Chemical Properties Tantalum

Tantalum is not affected by air or moisture at ordinary temperatures and does not "rust." When the metal is heated in the form of sheet or thick wire in air it becomes yellow at about 400° C., and with increasing temperature blue, and finally black. Above a dull red heat a film of the white pentoxide is produced, which to a large extent prevents further oxidation. Very thin tantalum wire can, however, be ignited in air by a match. In oxygen tantalum wire glows without flame at a white heat, and yields the pentoxide if the pressure of oxygen is greater than 20 mm. The reaction,

4Ta + 5O₂ ⇔ 2Ta₂O₅

appears to be reversible, and proceeds completely from right to left in vacuo at high temperatures; this enables pure tantalum metal to be produced directly from the pentoxide. Tantalum absorbs large volumes of hydrogen when heated in the gas, and yields a brittle product even when the amount of hydrogen present is less than 0.1 per cent.; the absorbed gas is completely removed by fusion in a good vacuum. Tantalum also absorbs nitrogen and, in minute quantities, helium and argon. It reacts slowly with sulphur and selenium, with probable formation of the sulphide and selenide; hydrogen sulphide is without action at 600° C. Tantalum is attacked readily by fluorine, and burns when gently heated in chlorine, the pentahalide being formed in each case; it can be made to combine with bromine, but does not react with iodine. Solutions of chlorine are also without action, but carbonyl chloride attacks the metal. The red-hot powder decomposes water with liberation of hydrogen.

Tantalum is remarkably resistant to corrosion by acids, and is, in fact, referred to as a "noble " metal. It is not attacked by hydrochloric acid, nitric acid or aqua-regia, whether hot or cold, dilute or concentrated; it is not attacked by hot dilute sulphuric acid, but boiling concentrated sulphuric acid dissolves it slowly. It dissolves in hydrofluoric acid, however, although when both metal and acid are very pure, solution takes place only very slowly. A mixture of hydrofluoric acid and nitric acid attacks the metal rapidly, and in contact with platinum or carbon it is readily dissolved by hydrofluoric acid with evolution of hydrogen. Tantalum excellently withstands exposure to sea air, sea-water, sulphur dioxide, and mine effluents.

The vapours of the alkali metals are without action even at 2000° C. Boiling solutions of the alkalis attack tantalum slowly; fusion with caustic potash in air yields a tantalate.

As in the case of niobium, the only well-defined tantalum compounds are those derived from the pentoxide, namely, the tantalates. Tantalum compounds display a much feebler tendency to undergo reduction than niobium compounds, and this is shown in the fact that only two oxides, Ta₂O₅ and TaO₂, are known, and even the existence of the latter has recently been questioned. When niobium pentoxide is heated to redness in hydrogen at 1250° C. reduction to the sesquioxide, Nb₂O₃, ensues, but under similar conditions tantalum pentoxide remains unchanged. Acid solutions of pentavalent niobium salts also undergo reduction with nascent hydrogen, whereas pentavalent tantalum salts are unaffected. It is. of some interest to note, however, that evidence for the existence of a dichloride has recently been obtained.

Tantalum pentoxide possesses only very weakly acidic properties. Its salts even with the strong alkalis are readily hydrolysed by boiling in aqueous solution. Its complex heteropoly-acids with other acids are ill-defined, but it takes up active oxygen to form a stable pertantalic acid, HTaO₄.xH₂O.

Tantalum adsorbs hydrogen directly, or when it is used as the cathode in the electrolysis of dilute sulphuric acid, but no definite hydrides have been isolated. One volume of tantalum in the form of wire, 0.3 mm. diameter, takes up 775 volumes of hydrogen at room temperatures and 46 volumes at about 800° C. The appended table gives the number of milligrams of hydrogen at 760 mm. pressure adsorbed by 100 grams of tantalum at different temperatures.

The amount of gas taken up decreases with increasing temperature, and at a given temperature (above 450° C.) is proportional to the square root of the gas pressure. The curve produced is similar to that given by palladium. Most of the hydrogen is expelled by heating to redness in

Adsorption of Hydrogen by Tantalum

Temperature, ° C.	17	100	183	263	314	417	474	530
Hydrogen, mgm.	419	400	377	327	297	212	157	107

a vacuum, but the small remaining quantity is removed only by fusing the material in a vacuum in the electric furnace. All the metallic tantalum now made is subjected to the latter treatment so as to drive off occluded gases and other volatile impurities.

When heated in hydrogen, tantalum wire undergoes a structural alteration, becoming brittle and crystalline; it retains these properties after the hydrogen has been removed by heating to a high temperature in vacuo. According to earlier investigators,1 the hydrogen absorbed at high temperatures is chemically combined with the tantalum, and it is stated that a hydride can also be obtained by the action of hydrogen on tantalum pentachloride.

The halides and oxyhalides of tantalum are set out in the following table

Halides and oxyhalides of Tantalum

Valency	Fluorine	Chlorine	Bromine	Iodine
"Di"-valent	. . .	HTa3Cl7.4H2O HTa3Cl6Br.4H2O	HTa3Br7.4H2O HTa3Br6Cl.4H2O HTa3Br6I.4H2O HTa3Br6OH.xH2O	. . .
Trivalent	. . .	TaCl3	TaBr3*	. . .
Pentavalent	TaF5 F3*	TaCl5 TaOCl3* TaO2Cl	TaBr5 TaOBr3	TaI6
* These compounds have not been isolated in the free state.

The pentavalent halides are the most stable, but even these can be prepared only in the dry way because of the readiness with which they undergo hydrolysis. The trichloride is obtained by reduction of the pentachloride with a powdered metal (lead, aluminium, zinc); the same process has also given a dichloride and perhaps a tetrachloride, but their formation awaits independent confirmation. The preparation of the chloroacid, HTa₃Cl₇.4H₂O, is of interest in that corresponding chloroacids of molybdenum, HMo₃Cl₇.4H₂O, and of tungsten, HWo₃Cl₇.4H₂O, have been obtained. The formation of the pentiodide is somewhat remarkable; niobium has not as yet yielded any iodine compounds, and vanadium has given only the tri-iodide. It is unusual for a metal falling in Groups IV. to VIII. to form an iodine derivative in which the maximum valency of the group is displayed.

Metallic tantalum and tantalum pentoxide are both dissolved by hydrofluoric acid, but evaporation of the solutions yields a residue which consists either of a tantalum oxyfluoride of variable composition or of the hydrated pentoxide.

Tantalum and iodine do not combine directly even when heated together for eight hours at 280° C. in a sealed tube. Attempts to prepare an iodide by the action of iodine on a mixture of tantalum pentoxide and carbon were also unsuccessful.

Only one sulphide of tantalum, namely, the disulphide, TaS₂, is known, and this is prepared by a dry method. Hydrogen sulphide does not precipitate any sulphides when passed into a solution of a tantalate, nor does reduction take place; it is without appreciable action on tantalum pentoxide even at 1200° C.

Tantalum reacts slowly with nitrogen when heated in the gas; combination commences at about 900° C. One gram of tantalum wire absorbed 2.2 cc. of nitrogen in one hour between 920° and 1030° C., and this was not expelled by heating in vacuo. In an earlier investigation thirty-one hours' exposure at 1000° C. gave an absorption equal to 17.3 per cent, of the weight of tantalum.

Reduction of tantalum pentoxide with carbon in the electric furnace yields carbides or alloys of variable composition. Traces of carbon render tantalum hard without affecting its ductility; when the carbon content exceeds 1 per cent, the product is extremely hard but brittle; tantalum containing 0.5 per cent, of carbon can, however, be drawn into wire 0.1 mm. diameter