Salts of Tungstic Acid-V

Solutions of tungstates containing ammonium sulphide yield with hydrochloric acid a brown precipitate of tungsten trisulphide. The addition of zinc chloride to a tungstate solution produces a yellow precipitate which becomes blue on warming with dilute hydrochloric or sulphuric acid. When excess of hydrochloric acid is added to a solution of alkali tungstate and the mixture reduced by means of zinc, brilliant colours, from red to blue, are produced; if phosphoric acid is used, a fine blue precipitate results.

The paratungstates are generally obtained by treating solutions of alkali normal tungstates with acid, or by double decomposition. They can only be obtained from solutions, and always contain water which appears essential to their constitution; it can only be removed with difficulty,.strong heating being necessary for complete dehydration, which is accompanied by decomposition of the salt into the soluble normal salt and the insoluble tetratungstate. From an investigation of the dehydration of the sodium and potassium salts the following results were obtained:
 

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