Encyclopædia Britannica, Ninth Edition/Tungsten

by William Dittmar

TUNGSTEN (Germ. wolfram, or, antiquated, scheel), one of the metallic elements of chemistry. The mineral tungsten (meaning in Swedish "heavy stone") used to be taken for a tin ore until this was disproved by Cronsted. Scheele showed in 1781 that it is a compound of lime with a peculiar acid, the metallic nature of which was recognized in the same year by Bergmann. It occurs only as a component of a number of relatively rare minerals, the most important of which are wolfram or wolframite, (Fe, Mn)O.WO₃, and scheelite (tungsten), CaOWO₃ (see Mineralogy). The metal is prepared from the pure oxide WO₃ by reduction with hydrogen in a platinum tube at a high temperature. It forms resplendent tin-white or grey plates, or a dull black powder similar to hydrogen-reduced iron. Sp. gr. = 19·129, water of 4° C. = 1 (Roscoe). It is more difficult to fuse than even Manganese (q.v.). It is unalterable in ordinary air; oxygen and even chlorine act upon it only at a high temperature. Hydrochloric and sulphuric acid do not attack it. Nitric acid attacks it slowly, aqua regia readily, with formation of the trioxide WO₃. Impure tungsten is now being prepared industrially for the production of a peculiar kind of steel (see Iron vol. xiii. p. 352).

Chlorides.—Tungsten forms four chlorides,—WCl₂ WC1₄, WC1₅, WCl₆. The highest, WCl₆, is obtained by heating the pure powdery metal in a current of absolutely pure chlorine. In the presence of moisture or air oxy-chlorides are produced. It sublimes off as a dark red liquid, freezing into crystals. These fuse at 275° C. and re-solidify at 270°; the liquid boils at 346°·7. The sp. gr. of the vapour is in accordance with the formula at 350°; at higher temperatures it dissociates into WC1₅ and free C1₂ (Roscoe). When the vapour of WC1₆ is passed over heated trioxide, the two bodies unite, W0₃ with 2WC1₆ into 3WOC1₄, forming magnificent red needles, which fuse at 210°·4 and boil at 227°·5 C. (Wöhler). Both compounds, WCl₆ and WOC1₄, are decomposed by water, the oxy-chloride more readily, with formation of hydrochloric acid and trioxide. For other chlorides and oxy-chlorides, see the ordinary hand-books of chemistry.

