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Thiotungstates

By substituting sulphur for oxygen in tungstates the following types of compounds have been obtained: R2WO3S, R2WO2S2, R2WOS3, R2WS4. These are produced:
  1. By dissolving the hydrate of tungstic acid in an alkali hydro- sulphide.
  2. By the action of hydrogen sulphide on solutions or suspensions of alkali or neutral tungstates.
  3. By fusing a tungstate with sulphur.
The compounds are generally yellow to yellowish brown in colour and yield well-defined crystals which are soluble in water. The solution, on addition of an acid, deposits tungsten trisulphide.

Potassium rnonothiotungstate, K2WO3S.H2O, is the only salt of this type which has been isolated. It is produced by passing hydrogen sulphide into a concentrated solution of potassium tungstate until a slight yellow precipitate is formed. A large bulk of alcohol is then added, and colourless deliquescent crystals are deposited. The salt is soluble in water, and the solution gives a white precipitate with zinc sulphate.

Ammonium dithiotungstate, (NH4)2WO2S2, is obtained as yellow crystals when hydrogen sulphide is passed into a strong ammoniacal solution of ammonium tungstate. The crystals are triclinic pinacoids, with axial ratio a:b:c = 0.703:1:0.3392. The salt is stable when dry, but decomposes when moist.

Potassium dithiotungstate, K2WO2S2, is formed by fusing together potassium tungstate and sulphur in absence of air and extracting with water. It yields lemon-yellow rectangular crystals, which lose water on heating, and if out of contact with air, melt without decomposition.

Potassium trithiotungstate, K2WOS3.H2O, is obtained in quadratic plates by the prolonged action of hydrogen sulphide on potassium tungstate. It is very soluble in water, but may be precipitated by means of alcohol. The crystals deliquesce in a damp atmosphere and effloresce in a dry one. The solution gives a lemon-yellow precipitate with zinc sulphate.

Normal ammonium thiotungstate, (NH4)2WS4, is prepared by the prolonged action of hydrogen sulphide on a dilute solution of ammonium tungstate. It forms transparent, orange-coloured crystals, rhombic bipyramidal, with axial ratio a:b:c = 0.778:1:0.5675, isomorphous with ammonium thiomolybdate. The salt is stable when dry. When heated in an atmosphere of carbon dioxide, tungsten disulphide remains; it is slowly reduced to the metal when heated in hydrogen.

Normal potassium thiotungstate, K2WS4, is prepared in a similar manner to the ammonium salt; or it may be precipitated by adding potassium hydrogen sulphide and alcohol to a solution of the ammonium salt. The crystals are rhombic bipyramids, with axial ratio a:b:c = 0.7495:1:0.5665. It is readily soluble in water. On heating, it melts without decomposition.

The compound, K2WS4.KNO3, is described by Berzelius.

Normal sodium thiotungstate, Na2WS4, is prepared by similar reactions to the above. The crystals are very hygroscopic, and are exceedingly soluble in water. The solutions of normal thiotungstates are not precipitated by zinc sulphate. Sodium thiotungstate is soluble in alcohol.

Thiotungstates of calcium, strontium, and barium have been prepared by the action of hydrogen sulphide on suspensions of the corresponding tungstates, whereby yellow solutions were obtained from which the thiotungstates could be crystallised.

The free acids corresponding to the above salts have not been isolated.

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