Tantalum Pentoxide, Ta₂O₅

Tantalum Pentoxide, Ta₂O₅, is one of the commonest compounds of tantalum. The anhydrous substance is produced by direct oxidation of the metal or by ignition of hydrated tantalum pentoxide, which is obtained by the methods described below. The removal of niobium and other metals has been described when dealing with the extraction of tantalum and niobium from their natural ores.

Anhydrous tantalum pentoxide, as usually prepared, is a white, nonvolatile, tasteless, odourless, amorphous powder, which remains white at high temperatures. When heated to dull redness it glows and assumes a crystalline form (rhombic prisms), isomorphous with niobium pentoxide; the same change takes place when the amorphous substance is fused with boric acid or microcosmic salt. Its melting-point has been determined under different conditions with discordant results: 1875° C., 1620° C. Its density is approximately 7.5, but the figure obtained varies with the method of preparation, increasing with rise in temperature of ignition and prolongation of duration of heating. The electrical resistivity has been measured. One gram of metallic tantalum evolves 1373 calories of heat when burnt directly to the pentoxide in a bomb calorimeter; 1 gram of aluminium under the same conditions evolves 6970 calories. The calculated heat of formation of tantalum pentoxide is as follows:

2Ta + ½(5O₂) = Ta₂O₅ + 498.3 Cals.

Tantalum pentoxide is a remarkably stable substance. It is not attacked by chlorine, hydrogen chloride, carbon tetrachloride, bromine or hydrogen bromide even at high temperatures, but if it is previously mixed with carbon these reagents do attack it. When heated with phosphorus pentachloride or phosphorus trichloride in an air-free sealed tube at 200° C. it is converted into the pentachloride. Sulphur is without action on it; hydrogen sulphide produces traces of tantalum disulphide at high temperatures; carbon disulphide produces tantalum disulphide at a white heat. Carbon, at high temperatures and in the absence of air, reduces the pentoxide to a carbide, TaC, or to a mixture of the lower oxides; in the presence of air a mixture of the carbide and nitride is produced. Reduction with aluminium yields an aluminium-tantalum alloy. The oxide is insoluble in all acids except hydrofluoric acid. According to some investigators the solution of tantalum pentoxide in hydrofluoric acid loses some of its tantalum content on being evaporated; according to others, however, no such loss takes place. It has recently been shown that pure tantalum pentoxide, prepared by oxidation of the metal, does not show any volatility when evaporated in hydrofluoric acid, and any loss that takes place has been attributed to the presence of traces of alkali in the pentoxide. The amount of the loss varies with the alkali content, and the pentoxide can be completely volatilised by heating with ammonium fluoride. No loss of tantalum takes place in the presence of concentrated sulphuric acid. The oxide is volatile in hydrogen chloride at 900° C. When prepared by precipitation in the presence of nitric acid or sulphuric acid the ignited material retains traces of these acids. Ignited tantalum pentoxide is also dissolved by fusion with potassium hydrogen sulphate, ammonium hydrogen sulphate, caustic potash, or a mixture of sodium carbonate and borax.

When tantalum pentachloride or pentabromide is treated with water, or when a solution of a tantalate is boiled with dilute acids, a gelatinous precipitate of more or less hydrated tantalum pentoxide is thrown down. Insoluble tantalates on fusion with potassium hydrogen sulphate and extraction of the melt with water give the gel, In dealing with double fluorides of tantalum it is necessary to remove all the hydrofluoric acid by evaporation with concentrated sulphuric acid, otherwise double fluorides are obtained.

As in the case of hydrated niobium and vanadium pentoxides, it is a difficult matter to remove traces of mineral acids from the precipitate. Treatment with water gives rise to a cloudy hydrosol which passes through an ordinary filter. This can be prevented by addition to the wash-water of a small quantity of ammonia or acetic acid. In the preparation of anhydrous tantalum pentoxide, traces of acids can be removed by igniting the gel in admixture with a small quantity of ammonium carbonate.

Hydrated tantalum pentoxide is a white, amorphous substance. A crystalline form is stated to be obtained when tantalum pentachloride is treated rapidly with water; the precipitate thrown down is dried slowly and again treated with water. A granular variety is produced when sodium tantalate solution is treated with sulphur dioxide and the flaky precipitate is dried. Tantalum pentoxide gel becomes incandescent and loses its water content when it is rapidly heated to 500° C., unless it has been previously aged by washing with water. This "glow " phenomenon is also displayed by hydrated chromium sesquioxide, by aluminium oxide, and by titanium dioxide. The composition of the gel, when dried at 100° C., varies with the method of preparation. Various hydrates have been reported, but their composition must be regarded as accidental, because recent investigation has shown that continuous variation in the water content takes place with variation in the vapour-pressure. The curves obtained were very similar to those given by gels of stannic oxide and silica. A true tantalic acid is, therefore, unknown, but the name is used for the more or less hydrated pentoxide.

Hydrated tantalum pentoxide or tantalic acid is very comparable in its properties to niobic acid. It is soluble in excess of strong acids to form a colloidal solution. The hydrate, which is prepared by precipitation from cold dilute solutions of alkali tantalates by means of dilute sulphuric acid, dissolves in hot concentrated sulphuric acid, and is reprecipitated by diluting the cooled solution with water; hydrated niobium pentoxide is not reprecipitated, however, under similar circumstances, and its solution remains clear. Hydrated tantalum pentoxide differs from titanium dioxide in that it yields a flesh-coloured insoluble addition compound with ammonium salicylate, whilst titanium dioxide passes into solution. Whereas niobic acid and titanic acid are practically completely soluble in hydrogen peroxide, tantalic acid, when precipitated hot, is almost insoluble, and when precipitated cold is only partly soluble. Certain weak acids, for example oxalic acid and tartaric acid, dissolve tantalum pentoxide, probably because of the formation of soluble heteropoly-acids.