Niobium Pentoxide, Nb₂O₅

Niobium Pentoxide, Niobic Anhydride, Nb₂O₅, is probably the commonest compound of niobium. It is obtained in the treatment of niobium-bearing minerals, and is the starting material in the preparation of other niobium compounds. It is precipitated in the hydrated state by the hydrolysis of nearly all pentavalent niobium salts, and is formed in the gravimetric estimation of niobium.

It can be prepared by heating metallic niobium or any of the lower oxides, or the sulphide, carbide, or nitride of niobium, in air or oxygen. Ammonium niobium oxyfluoride, (NH₄)₂NbOF₅, also yields the pentoxide on being heated in air. Solutions of the alkali niobates yield a white gel of the hydrated pentoxide when treated with mineral acids, sulphuric acid being usually preferred. Insoluble niobates on fusion with potassium hydrogen sulphate, and extraction with water, yield a residue of niobic acid, which is repeatedly washed with water containing a little ammonia to remove the adhering sulphuric acid. The gel is then ignited, preferably with the addition of a small quantity of ammonium carbonate, and a residue of pure, dry niobium pentoxide is obtained. The pentafluoride, oxyfluorides, and double alkali fluorides similarly yield the pentoxide if they are evaporated with concentrated sulphuric acid until all the hydrogen fluoride has been evolved; the residue is carefully washed and ignited. It is a difficult matter to wash precipitated niobic acid free from adsorbed mineral acids because of its colloidal character. Traces of alkali, which may also be present in the product, are best avoided by starting with the ammonium salt.

Anhydrous niobium pentoxide is a white, amorphous, tasteless, odourless powder which remains white on being heated, although some samples assume a temporary yellow colour while hot. It is a remarkably stable substance and can be heated to 1750° C. without undergoing decomposition. When precipitated niobium pentoxide is heated to dull redness it glows brightly for a short time, the extent of the glow and the temperature at which it occurs depending on the method of preparation. Various explanations of this have been put forward: (a) Allotropic transformation; (b) increase in size of particles; (c) transition from amorphous to crystalline state. Recently, Debye-Scherrer X-ray spectrograms, made on samples of the oxide before and after glowings, have shown that transition from the amorphous to a crystalline state takes place. The same explanation is found to hold good for the glowing of tantalum pentoxide, ferric oxide, chromium sesquioxide, titanium dioxide, and other oxides, and is consistent with the fact that niobium pentoxide assumes a crystalline form on being very strongly heated. Fusion with borax also yields green, rhombic prisms. Microscopic, biaxial, rhombic plates and cubes have also been reported. The melting-point is about 1520° C. The density varies with the method of preparation; it usually lies between 4.37 and 4.53, but decomposition of the pentachloride in air and subsequent ignition gave a product the density of which varied between 4.88 and 5.05, and hydrolysis of the oxychloride followed by ignition gave samples the densities of which were as high as 5.26. The crystalline modifications have densities varying between 4.57 and 4.76. The specific heat of niobium pentoxide between 0° and 100° C. is 0.1184, and increases at higher temperatures. The magnetic susceptibility and electrical conductivity have been measured. When 1 gram of metallic niobium burns to the pentoxide, 2350 calories of heat are evolved. The calculated heat of formation of the pentoxide is given by the equation

2Nb + ½(5O₂) = Nb₂O₅ + 441,330 calories.

Niobium pentoxide is reduced to the dioxide, NbO₂, and the sesquioxide, Nb₂O₃, on being strongly heated in a stream of hydrogen at ordinary pressures. Reduction in the carbon electric vacuum furnace at high temperatures and pressures yields a mixture of the lower oxides. Ammonia at a white heat brings about partial conversion into the nitride. Heated in a current of hydrogen chloride, the oxide is completely volatilised without undergoing reduction, and yields a white hydrochloride, Nb₂O₅.a?HCl. Vanadium pentoxide, V₂O₅, molybdenum trioxide, MoO₃, and tungsten trioxide, WO₃, are also completely volatilised when heated in hydrogen chloride. The oxide is readily attacked by carbon tetrachloride vapour at 220° C. with formation of niobium pentachloride; tantalum pentoxide is unaffected under the same conditions. Sulphur monochloride and phosphorus pentachloride produce the pentachloride or oxychloride, according to the conditions, and chlorine reacts similarly if the pentoxide is previously mixed with charcoal. Hydrogen sulphide and carbon bisulphide vapour yield an oxysulphide at high temperatures. Niobium pentoxide is stable towards light, but becomes markedly photo-sensitive and undergoes reduction in the presence of certain organic liquids and reducing solutions, particularly glycerol. The process depends to some extent on the presence of impurities, notably stannic and tungstic acids, zirconium compounds, and titanic acid.

