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.