Potassium Metatungstate, K₂W₄O₁₃

Potassium Metatungstate may be prepared by the following methods:

1. A solution of the paratungstate is boiled with excess of the yellow tungstic acid until the filtered solution no longer gives a precipitate with hydrochloric acid. It is then concentrated and allowed to crystallise slowly over sulphuric acid.
2. A large excess of acetic acid is added to a saturated solution of potassium paratungstate. A heavy oily layer, consisting of a concentrated solution of the metatungstate, forms at the bottom of the vessel.
3. A mineral acid in concentrated solution is added to the paratungstate solution until the precipitate no longer redissolves. The solution is then concentrated and allowed to crystallise.
4. An aqueous solution of normal potassium tungstate is electrolysed in a cell in which the electrodes are separated by a diaphragm. When the anodic solution contains sufficient tungstic acid the metatungstate is formed.

Potassium metatungstate usually crystallises as the octahydrate, K₂W₄O₁₃.8H₂O, i.e. K₆H₄[H₂(W₂O₇)₆].21H₂O, yielding tetragonal octahedra, which effloresce and at 100° C. lose seven-eighths of the water present.

Long needle-shaped crystals are also occasionally obtained which are probably identical with the hydrate, K₂W₄O₁₃.6.5H₂O, obtained by Copaux in hexagonal prisms, with axial ratio a:b = 1:0.659, isomorphous with potassium boro- and silico-tungstates. These crystals, like their isomorphs, exhibit dextrorotatory power, [α]_D = + 9°.

The pentahydrate, K₂W₄O₁₃.5H₂O, is obtained by adding alcohol to a solution of the metatungstate, and warming the mixture until the precipitate redissolves; on cooling the resulting solution, monoclinic prisms separate. These crystals are not efflorescent and are readily soluble in water. Their crystallographic data are

a:b:c = 0.5945:1:0.6609; β = 123° 0'.

When heated at 100° C. they lose 4 molecular proportions of water.