atomic

weight 184.o

See also:

• atomic weight 184.o TUNGSTEN [symbol W (0=16)]

TUNGSTEN [

See also:

• SYMBOL (Gr. o-uµ(3oXov, a sign)

symbol W (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, 112WO4 . 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 nitricand 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 hydro- gen 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, WO3, 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, H2WO4, and metatungstic acid, H2W4O13; 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),,, where n=1, 2, 3, 4, 5, 6, 7, 8, and also (M20),,,(W03),,, where in, 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, HZW04, is obtained as H2WO4•H2O 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, H2W4O13.7H20, 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 1o0°, 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, NaioWi2041.28H2O . 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 H2O 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 CaWO4, stolzite is PbWO4, farberite is FeWO4, wolfram is (Fe,Mn)WO4, whilst hiibnerite is MnWO4 . 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, Na2W5O15, forming dark blue cubes with a red reflex, is obtained by electrolysing fused sodium paratungstate; a purple-red variety, Na2W3O9, 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, H3PWi204o•nH2O, obtained in quadratic pyramids by crystallizing mixed solutions of orthophosphoric and metatungstic acids . Two sodium salts, viz . NasHPW13Ogo•nHsO and Na3PWisO4o•nH2O, are obtained by heating sodium hydrogen phosphate with a tungstate . The most important silicotungstic acids are silicodecitungstic acid HsW1oSiO3s 3H2O, tungstosilicic acid, H8W,2SiO42.2oH2O, and silicoduodecitungstic or silicotungstic acid, HsW1iSiO42.29H2O . On boiling gelatinous silica with ammonium polytungstate and evaporating with the occasional addition of ammonia, ammonium sihcodecitungstate 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 glass, 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, HWO4.—The sodium salt, NaWO4•H2O, 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 . 2KF•WO1F2•H2O, 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, WF6, is a very active gaseous compound, which attacks glass and metals, obtained from tungsten hexachloride and hydrofluoric acid (Ruff and Eisner, Per., 1905, 38, p . 742) . Oxyfluorides of the formulae WOF4 and WO2F2 are also known . Tungsten forms four chlorides, viz . WCl2, WCI4, WCI6, WCI6 . The dichloride, WCl2, 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, WCI4, is obtained by partial reduction of the higher chlorides with hydrogen; a mixture of the pentaand hexes-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 greyish-brown crystalline powder . It is very hygroscopic and with cold water gives the oxide and hydrochloric acid . On heating it gives the di- and penta-chlorides . At a high temperature hydrogen reduces it to the metal partly in the form of a black pyrophoric powder . The pentachloride, WCI6, 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 275.60; 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, WCI6, 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° hand boils at 346.7° (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 moils at 227.5 forming a red vapour . Moist air brings about the immediate formation of a yellowish crust of tungstic acid . The dioxychloride, WO2Cl2, 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 penta- and 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, WBr2, is a non-volatile bluish-black powder obtained by reducing the pentabromide with hydrogen . By passing bromine vapour over red-hot tungsten dioxide a mixture of WO2Br2 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 monoxybromideforms brownish-black needles, which melt at 277° and boil at 327-5; it is decomposed by water . The di-iodide is obtained as green metallic scales on passing iodine over red-hot tungsten . Tungsten disulphide, WS2, 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 W5H6N3O5 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 .