-red precipitate with an acid solution of niobic acid, and a yellow or light brown precipitate with acid solutions of tantalic acid. Pyrogallol and other polyhydroxy derivatives of benzene behave similarly.
  8. Addition of ammonium hydroxide or ammonium sulphide to solutions of niobic acid and tantalic acid in mineral acids reprecipitates the niobic and tantalic acids, which may, however, retain some of the ammonia. This test does not distinguish between niobium and tantalum, and it does not proceed in the presence of tartaric acid.

Dry Reactions

When heated in the reducing flame a bead of microcosmic salt assumes a blue, violet, or brown colour with increasing quantities of niobic acid; the heated bead becomes red on the addition of ferrous sulphate. With tantalic acid under these conditions the bead remains colourless.

Borax beads do not produce colorations either with niobium or tantalum.

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