Tungsten

Tungsten [[[symbol]] W, atomic weight 184o (0=16)], a metallic chemical element found in the minerals wolfram, an iron and manganese tungstate, scheelite, a calcium tungstate, stolzite, a lead tungstate, and in some rarer minerals. Its presence in scheelite was detected by Scheele and Bergman in 1781, and in 1783 Juan, Jose and d'Elhuyar showed the same substance occurred in wolfram; they also obtained the metal. Tungsten may be prepared from wolfram by heating the powdered ore with sodium carbonate, extracting the sodium carbonate with water, filtering and adding an acid to precipitate tungstic acid, H 2 W0 4. This is washed and dried and the oxide so obtained reduced to the metal by heating with carbon to a high temperature (Hadfield, Journ. Iron and Steel Inst., 1903, ii. 38). On a small scale it is obtained by reducing the trioxide in a current of hydrogen, or the chloride by sodium vapour, or the oxide with carbon in the electric furnace; in the last case the product is porous and can be welded like iron. In the form of a powder, it is obtained by reducing the oxide with zinc and extracting with soda, or by dissolving out the manganese from its alloys with tungsten. The metal may be used uncombined, but large quantities of ferrotungsten are made in the electric furnace; other alloys are prepared by acting on a mixture of the oxides with aluminium. Tungsten has been applied in the manufacture of filament electric lamps. The metal has a crystalline structure, and melts at about 2800°. The powdered metal burns at a red heat to form the trioxide; it is very slowly attacked by moist air. It combines with fluorine with incandescence at ordinary temperatures, and with chlorine at 250-300°; carbon, silicon, and boron, when heated with it in the electric furnace, give crystals harder than the ruby. It is soluble in a mixture of nitric and hydrofluoric acids, and the powdered metal, in aqua regia, but slowly attacked by sulphuric, hydrochloric and hydrofluoric acids separately; it is also soluble in boiling potash solution, giving a tunstate and hydrogen.

Tungsten dioxide, W02, formed on reducing the trioxide by hydrogen at a red heat or a mixture of the trioxide and hydrochloric acid with zinc, or by decomposing the tetrachloride with water, is a brown strongly pyrophoric powder, which must be cooled in hydrogen before being brought into contact with air. It is slightly soluble in hydrochloric and sulphuric acids, giving purple solutions. It dissolves in potash, giving potassium tungstate and hydrogen, and is readily oxidized to the trioxide.

Tungsten trioxide, W0 31 occurs in nature as wolframine, a yellow mineral found in Cumberland, Limoges, Connecticut and in North Carolina. It is prepared as shown above, or by other methods. It is a canary-yellow powder, which becomes a dark orange on heating; the original colour is regained on cooling. On exposure to light it assumes a greenish tinge. A crystalline form was obtained by Debray as olive-green prisms by igniting a mixture of sodium tungstate and carbonate in a current of hydrochloric acid gas, and by Nordenskjold by heating hydrated tungstic acid with borax. Partial reduction of tungsten trioxide gives blue or purple-red products which are intermediate in composition between the dioxide and trioxide. Tungsten trioxide forms two acids, tungstic acid, H 2 WO 4, and metatungstic acid, H2W4013; it also gives origin to several series of salts, to which the acids corresponding are unknown. Thus we have salts of the following types M20(W03)n, where n=1, 2, 3, 4, 5, 6, 7, 8, and also (M20)m(W03)n, where m, n=2, 5; 3, 7; 4, 3; 5, 12; M standing for a monovalent metal. The (M20)5(W03)12 or M10W12041 salts are called paratungstates. Tungstic acid, H 2 W0 4, is obtained as H 2 W0 4 H 2 0 by precipitating a tungstate with cold acid; this substance has a bitter taste and its aqueous solution reddens litmus. By using hot acid the yellow anhydrous tungstic acid is precipitated, which is insoluble in water and in all acids except hydrofluoric. It may be obtained in a flocculent form by exposing the hexachloride to moist air. Metatungstic acid, H2W4013.7H20, is obtained by decomposing the barium salt with sulphuric acid or the lead salt with hydrochloric acid. It forms yellow octahedra, which become anhydrous at 100°, and are converted into the trioxide on ignition. It is readily soluble in water, and on boiling the aqueous solution a white hydrate is first deposited which after a time is converted into the trioxide. Graham obtained a colloidal tungstic acid by dialysing a dilute solution of sodium tungstate and its equivalent of hydrochloric acid; on concentrating in a vacuum a gummy product is obtained, which still remains soluble after heating to 200°, but it is converted into the trioxide on heating to redness. When moistened it becomes adhesive. The solution has a bitter taste and does not gelatinize, even under the influence of boiling acids.

