Preparations of Tungsten Acid

Tungstic acid, H2WO4, is formed as an amorphous yellow precipitate when an excess of hot hydrochloric acid is added to a solution of an alkali tungstate. If cold acid is used a white precipitate of the hydrate, H2WO4.H2O, results, from which the acid may be obtained either by boiling the mixture or by drying over sulphuric acid. It may also be prepared by the following methods:

1.By digesting a tungsten mineral with hydrochloric acid and then with aqua regia until the iron and manganese are dissolved and a yellow residue remains. This, after washing, is shaken with ammonia, which dissolves the free tungstic acid. The liquid is filtered, and on concentration tungstic acid crystallises out.

2.By heating the mineral under pressure with concentrated potash solution and lime, the tungstic acid being subsequently separated from the resulting solution by fractional precipitation.

3.By fusion of the finely powdered mineral with alkali carbonates, sodium chloride, or calcium chloride. The residue is treated with water and then with nitric or hydrochloric acid to decompose any insoluble tungstates. The acid may be further purified by solution in ammonia and precipitation with dilute nitric acid, or by means of chlorine.

The white hydrate, H2WO4.H2O, is produced by the decomposition of tungsten pentachloride, or the oxychlorides, in presence of moist air.

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