Tungstic Acid, H₂WO₄

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

1. A yellow monohydrate, WO₃.H₂O, and
2. 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, H₂WO₄.H₂O. Both are insoluble in water, but colloidal forms of the acid exist. Several other hydrates have been described, but except in the case of the complex hydrate known as metatungstic acid 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:

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.

Both hydrates on heating to 100° to 110° C. lose water and leave a residue of composition 2WO₃.H₂O. This does not appear to be a true hydrate; on further heating, anhydrous WO₃ is obtained, and the process of dehydration has been investigated by means of the Huttig tensi-eudiometer, an instrument which is able to determine the pressure and volume of a gas liberated in any reaction at any moment. The results indicate that between the dihydrate, WO₃.2H₂O, and the anhydrous compound only one definite hydrate, WO₃.H₂O, exists. These results are supported by X-ray examination, both hydrates exhibiting characteristic crystalline forms.

The yellow hydrate dissolves very slightly in water; the specific conductivity of the saturated solution at 25° C. is k = 10.3×10^-6.

Tungstic acid is insoluble in most acids, but dissolves slightly in hydrochloric acid and is readily soluble in hydrofluoric acid, as is shown in the following table:

Solvent.	Temperature, °C.	Grams WO3 in 100 Grams Solution.
Hydrofluoric acid (40 per cent. HF)	25	44.75
Hydrofluoric acid (40 per cent. HF)	50	53.7
Hydrochloric acid (38 per cent. HCl)	50	0.36
Hydrochloric acid (38 per cent. HCl)	80	0.75

It is readily soluble in alkalies.

When dilute hydrochloric acid is added to a solution of sodium tungstate until the liquid becomes slightly acid, a colloidal solution of tungstic acid is obtained. If the solution of sodium tungstate is concentrated, a white gelatinous precipitate is obtained on the addition of the acid. This precipitate, after washing by decantation at a low temperature (0° to 5° C.) with as little exposure to air as possible, may be dissolved in a concentrated solution of oxalic acid and the liquid subjected to dialysis. If the outer water is repeatedly changed, the oxalic acid may be completely removed, leaving a colloidal solution of tungstic acid.

The colloidal solution may also be prepared by dissolving tungsten tetrachloride in alcohol and ether (equal volumes) and then diluting with alcohol and water. The solution obtained acts as a positive colloid and coagulates immediately when small quantities of neutral salts, hydroxides, or strong acids are added. On passing an electric current through the solution, a deep blue precipitate separates at the cathode.

A hydrosol of tungsten hydroxide is readily produced by the electrolysis of a 2 per cent, solution of sodium tungstate between a mercury cathode and a silver anode in a Hildebrand cell. The solution must not be allowed to become acid, or blue compounds are produced. The hydrosols obtained in this way are clear and transparent but brown in colour. The addition of potassium chloride causes coagulation, a black powder, resembling the lower oxides of tungsten, being formed.

Colloidal solutions of tungstic acid, in presence of various organic reducing agents such as formaldehyde, sucrose, glucose, dextrin, etc., yield intensely blue solutions on exposure to light. If the solution is kept for some time, it does not undergo this reduction on being exposed to light; but on raising the temperature the blue reduction products are obtained. In order to account for this it has been suggested that two forms of colloidal tungstic acid exist, one being photochemically sensitive and the other not. The former changes spontaneously into the latter, the reverse change being brought about by rise in temperature, and the absorption spectra of the two modifications differ considerably.

The composition of the hydrosol has not been determined, but it is thought to consist of tungstic acid in combination with water, or possibly with sodium tungstate, since Sabaneeff obtained an amorphous powder, of composition Na₂O.4WO₃, from the dialysed solution.

When the solution is evaporated to dryness, transparent vitreous scales remain, strongly adherent to the crucible; on heating this residue to redness, the trioxide WO₃ results. The aqueous solution has a bitter astringent taste; its density at 19° C. is as follows:

Per cent. WO3	5	20	50	66.5	79.8
Density	1.0475	1.2168	1.8011	2.396	3.243

The gelatinisation of silicic acid is retarded by the presence of colloidal tungstic acid.