Of the salts, the normal tungstates are insoluble in water with the exception of the alkaline tungstates; they are usually amorphous, but some can be obtained in the crystalline form. The metatungstates of the alkalis are obtained by boiling normal tungstates with tungstic acid until the addition of hydrochloric acid to the filtrate gives no precipitate. The most important tungstate is the so-called tungstate of soda, which is sodium paratungstate, NaloW12041.28H20. This salt is obtained by roasting wolfram with sodium carbonate, lixiviating, neutralizing the boiling filtrate with hydrochloric acid and crystallizing at ordinary temperatures. The salt forms large monoclinic prisms; molecules containing 25 and 21 H 2 O separate from solutions crystallized at higher temperatures. The salt is used as a mordant in dyeing and calico printing, and also for making textiles non-inflammable. Several other sodium tungstates are known, as well as potassium and ammonium tungstates. Many salts also occur in the mineral kingdom: for example, scheelite is CaWO 4, stolzite is PbWO 4, farberite is FeWO 4, wolfram is (Fe,Mn)WO 4, whilst hi bnerite is MnW04.

By partial reduction of the tungstates under certain conditions products are obtained which are insoluble in acids and alkalis and present a bronze-like appearance which earned for them the name of tungsten bronzes. The sodium compound was first obtained by Wohler on reducing sodium tungstate with hydrogen; coal-gas, zinc, iron or tin also effect the reduction. It forms golden cubes which are unattacked by alkalis or by any acid except hydrofluoric. It appears to be a mixture of which the components vary with the materials and methods used in its production (Philipp, Ber., 1882, 15, p. 499). A blue bronze, Na2W5015, forming dark blue cubes with a red reflex, is obtained by electrolysing fused sodium paratungstate; a purple-red variety, Na2W309, and a reddish yellow form result when sodium carbonate and sodium tungstate are heated respectively with tungsten trioxide and tinfoil. Similar potassium tungsten bronzes are known.

Tungstic acid closely resembles molybdic acid in combining with phosphoric, arsenious, arsenic, boric, vanadic and silicic acids to form highly complex acids of which a great many salts exist. Of the phosphotungstic acids the most important is phosphoduodecitungstic acid, H 3 PW, 2040nH 2 O, obtained in quadratic pyramids by crystallizing mixed solutions of orthophosphoric and metatungstic acids. Two sodium salts, viz. Na2HPW12040nH 2 O and Na3PW12040 nH20, are obtained by heating sodium hydrogen phosphate with a tungstate. The most important silicotungstic acids are silicodecitungstic acid H8W10S1036.3H20, tungstosilicic acid, H 8 W 12 SiO 42.20H 2 O, and silicoduodecitungstic or silicotungstic acid, H8W12S1042.29H20. On boiling gelatinous silica with ammonium polytungstate and evaporating with the occasional addition of ammonia, ammonium silicodecitungstate is obtained as short rhombic prisms. On adding silver nitrate and decomposing the precipitated silver salt with hydrochloric acid, a solution is obtained which on evaporation in a vacuum gives the free acid as a glassy mass. If this be dissolved in water and the solution concentrated, some silicic acid separates and the filtrate deposits triclinic prisms of tungstosilicic acid. Silicotungstic acid is obtained as quadratic pyramids from its mercurous salt which is prepared from mercurous nitrate and the salt formed on boiling gelatinous silicic acid with a polytungstate of an alkali metal.

Pertungstic Acid, HWO 4. - The sodium salt, NaW0 4 H 2 O, is obtained by evaporating in a vacuum the product of boiling a solution of sodium paratungstate with hydrogen peroxide. Its solution liberates chlorine from hydrochloric acid and iodine from potassium iodide.

