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Barium Tungstates

The anhydrous normal salt, BaWO4, may be obtained by fusing together sodium tungstate, sodium chloride, and barium chloride. It crystallises in colourless octahedratetragonal bipyramidal; a:c = 1:1.6046, of density, according to Zambonini, 6.35. It is only slightly soluble in water. On heating, it first fuses and at high temperatures glows brightly.

Various hydrates have been described. When a solution of barium metatungstate or metatungstic acid is treated with barium hydroxide solution a white gelatinous precipitate is formed which slowly becomes crystalline. According to Pechard, the gelatinous precipitate has the composition BaWO4.2H2O, whilst the crystalline precipitate is BaWO4.H2O. A precipitate, of composition BaWO4.2.5H2O, is obtained by adding barium chloride to a solution of sodium paratungstate containing acetic acid. If barium hydroxide solution is added to a boiling solution of sodium paratungstate until the precipitate no longer redissolves, and the liquid is then allowed to cool, a double salt of sodium and barium (see below) separates; on adding excess of barium hydroxide solution to the mother-liquor a white crystalline precipitate of the pure subhydrated normal tungstate, 2BaWO4.H2O, is obtained.

Barium tungstate has been recommended for use instead of white lead and for colouring porcelain.

Barium ditungstate, BaO.2WO3.H2O, is obtained according to Lefort by double decomposition from solutions of sodium ditungstate and barium acetate, as a white amorphous precipitate, slightly soluble in water. The existence of this compound has been disputed.

Barium paratungstate, 3BaO.7WO3.xH2O, is precipitated on adding excess of a barium salt to a solution of an alkali paratungstate. When dried in the air the salt contains 16H2O; dried over sulphuric acid, 8H2O; dried at 100° C., 4H2O. The double salt, 3Na2O.2BaO.12WO3.24H2O, or 2Na2O.BaO.7WO3.14H2O, separates when excess of barium hydroxide solution is added to a boiling solution of sodium paratungstate and the mixture is allowed to cool.

The acid salt, 4BaO.10WO3.22H2O, is obtained as a white insoluble granular powder when barium chloride is added to a boiling aqueous solution of the corresponding sodium salt.

Barium tritungstate, BaO.3WO3.6H2O, is obtained as a white amorphous precipitate by the addition of cold water to barium metatungstate; a similar precipitate is obtained on mixing solutions of barium acetate and sodium tritungstate, which on drying at 100° C. gives a product of composition BaO.3WO3.4H2O. It dissolves in 300 parts of water at 15° C. Boiling water decomposes it with formation of the insoluble ditungstate and the soluble metatungstate.

An acid tungstate, of composition BaO.8WO3.8H2O, is described by Zettnow, who obtained it by the addition of hydrochloric acid and barium chloride , to a solution of sodium paratungstate which contained sufficient phosphoric acid to prevent precipitation by hydrochloric acid alone; according to Scheibler the precipitate so formed is not of definite composition.

Complex beryllium tungstates corresponding with the formulae: xBeCl2.yBeWO4.zBeO and xBe(NO3)2.yBeWO4.zBeO, have been prepared.

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