After being strongly ignited, niobium pentoxide becomes insoluble in all acids other than hydrofluoric acid, but is dissolved by molten potassium hydrogen sulphate, ammonium hydrogen sulphate, and borax. It is also insoluble in solutions of alkalis, but is converted into the alkali niobates by fusion with alkali hydroxides and carbonates.

Hydrates of Niobium Pentoxide. Colloidal Niobium Pentoxide. - Niobium pentoxide does not combine directly with water to form acids of definite composition. Two hydrates of the oxide, namely, 3Nb₂O₅.4H₂O and 3Nb₂O₅.7H₂O, have been reported, but their existence is very improbable. The term niobic acid is applied to the more or less hydrated pentoxide. When niobium pentachloride or niobium oxytrichloride, NbOCl₃, is hydrolysed with excess of water there is produced a white, amorphous, hydrated gel, which can also be obtained by the action of sulphuric acid or hydrochloric acid on alkali niobates; the precipitate is redissolved by excess of acid. Similar solutions are obtained by fusing a niobate with potassium hydrogen sulphate and extracting the melt with water; the extract, however, readily clouds, with precipitation of the acid.

The gel can be dried at room temperature to a horny mass; at 100° C. it becomes a white powder which still contains varying proportions of water, according to the history of the sample; it is completely dehydrated at about 300° C. The gel is insoluble in water, but on being washed undergoes peptisation and passes through the filter-paper. This can be prevented by the addition of a small quantity of an electrolyte (for example, ammonium nitrate or ammonium carbonate) to the water. The gel is only slightly soluble in concentrated hydrochloric acid, with probable formation of niobium oxytrichloride, but the residue, after decantation, undergoes peptisation very readily to yield a clear hydrosol from which all the hydrochloric acid can be removed by dialysis. The hydrosol prepared in this manner clouds if allowed to stand overnight or if boiled. The gel is more readily soluble in concentrated sulphuric acid; the solution remains clear on being diluted, but it is precipitated by sulphur dioxide. The gel dissolves also in hydrobromic acid and perchloric acid, and is readily taken up by hydrofluoric acid.

Hydrosols of both niobium pentoxide and tantalum pentoxide have been prepared by fusing each of these oxides with alkali in a silver crucible, dissolving the melt in water and dialysing the product for about ten days. Concentration over sulphuric acid yields a hydrosol containing 2.571 grams of niobium pentoxide per litre; the dispersed phase is negatively charged. The sols so obtained are quite stable, and are not coagulated even when heated, but they are precipitated by all strong electrolytes except bases, which impart stability through preferential adsorption of hydroxyl ions. They are very sensitive to chloride, sulphate, sulphite and nitrate ions. Hydrosols of niobium pentoxide and tantalum pentoxide differ in their behaviour towards carbon dioxide; whereas the tantalum pentoxide coagulates and is precipitated fairly rapidly, the niobium pentoxide is not precipitated after a day under the same conditions. This difference in respective rates of coagulation has been used to separate niobium from tantalum, but the separation is not quantitative.

The remarkable optical properties displayed by hydrosols of vanadium pentoxide have not hitherto been observed with niobium pentoxide or tantalum pentoxide.

In addition to dissolving in acids, freshly precipitated niobium pentoxide dissolves in caustic soda and in caustic potash, and hence it appears to possess weakly amphoteric character. Its colloidal state in solution has hitherto prevented any direct determination of its basicity or acidity. According to Weinland and Storz it is comparable to silicic acid, and is more strongly acid than titanic acid. That niobic acid is a very weak acid is shown by the readiness with which the niobates are (a) hydrolysed and (b) decomposed by mineral acids, sulphur dioxide, and even so weak an acid as carbon dioxide, with precipitation of niobic acid. Solutions of vanadates, on the other hand, on being treated with dilute acids yield the complex acid vanadates. Niobic acid combines in varying proportions with basic oxides of metals, which suggests that niobium pentoxide forms condensed complex anions containing several niobium atoms. It is most probable, however, that many of these niobates are not distinct chemical identities, but consist of isomorphous mixtures of simple salts. Their general insolubility in water and the readiness with which they undergo hydrolysis in contact with water have prevented any close investigation into their constitution, and hence into the constitution of niobic acid.

Niobic acid possesses the ability to form complex poly-acids with other acids (for example, with tungstic and oxalic acids). There are by no means as many of these known as in the case of vanadic acid, as they have not hitherto received much attention.