Halogen Compounds. - Although the trioxide is soluble in hydrofluoric acid, evaporation of the solution leads to the recovery of the oxide unchanged. A double salt of the oxyfluoride, viz. 2KFWO 2 F 2 H 2 O, is obtained as crystalline scales by dissolving normal potassium tungstate in hydrofluoric acid and adding potassium hydroxide till a permanent precipitate is just formed. Other oxyfluorides are known. The hexafluoride, WF 6, is a very active gaseous compound, which attacks glass and metals, obtained from tungsten hexachloride and hydrofluoric acid (Ruff and Eisner, Ber., 1905, 38, p. 74 2). Oxyfluorides of the formulae WOF 4 and WO 2 F 2 are also known. Tungsten forms four chlorides, viz. WC1 2, WC1 4, WC1 5, WC1 6. The dichloride, WC1 2, is an amorphous grey powder obtained by reducing the hexachloride at a high temperature in hydrogen, or, better, by heating the tetrachloride in a current of carbon dioxide. It changes on exposure to air and dissolves slightly in water to give a brown solution, the insoluble portion gradually being converted into an oxide with evolution of hydrogen. The tetrachloride, WC1 41 is obtained by partial reduction of the higher chlorides with hydrogen; a mixture of the pentaand hexa-chloride is distilled in a stream of hydrogen or carbon dioxide, and the pentachloride which volatilizes returned to the flask several times. This gives the tetrachloride as a greyishbrown crystalline powder. It is very hygroscopic and with cold water gives the oxide and hydrochloric acid. On heating it gives the diand penta-chlorides. At a high temperature hydrogen reduces it to the metal partly in the form of a black pyrophoric powder. The pentachloride, WC1 5, is obtained as a product in the preparation of the tetrachloride. It forms black lustrous crystals, or when quickly condensed, a dark green crystalline powder. It melts at 248° and boils at 275.6°; the vapour density corresponds to the above formula. It is more hygroscopic than the tetrachloride; and when treated with much water the bulk is at once decomposed into the blue oxide and hydrochloric acid, but an olive-green solution is also produced. The hexachloride, WC1 6, is obtained by heating the metal in a current of dry chlorine in the absence of oxygen or moisture, otherwise some oxychloride is formed; a sublimate of dark violet crystals appear at first, but as the hexachloride increases in quantity it collects as a very dark red liquid. When perfectly pure, the hexachloride is stable even in moist air, but the presence of an oxychloride brings about energetic decomposition; similarly water has no action on the pure compound, but a trace of the oxychloride occasions sudden decomposition into a greenish oxide and hydrochloric acid. It melts at 275° and boils at 34 6.7° (759.5 mm.). Vapour density determinations indicate that dissociation occurs when the vapour is heated above the boiling point.

Several oxychlorides are known. The monoxychloride, WOC14, is obtained as red acicular crystals by heating the oxide or dioxychloride in a current of the vapour of the hexachloride, or from the trioxide and phosphorus pentachloride. It melts at 210.4° and boils at 227.5° forming a red vapour. Moist air brings about the immediate formation of a yellowish crust of tungstic acid. The dioxychloride, WO 2 C12, is obtained as a light lemon-yellow sublimate on passing chlorine over the brown oxide. It is unaffected by moist air or cold water, and even when boiled with water the decomposition is incomplete. Tungsten combines directly with bromine to give, when the bromine is in excess, the pentaand not a hexabromide. This substance forms crystals resembling iodine, which melt at 276° and boil at 333°. It slowly evolves bromine on standing, and is at once decomposed by water into the blue oxide and hydrobromic acid. The dibromide, WBr 2, is a non-volatile bluishblack powder obtained by reducing the pentabromide with hydrogen. By passing bromine vapour over red-hot tungsten dioxide a mixture of WO 2 Br 2 and WOBr4 is obtained, from which the latter can be removed by gently heating when it volatilizes. The dioxybromide forms light red crystals or a yellow powder; it volatilizes at a red heat, and is not acted upon by water. The monoxybromide forms brownish-black needles, which melt at 277° and boil at 3 2 7.5; it is decomposed by water. The di-iodide is obtained as green metallic scales on passing iodine over red-hot tungsten.

Tungsten disulphide, W52, is obtained as soft black acicular crystals by the action of sulphur, sulphuretted hydrogen or carbon bisulphide on tungsten. The trisulphide, WS3, is obtained by dissolving the trioxide in ammonium sulphide or by passing sulphuretted hydrogen into a solution of a tungstate and precipitating by an acid in both cases. When dry it is a black mass which yields a liver-coloured powder. It is sparingly soluble in cold water, but is easily dissolved by potassium carbonate or ammonia. By dissolving it in a hydrosulphide a sulphotungstate is produced; these salts can also be obtained by passing sulphuretted hydrogen into a solution of a tungstate.

A nitride, W2N3, is obtained as a black powder by acting with ammonia on the oxytetrachloride or hexachloride; it is insoluble in sodium hydroxide, nitric and dilute sulphuric acids; strong sulphuric acid, however, gives ammonia and tungstic acids. Ammonia does not react with tungsten or the dioxide, but with trioxide at a red heat a substance of the formula W 5 H 6 N 3 0 5 is obtained, which is insoluble in acids and alkalis and on ignition decomposes, evolving nitrogen, hydrogen and ammonia. Phosphorus combines directly with the metal to form W3P4; another phosphide, W2P, results on igniting a mixture of phosphorus pentoxide and tungsten. trioxide.

The atomic weight has been determined by many investigators; the chief methods employed being the analysis and synthesis of the trioxide and the analysis of the hexachloride. The former was employed by Pennington and Smith and Desi (Zeit. anorg. Chem., 1895, 8, pp. 198, 205) who obtained the value 183.42.