Tantalum

Tantalum [[[symbol]] Ta, atomic weight 181o (0=16)], a metallic chemical element, sparingly distributed in nature and then almost invariably associated with columbium. Its history is intermixed with that of columbium. In 1801 C. Hatchett detected a new element, which he named columbium, in a mineral from Massachusetts, and in 1802 A. G. Ekeberg discovered an element, tantalum, in some Swedish yttrium minerals. In 1809 W. H. Wollaston unsuccessfully endeavoured to show that columbium and tantalum were identical. In 1844 H. Rose detected two new elements in the columbites of the Bodenmais, which he named niobium and pelopium; dianium was discovered by W. X. F. von Kobell in various columbites; and ilmenium  and neptunium were discovered by R. Hermann. The researches of C. W. Blomstrand, and others, especially of Marignac, proved the identity of columbium, dianium and niobium, and that ilmenium was a mixture of columbium and tantalum. It is very probable that neptunium is a similar mixture. Berzelius, who prepared tantalic acid from the mineral tantalite in 1820, obtained an impure metal by heating potassium tantalofluoride with potassium. In 1902 H. Moissan obtained a carbon-bearing metal by fusing the pentoxide with carbon in the electric furnace. The preparation of the pure metal was successfully effected by Werner von Bolton in 1905, who fused the compressed product obtained in the Berzelius process in the electric furnace, air being excluded. An alternative method consisted in passing an electric current through a filament of the tetroxide in a vacuum. The metal is manufactured, for use as filaments in electric lamps, by the action of sodium on sodium tantalofluoride.

The pure metal is silver-white in colour, is very ductile, and becomes remarkably hard when hammered, a diamond drill making little impression upon it. Its tensile strength is higher than that of steel. It melts between 2250° and 2300°, its specific heat is 0.0365, coefficient of expansion o0000079, and specific gravity 16.64. When heated in air the metal burns if in the form of thin wire, and is superficially oxidized if more compact. At a red heat it absorbs large volumes of hydrogen and nitrogen, the last traces of which can only be removed by fusion in the electric furnace. These substances, and also carbon, sulphur, selenium and tellurium, render the metal very brittle. Tantalum is not affected by alkaline solutions, but is disintegrated when fused with potash. Hydrofluoric acid is the only acid which attacks it. It alloys with iron, molybdenum and tungsten, but not with silver or mercury.

In its chemical relationships tantalum is associated with vanadium, columbium and didymium in a sub-group of the periodic classification. In general it is pentavalent, but divalent compounds are known.

Tantalum tetroxide, Ta 2 0 4, is a porous dark grey mass harder than glass, and is obtained by reducing the pentoxide with magnesium. It is unaffected by any acid or mixture of acids, but burns to the pentoxide when heated.

Tantalum pentoxide, Ta205, is a white amorphous infusible powder, or it may be crystallized by strongly heating, or by fusing with boron trioxide or microcosmic salt. It is insoluble in all acids. It is obtained from potassium tantalofluoride by heating with sulphuric acid to 400°, boiling out with water, and decomposing the residual compound of the oxide and sulphuric acid by ignition, preferably with the addition of ammonium carbonate.

Tantalic acid, HTa03,, is a gelatinous mass obtained by mixing the chloride with water. It gives rise to salts, termed the tantalates. The normal salts are all insoluble in water; the complex acid, hexatantalic acid, H $ Ta 6 0, 9 (which does not exist in the free state), forms soluble salts with the alkaline metals. Pertantalic acid, HTaO 4, is obtained in the hydrated form as a white precipitate by adding sulphuric acid to potassium pertantalate, K 3 Ta0 5. ZH 2 O, which is formed when hydrogen peroxide is added to a solution of potassium hexatantalate.

Tantalum pentafluoride, TaF5, for a long time only known in solution, may be obtained by passing fluorine over an alloy of tantalum and aluminium, and purifying by distillation in a vacuum. It forms colourless, very hygroscopic prisms, which attack glass, slowly at ordinary temperatures, more rapidly when heated (Ber., 1909, 4 2, p. 49 2). Its double salts with the alkaline fluorides are very important, and serve for the separation of the metal from columbium and titanium. Tantalum pentachloride, TaC1 5, is obtained as light yellow needles by heating a mixture of the pentoxide and carbon in a current of chlorine. By heating with sodium amalgam and separating with hydrochloric acid, the dichloride, TaC1 2.2H 2 O, is obtained as emerald green hexagonal crystals. The pentabromide exists, but tantalum and iodine apparently do not combine. Tantalum forms a sulphide, TaS 2, and two nitrides, TaN 2 and Ta3N5, have been described.

Marignac determined the atomic weight to be 181, but Henrichsen and N. Sahlbom (Ber., 1906, 39, p. 2600) obtained 179.8 (H =1) by converting the metal into pentoxide at a dull red heat.