Barium Metatungstate, BaW₄O₁₃

Barium Metatungstate, BaW₄O₁₃.9H₂O or Ba₃H₄[H₂(W₂O₇)₆].25H₂O, is prepared by mixing boiling concentrated solutions of sodium metatungstate and barium chloride in the presence of a little hydrochloric acid and allowing to cool. The resulting crystals contain some tritungstate, but this can be removed by recrystallisation. The crystals are lustrous, milky-white, rhombic bipyramids, with axial ratio a:b:c = 0.9962:1:1.5070, and isomorphous with barium borotungstate. Density 4.298 at 14° C. The salt is decomposed by water. When heated to 100° C. it loses two-thirds of the water present.