Chemical Properties

When heated in hydrogen, pure, finely divided niobium is converted into a dark grey powder containing a maximum of 1.12 per cent, of hydrogen, which corresponds to a hydride NbH An impure sample of niobium absorbed 7.5 per cent, of its weight of hydrogen at a red heat. Niobium does not tarnish in the air at ordinary temperatures. When a compact piece of the metal is gradually heated in air it first becomes yellow, then blue, and finally becomes coated with a brownish-blue film of oxide which hinders further oxidation; the finely divided metal yields the pentoxide only slowly when strongly heated in air or oxygen. The glowing, finely divided metal decomposes water vigorously, with evolution of hydrogen. Heated in nitrogen at 1000° C. it becomes coated with a nitride. Filings of the metal decompose ammonia at a red heat with formation of a nitride which yields ammonia and niobic acid with caustic potash. The metal is attacked by chlorine at about 200° C. with formation of the pentachloride; bromine gives the pentabromide at higher temperatures; iodine is without action. Sulphur and selenium are absorbed with considerable evolution of heat and the formation of a black sulphide or selenide. The pure metal is insoluble in sulphuric acid, hydrochloric acid, nitric acid, and aqua-regia, but it is attacked by these acids when it is alloyed with other metals. It is slowly dissolved by hot hydrofluoric acid, more rapidly in the presence of platinum. Alkali solutions are without action on niobium, but the metal is converted into a niobate when fused with solid caustic potash, potassium carbonate, etc. Carbon dioxide, sulphur dioxide, phosphorus pentoxide, arsenic pentoxide, chromium sesquioxide, iodic acid, lead oxide, and mercuric chloride are all reduced by niobium when heated with it at high temperatures.

The compounds of niobium are not so numerous as those of vanadium. The following oxides, Nb₂O₃, NbO₂, Nb₂O₅, are known, but only the pentoxide gives rise to salts, viz. the niobates. The acid character of niobium pentoxide or "niobic acid" is very weak; the niobates are decomposed, for instance, by carbon dioxide, and are readily hydrolysed to the pentoxide. Niobic acid is, in fact, very comparable in its method of preparation and behaviour to silicic acid and stannic acid.

Reduction of pentavalent niobium compounds in acid solution with zinc yields solutions which appear to contain the niobium in the tetravalent state and probably also the trivalent state. The solution first becomes blue, then olive-green, and finally dark brown. Reduction of a boiling, dilute solution of niobic acid in concentrated hydrochloric or sulphuric acid may yield the brown solution immediately. The course of the reaction is considerably affected by such conditions as the acidity of the solution, the reducing agent employed, the physical condition of the reducing agent, and the presence of foreign substances. The final stage of the reduction can usually be depended on as being only approximately trivalent. Electrolytic methods of reduction, using platinum and amalgamated lead electrodes, have also been employed. The blue solutions gradually precipitate brown flakes, which are thereafter slowly converted into white niobic acid. Similar brown precipitates are obtained by the addition of ammonium hydroxide to the brown solutions. Marignae concluded from the amount of potassium permanganate required to oxidise the brown precipitate that its composition was Nb₃O₅, which can be alternatively written NbO.2NbO₂. This formula cannot, however, be definitely accepted, as the experimental data do not exclude the approximate empirical formula Nb₃O_4½, from which it would follow that the brown precipitate is composed of niobium sesquioxide together with small proportions of unreduced pentoxide. The blue solution functions as a powerful reducing agent, and will, for instance, precipitate copper from copper sulphate solution, and generally is a stronger reducing agent than trivalent titanium solution. On being evaporated in vacuo it leaves a damp blue mass, which on being dissolved in concentrated hydrochloric acid and treated with ammonium chloride gives a precipitate of " niobium blue," very similar in appearance to " molybdenum blue." No definite salts have hitherto been isolated, however, from reduced niobium solutions, but an ammonium niobium sulphate, which has the probable formula (NH₄)₂SO₄.Nb₂(SO₄)₃.6H₂O, and an acid ammonium niobium sulphate, (NH₄)₂SO₄.Nb₂(SO₄)₃.H₂SO₄. 6H₂O, have been prepared. The halides of niobium are devoid of saline character.

Niobic acid displays a much less pronounced tendency than vanadic acid to form heteropoly-compounds with other acids, but oxaloniobates are known. It reacts with hydrogen peroxide to form perniobic acid, HNbO₄.xH₂O, salts of which are known. The double niobium oxy- fluorides also take up active oxygen.