Oxides, WO₂ and WO₃ and Compounds of These with Each Other.—The trioxide, popularly known as tungstic acid, is the more important. Impure trioxide is producible by treating scheelite (WO₃CaO) with hot hydrochloric acid. Wolframite is not so readily decomposed; but when fused with twice its weight of chloride of calcium it passes into lime salt, obtainable as an insoluble residue by lixiviation of the fuse with water. The oxide obtained forms a yellow powder insoluble in water and in hydrochloric acid. To purify it, it is washed, dissolved in aqueous ammonia, aud the filtered solution evaporated, when an acid tungstate of ammonia separates out in scales of great purity. These, when heated in air, leave behind them a pseudo-morphose of pure yellow oxide. Trioxide of tungsten combines with basic oxides into tungstates; but the pro portion in which it unites with a given base is subject to great variation: for instance, the quantity Na₂O of soda unites into so many definite tungstates with 1, 11/2, 2, 21/3, 22/5, 21/2, 4 times W0₃ and in each case more or less of water. To each of these soda salts corresponds theoretically a certain tungstic acid,—to the salt Na₂OW₂O₆, for instance, the acid H₂OW₂O₆ or H₂W₂O₇. But few of these hydrates actually exist, and they are not individual acids in the sense in which the three phosphoric acids are, except perhaps that remarkable substance known as metatungstic acid, H₂W₄O₁₃ + 7H₂O. This acid forms crystals of the stated composition; it dissolves in water and the solution unites with bases into meta-tungstates. Most meta-tungstates are soluble in water; of the tungstates proper only the alkali salts are so soluble. The soda tungstate, 5Na₂O.12WO₃ + xH₂O, known as para-tungstate of soda, is made industrially by fusing wolframite with carbonate of soda and lixiviating the fuse with water. The insoluble oxides of iron and manganese are filtered off; the filtrate, while still hot, is nearly neutralized with hydrochloric acid and allowed to crystallize. It forms large crystals containing twenty-one, twenty-five, or twenty-eight times H₂O according to the temperature at which they are formed. The salt has been recommended as a mordant in dyeing and calico-printing, but has not taken root in these industries. Oppenheim and Versmann recommended it before 1862 as the best means for rendering textile fabrics uninflammable. If a solution of the para-tungstate is boiled with hydrated tungstic acid (as obtained by precipitating any ordinary alkaline tungstate solution with hydrochloric acid in the heat), or is simply mixed with excess of acetic acid, the meta-tungstate is formed; in the latter case it separates out as a heavy oil. Meta-tungstate of soda forms octahedral crystals of the composition Na₂O.4WO₃ + 10H₂O. If concentrated warm solutions of this salt and the equivalent quantity of chloride of barium are mixed and allowed to cool after addition of a little hydrochloric acid, meta-tungstate of barium crystallizes out as BaO.4WO₃ + 9H₂O, in large quadratic pyramids which are very easily soluble in water. From this salt the free acid is easily produced by addition of the exact quantity of sulphuric acid required to precipitate the baryta, and from it any other meta-tungstate is easily produced. Meta-tungstic acid solution is a sensitive and characteristic precipitant for almost all alkaloids (strychnine, quinine, &c.). The alkaloid, whatever its name, goes down as a flocculent insoluble meta-tungstate. Tungstic acid combines with phosphoric acid and with silicic acids into highly complex phospho-tungstic acids and silico-tungstic acids. Of the former there is quite a series, each consisting of one P₂O₅ united with respectively fourteen, sixteen, eighteen, twenty, twenty-two, twenty-four times WO₃ and six H₂O of basic water. Of silico-tungstic acids three are known, namely, one 4H₂O.SiO₂.10WO₃ + 3Aq and two SiO₂.12WO₃xH₂O. All these complex acids (both kinds) are easily soluble in water. The phospho-acids are delicate precipitants for all alkaloids.

The binoxide, WO₂, is obtained when the trioxide is reduced by hydrogen at a dull red heat. This oxide is very prone to pass into trioxide or tungstate. An interesting and beautiful class of compounds of WO₃, WO₂, and bases are known as tungsten bronzes. The first of these was discovered by Wöhler. Normal tungstate of soda, Na₂OWO₃, is fused, and trioxide added to it as long as it dissolves. The product is then heated in hydrogen as long as water goes away, and the substance thus reduced is exhausted successively with water, hydrochloric acid, caustic potash ley, and again with water. A residue of the composition Na₂O.W₂O₆ + WO₂ remains in the shape of magnificent gold-like lustrous cubes, of specific gravity 6·617, which conduct electricity like a metal. Only hydrofluoric acid dissolves this soda-tungsten bronze. There are a number of other tungsten bronzes, all distinguished by metallic lustre and magnificent purple, red, yellow, or blue colours.

Analysis.—Oxides of tungsten dissolve in fused microcosmic salt, Na₂OP₂O₅; the bead becomes blue in the reducing flame, in the presence of iron blood-red, and in the oxidizing flame colourless. When heated on charcoal with (not too much) carbonate of soda or cyanide of potassium in the reducing flame, they yield a grey heavy powder of metal, obtainable by elutriation. The process fails in the presence of too much alkali. Insoluble tungstates (e.g., the ordinary tungsten minerals) are disintegrable by fusion with alkaline carbonate; the fuse, when treated with water, yields a solution of alkaline tungstate. This solution, when mixed with excess of hydrochloric acid, gives a white precipitate of hydrated trioxide, which on boiling becomes yellow by partial dehydration. The yellow unignited precipitate is soluble in aqueous ammonia. If tungstate of alkali solutions are mixed with hydrochloric acid and then treated with metallic zinc, they become blue through the formation of a compound of WO₃ and WO₂ or rather the respective chloride; this reaction gains in definiteness through the presence of phosphoric acid.(w. d.)