Tungstic acid hydrate

Ammonium tungstate [11140-77-5] (NH 2 0cannot be obtained from an aqueous solution because it decomposes when such a solution is concentrated. It is prepared by the addition of hydrated tungstic acid to Hquid ammonia. 

Molybdic and Tungstic Acids. What hydrates do molybdenum and tungsten trioxides form ... 

What is the colour and composition of the tungsten trioxide hydrates Test the reaction of tungstic acids with 10% solutions of alkalies and acids. 

Silver nitrate solution pale yellow precipitate of silver tungstate, soluble in ammonia solution, decomposed by nitric acid with the formation of white hydrated tungstic acid. 

Hydrates of Tungsten Trioxide, Tungstic Acid.—Two well-defined hydrates of tungsten trioxide are knovm ... 

A white dihydrate, WO3.2H2O. With bases, both hydrates produce the same series of salts, and the first appears to be the true acid, tungstic acid, H2WO4, whilst the second is the hydrate, HjWOi.HaO. Both are insoluble in water, but colloidal forms of the acid e.xist. Several other hydrates have been described, but except in the case of the complex hydrate known as metatungstic acid (see p. 238) their existence has not been established. 

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 ... 

Ammonium Tungstates.— The normal salt cannot be obtained by dissolving tungstic acid in aqueous ammonia, since on concentration ammonia is lost and an acid salt remains. That the normal tungstate is present in solution would appear evident from the fact that calcium chloride precipitates the normal calcium tungstate. It may be obtained as a white mass by the addition of hydrated tungstic acid to liquid ammonia. The normal tungstate is very soluble in water and readily loses ammonia. 

The aqueous solution, yvhich is alkaline, when allowed to crystallise at temperatures above 0 C., yields slender nacreous crystals of the dikydrate, Xa2W04.2H20, in the form of rhombic bipyramidal scales, a b e=0-8o 02 1 0-6470, of density 23-259 at 17-5 C. and 3-231 at 19 C. This hydrate is stable in the air, and it is in this form that the salt is generally used. When heated, it loses water at 200° C., becomes opaque, and finally melts. It dissolves readily in hot water, but may be precipitated by means of alcohol. The solution yields white tungstic acid on the addition of mineral acids. 

Metatungstic acid is readily soluble in water and is referred to in the earlier literature as the soluble hydrate of tungstic acid. The aqueous solution, which is colourless, has a marked acid reaction and a pronounced bitter taste. The following table gives the solubility of the acid at different temperatures and the density of the solutions (Soboleff) ... 

By dissolving the hydrate of tungstic acid in an alkali hydrosulphide. 

Solutions of alkali tungstates are precijhtated h>y dilute mineral acids, yellow tungstic acid HgWOj or the white hydrate HgWO. HjO being formed (see p. 205). The precipitate readily dissolves in alkalies, and on reduction yields the blue oxide. 

In principle, the polycondensation may also be regarded as a dehydration process caused by the increase in proton concentration. In the starting species of the polycondensation process, the fully hydrated monotungstate, the W H20 ratio is 1 4 (pH > 6.2). In the various polytungstates prevailing between pH 6.2 and 1, the ratio is somewhere between 1 4 and 1 2, and attains exactly 1 2 in the precipitated tungstic acid at pH 1. 

Ammonium tungstate is produced by reaction of liquid ammonia with tungstic acid in a nonhydrated form. A hydrated form does not exist, because ammonium ions... 

Hydrated tungstic acid is dissolved in excess ammonia, and the solution is concentrated on a steam bath or at room temperature, whereby the excess ammonia evaporates. 

Dilute hydrochloric acid white precipitate of hydrated tungstic acid, H2WO4 H2O, in the cold upon boiling the mixture, this is converted into yellow tungstic acid, H2WO4, insoluble in dilute acids. Similar results are... 

Therefore, of the ions forming whife insoluble chlorides, only thallium cannot be excluded as a cause of a false positive reaction. The risk of finding thallium in a pharmaceutical sample must be considered very small and the same could be said about another possible cause of a false positive reaction, the tungstate ion. By the addition of dilute hydrochloric acid it will form, not an insoluble chloride salt, but a white precipitate of hydrated tungstic acid. 

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