The absorption of hydrogen by pure metallic niobium under different conditions of temperature and pressure has not been investigated. An impure sample absorbed 8 per cent, of hydrogen after fifteen hours' exposure to the gas at a high temperature. In another experiment a substance of empirical formula NbH was obtained. This substance has also been prepared by fusing potassium niobium lluoride, K₂NbF₇, with potassium fluoride (which has the effect of rendering the subsequent reaction less violent) and then reducing the product with sodium by heating strongly in a wrought-iron crucible. The excess of sodium is distilled off and the residue extracted repeatedly with water, and finally with water containing a small quantity of hydrofluoric acid. The hydride is left behind as a black or dark grey powder. Its density varies from 6.0 to 6.6. The specific heat of the hydride appears to decrease with increase of temperature; an impure sample gave the value 0.0834 between 0° and 440° C. It resembles the metal in that it is insoluble in hydrochloric acid, nitric acid, dilute sulphuric acid, and aqua-regia, but it is attacked by concentrated sulphuric acid, hydrofluoric acid, and molten potassium hydrogen sulphate. On being heated in air it burns readily with incandescence to niobium pentoxide and water. It is scarcely affected by being heated in hydrogen, but it reacts with sulphur to form a black sulphide, and with chlorine and hydrochloric acid to form various niobium chlorides.

Other niobium-hydrogen compounds or alloys of doubtful composition have been obtained by reduction of niobium oxytrichloride, NbOCl₃, with hydrogen, and by using metallic niobium as the cathode in the electrolysis of dilute sulphuric acid.

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

Halides and oxyhalides of Niobium

Valency	Fluorine	Chlorine	Bromine	Iodine
"Di"-valent	. . .	Nb6Cl14.7H2O Nb6Cl12(OH)2.8H2O Nb6Cl12Br2.7H2O	. . .	. . .
Trivalent	. . .	NbCl3 Nb2Cl4(OH)2.5H2O Nb2Cl3(OH)3.3H2O	. . .	. . .
Pentavalent	NbF5 NbOF3 NbO2F	NbCl5 NbOCl3	NbBr5 NbOBr3	. . .

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 niobic 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.

Definite compounds of niobium and iodine are unknown, although tantalum pentiodide and vanadium tri-iodide have been prepared.

The divalent chloro-compounds are probably more correctly represented as chloroniobium acid, HNb₃Cl₇.4H₂O, and its derivatives, in analogous manner to chlorotantalum acid, HTa₃Cl₇.4H₂O, the composition of which has recently been reinvestigated.

The most important niobium oxychlorine compound is niobium oxytrichloride, NbOCl₃.

By the electrolytic reduction of a solution of niobium pentoxide in (a) concentrated hydrochloric acid and (b) dilute hydrochloric acid, and subsequent evaporation in vacuo or in an atmosphere of carbon dioxide, two compounds having the following compositions have been reported

(a) Nb₂(OH)₂Cl₄.5H₂O or NbOCl.[Nb(OH₂)₆]Cl₃.
(b) Nb₂(OH)₃Cl₃.3H₂O or NbOCl.[Nb(OH₂)₄Cl₂].

Definite compounds of niobium and iodine are unknown. The preparation of an iodide from the pentabromide has been reported but no details are supplied. A pyridine addition compound of the pentiodide, NbI₅.(C₅H₅N.HI)₆, has been described, but its existence lacks confirmation.

The absorption of nitrogen by metallic niobium under different conditions of temperature and pressure has not been investigated. Two substances have, however, been prepared, to which the formulae NbN and Nb₃N₅ have been ascribed, from their analytical data.

Metallic niobium in the molten state absorbs graphite slowly to yield carbides of unknown composition. These are insoluble in all acids, including hydrofluoric acid, and are brittle and very hard; they scratch quartz and glass. Reduction of niobium pentoxide with carbon in the electric furnace gives rise to similar products. A definite carbide having the composition NbC has recently been prepared by heating niobium sesquioxide, Nb₂O₃, mixed with carbon, in hydrogen at 1200° C. It is a greyish-violet powder which melts at about 3650° abs. Its hardness after fusion lies between 9 and 10. Its specific electrical resistance at ordinary temperatures is 1.47×10^-4 ohms per cc. Its density is 7.56. The crystal structure has been studied by the Debye-Scherrer method. Long, dark blue needles of a carbide having the same composition were also obtained by heating a mixture of potassium niobate, K₂O.3Nb₂O₅, potassium carbonate and sugar charcoal to a high temperature in a graphite crucible. The product was purified by washing with sulphuric acid and water.

The last-mentioned reaction at lower temperatures has also furnished greyish-violet crystalline niobium compounds containing both carbon and nitrogen, and which appear to contain varying proportions of nitride and carbide of niobium.

A substance which had the probable composition Nb₂O₅.N₅C was obtained as a black powder by the action of cyanogen on niobium pentoxide at a red heat.