Tungsten Compounds

In Tungsten Compounds, tungsten resembles molybdenum very closely. Like all the metals of the Group, it exhibits valency varying from 2 to 6, and its most stable derivatives are those containing hexavalent tungsten. Halogen compounds containing the element in all the various stages of oxidation are known, but in the oxygen derivatives the tungsten is found almost exclusively in the tetra- or hexa-valent condition. The dioxide WO₂ is basic and gives with acids the corresponding series of salts, which, however, are unstable and readily undergo oxidation. With alkalies the dioxide liberates hydrogen and forms tungstates. The trioxide WO₃ is an amphoteric oxide, but its acidic properties predominate. Its hydrate WO₃.H₂O or H₂WO₄ is known as tungstic acid, and gives rise not only to a very stable series of normal tungstates, but also to several other series of salts which contain the ratio R^•₂O:WO₃ in varying proportions. The metatungstates, of composition R^•₂W₄O₁₃, are known to be derived from a definite metatungstic acid, H₂W₄O₁₃.

Tungsten Compounds intermediate between the di- and tri-oxides, and obtained from the latter by reduction, are known, and probably contain both tetra- and hexa-valent tungsten. These derivatives are generally blue in colour, and the existence of a definite blue oxide, of composition W₂O₅, appears to be established. Many of the blue reduction products appear to be mixtures of compounds in various stages of oxidation.

A remarkable series of reduction products is obtained from the tungstates of the alkali and alkaline earth metals. These have the empirical formula R^•₂O.(WO₃)_x.WO₂, and are known as tungsten bronzes, since they are vividly coloured and usually possess a bronze-like superficial lustre.

More highly oxidised compounds corresponding to the persulphates and containing peroxidic oxygen are obtained by the action of hydrogen peroxide on tungstic acid and the tungstates.

Only two sulphides of tungsten, WS₂ and WS₃, are known, but these dissolve in alkali solutions with formation of a series of soluble thio-salts.

Tungsten trioxide, like molybdenum trioxide, possesses in marked degree the property of combining with other acidic oxides, such as phosphorus pentoxide, arsenic pentoxide, silica, and boron trioxide, in varying proportions, producing heteropolyacids which contain the tungsten in a complex anion, and which yield well-defined crystalline salts with basic oxides.

Tungstic oxide and certain tungstates are used for imparting a yellow colour to glass and porcelain. Sodium tungstate has found application as a mordant, and for rendering fabrics more or less incombustible. The tungsten bronzes are used for decorative purposes.

Three well-defined oxides of tungsten exist - the brown, essentially basic, dioxide, WO₂; the yellow, essentially acidic, trioxide, WO₃; and a blue intermediate oxide, ditungsten pentoxide, W₂O₅. Several other oxides have been described, for example - W₄O₃, W₂O₃, W₅O₁₄; W₅O₈; W₃O₈; and W₄O₁₁; but the identity of none of these has been proved, and they appear to be either identical with, or mixtures containing, the blue oxide. By measurement of the equilibrium constants during reduction of the trioxide by a gradually increasing amount of hydrogen, it has been shown that between the trioxide and the metal only the two oxides, W₂O₅ and WO₂, exist as stable phases.

Metallic tungsten may be heated in nitrogen gas to a temperature of 1500° C. without any formation of nitride. In ammonia gas, tungsten is slightly affected at 850° C., a mere trace of nitride being formed. However, if the oxide of tungsten is heated under pressure in a mixture of hydrogen and nitrogen, tungsten nitride is produced. According to Langmuir, nitrogen does not, at any temperature, react with solid tungsten, but when a tungsten lamp containing nitrogen at low pressures is run for some time, the nitrogen gradually disappears. This takes place in three ways -

1. Nitrogen combines with tungsten vapour to form the nitride WN₂.
2. As the electric discharge passes through the nitrogen, the nitride WN₂ is formed at the hot tungsten cathode.
3. At very low pressures and high voltages nitrogen is driven on to the glass in such a form that it can be recovered by heating.

The dinitride, WN₂, is a clear brown powder, stable in vacuo at 400° C., but is decomposed at 2150° C. It is decomposed by water, yielding ammonia and probably the oxide WO₃.

Other nitrides have been described, but the literature concerning them is somewhat contradictory. By the action of ammonia on tungstic anhydride and tungsten hexachloride various products have been obtained. Wohler found that when the anhydride was heated in a stream of dry ammonia a black compound was obtained, which he named tungsten nitretamidoxide, considering it to be a mixed compound of nitride, amide, and oxide, of composition 4WN₂.W₂(NH₂)₂.2WO₂ = W₈N₁₀H₄O₂. Rideal, however, by similar means, obtained a black amorphous product of composition W₅N₆H₃O₅. At a higher temperature this compound yields metallic tungsten. Liquid ammonia does not react with tungstic anhydride even when the mixture is heated to 108° C. under pressure.

If tungstic anhydride is heated in a crucible with ammonium chloride, repeated additions of the latter compound being made until a product of constant weight is obtained, a black substance results which appears to be an oxynitride of composition WO₃.WN₂.

The sesquinitride, W₂N₃, is, according to Rideal, obtained as a black powder by the action of dry ammonia on tungsten hexachloride. The reaction takes place in the cold, and the product is freed from ammonium chloride by washing with water. The nitride has a semi-metallic lustre, is insoluble in nitric acid, but is oxidised by aqua regia to tungstic acid. When heated in the air it glows and is converted into yellow tungstic anhydride. Wohler regarded the product obtained by the action of ammonia on tungsten hexachloride as a mixture of two compounds to which he gave the formulae W₃N₄H₄ = 2WN.W(NH₂)₉ and W₃N₆H₄ = 2WN₂.W(NH₂)₂. The latter can be converted to the former by heating in hydrogen, and both at 1000° C. yield metallic tungsten. If the compound W₃N₆H₄ is heated just above its temperature of formation in ammonia, a nitride of composition W₃N₂ is obtained.

By heating tungstyl chloride, WO₂Cl₂, with dry ammonia, a black compound of composition W₄O₄N₄H₂ is obtained. This is insoluble in water, dilute alkalies, or hydrochloric acid. It is decomposed by concentrated nitric acid, and yields ammonia with potassium hydroxide. It reduces silver nitrate solution to metallic silver. By the action of liquid ammonia on tungstyl chloride, a brown compound, WO₃.3NH₃, is obtained.

By boiling a solution of potassium paratungstate with excess of antimonic acid a solution is obtained from which crystals of a potassium antimoniotungstate, of composition 2WO₃.3KSbO₃.8H₂O, may be obtained. The corresponding silver salt, treated with hydrochloric acid and the solution evaporated in a vacuum, yields a vitreous residue of the acid, 4WO₃.3Sb₂O₅.3H₂O + 8H₂O. A barium salt of composition 2BaO.3Sb₂O₃.11WO₃.18H₂O is described by Gibbs.

Several complex bismuthotungstates, of the type 3R^•₂O.2Bi₂O₃.11WO₃.xH₂O, have been obtained. The salts of potassium, ammonium, and strontium are oily substances which when dried yield yellow vitreous masses. By treating the potassium salt with a mercurous salt a yellow, well-defined, and stable bismuthotungstate, of composition 3Hg₂O.2Bi₂O₃.11WO₃.15H₂O, is obtained. By the action of hydrochloric acid on this substance a greenish-yellow solid of the formula 2Bi₂O₃.6HCl.11WO₃ may be isolated.

Three well-defined series of complex salts containing tungsten and vanadium have been prepared. They are obtained (1) by saturation of a solution of a paratungstate with vanadium pentoxide, (2) by the action of acetic acid on solutions of mixed alkali tungstates and vanadates. or (3) by the addition of a paratungstate to a metavanadate. The salts are characteristically coloured and all contain water of constitution. The dichromate coloured series have composition 5R^•₂O.3V₂O₅.6WO₃.xH₂O (R = NH₄, K, Na). The yellowish-red salts have composition 2R^•₂O.V₂O₅.4WO₃.xH₂O, and salts of the alkali metals, alkaline earth metals, and silver have been prepared. The salts of both these series have properties similar to the paratungstates. The salts of the third series are deep red in colour, of composition 5R^•₂O.3V₂O₅.14WO₃.xH₂O, and resemble the metatungstates. The ammonium, potassium, caesium, and barium salts have been prepared.

More complex compounds have also been prepared; for example, an ammonium phosphovanadotungstate, 13(NH₄)₂O.2P₂O₅.8V₂O₅.34WO₃.86H₂O; corresponding potassium and barium salts have also been prepared. Three series of silico-vanadotungstates of composition

3R^•₂O.SiO₂.V₂O₅.9WO₃.xH₂O,
3R^•₂O.SiO₂.V₂O₅.10WO₃.xH₂O, and
7R^•₂O.2SiO₂.3V₂O₅.18WO₃.xH₂O,

have been described.

When metallic tungsten is heated with carbon in an electric furnace carbide formation takes place, and the carbides W₃C, WC, and probably W₂C appear capable of existence. A carbide of composition W₃C₄ has also been described as being formed when finely powdered tungsten is heated in carbon monoxide at 1000° C., but its existence as a chemical entity is doubted by Ruff.

Simple cyanides of tungsten have not been isolated, but two well defined series of complex cyanides are known, one containing tetravalent tungsten and of composition R^•₄[W(CN)₈], the other with pentavalent tungsten and general formula R^•₃[W(CN)₈].

By dissolving zirconium hydroxide in solutions of ammonium and potassium paratungstates Hallopeau claims to have prepared 10-tungstozirconates of composition 4R^•₂O.ZrO₂.10WO₃.xH₂O. According to other investigators, however, zirconium hydroxide appears to be quite insoluble in paratungstate solutions.