Tungstic acid resembles molybdic acid in that it reacts with bases to form many different types of salts. A satisfactory classification of these salts has long been, and still is, a matter of difficulty, owing to the fact that many of the compounds are only described by individual investigators, whose work is either insufficient in detail or has lacked confirmation by later workers. The existence of the di- and tri-tungstates prepared by Lefort has not been established. Laurent, in 1847, suggested that there were at least five distinct acids (combinations of tungstic anhydride with water in different proportions) with corresponding series of salts, and although the preparation and properties of all these acids have been described, it is now recognised, largely owing to the work of Riche, Scheibler, and Marignac, that only two different acids are definitely known to exist, namely, ordinary tungstic acid, H₂WO₄, and metatungstic acid, H₂W₄O₁₃.aq. The former is insoluble in water and reacts with bases to form normal di-, tri-, and para-tungstates; the latter is soluble in water and yields a well-defined series of salts, the metatungstates.

This division into only two groups is justified by the fact that the metatungstates show marked differences both in properties and in ionic reactivity from those of the ordinary normal and acid tungstates, whilst the latter are very similar in their reactions. The transformation of normal tungstates into ordinary acid tungstates takes place readily, whereas the formation of metatungstates - by the addition of tungstic acid or other acids to tungstates - takes place only slowly and incompletely at ordinary temperatures. Further differences between the two types of salts are found in the peculiar behaviour of metatungstates on dehydration, and in the fact that whilst normal and para-tungstates increase the specific rotatory power of tartaric acid, the metatungstates do not act in this way.

Of the numerous types of salts of ordinary tungstic acid only two (the normal, of composition R₂O.WO₃.xH₂O, and the so-called para-tungstates in which the ratio R₂O:WO₃ = 3:7 or 5:12) have been accurately investigated and their existence established beyond doubt. The readiness with which one type of salt is transformed into another, and the fact that paratungstates decompose on prolonged contact with water or on heating, make exact analysis almost impossible; and an added difficulty lies in the high atomic weight of tungsten, the difference in composition of various compounds with high tungsten content being very small. It is from such causes that, although the paratungstates are recognised as a well-defined series of salts, their actual constitution and relation to the normal tungstates remains unestablished. Further, the higher acid salts such as hexa- and octa-tungstates appear to show a closer relation to meta-tungstates than to ordinary tungstates, but the nature of this has not been determined. According to Smith, tungstates of the type 4R₂O.10WO₃.xH₂O constitute another very definite series of salts.

The normal tungstates of the alkali metals are usually obtained by fusing together tungstic anhydride and the alkali hydroxide or carbonate in equivalent proportions. Those of the heavier metals are produced either by double decomposition in solution, or by fusing together an alkali tungstate and the chloride of the metal, often in the presence of sodium chloride. The tungstates of the alkali metals and of magnesium are soluble in water, those of other metals being insoluble, or only slightly soluble, not only in water but also in dilute acids. Concentrated mineral acids (except phosphoric acid) decompose them, with separation of tungstic acid. In this reaction the paratungstates behave similarly, whereas the metatungstates are not decomposed.

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:

Temperature, t° C.	Water remaining after heating at t° C.
	Na10W12O41.28H2O.	K10W12O41.11H2O.
110	5.0 molecules	5.4 molecules
150	4.0 molecules	4.4 molecules
200	2.4 molecules	2.4 molecules
250	1.4 molecules	1.4 molecules

The content of base to acid in paratungstates was first given by Laurent as 5R₂O:12WO₃, whilst Lotz and Scheibler suggested the formula 3R₂O.7WO₃.xH₂O. Marignac, after careful analysis, concluded that most paratungstates contained 5R₂O:12WO₃, but that a few contained 3R₂O:7WO₃. Other investigators, for reasons mentioned above, were unable to decide between the two formulae. Copaux, from the behaviour of the salts towards dehydration, considered them to be hydrotungstates and gave them the co-ordinative formula R^•₅[H(W₂O₇)₃].aq. In support of this he points to the fact that the paratungstates resemble the complex tungstates in absorbing ultraviolet light, whereas normal tungstates do not do so. Rosenheim suggests that they are 6-tungsto-aquates of composition R₅H₅[H₂(WO₄)₆]aq.

Such formulation suggests a closer relation to the metatungstates than appears to be justified, and would not account for the very essential differences between the two types of compounds.

Paratungstates gradually decompose in aqueous solution with formation of the normal and metatungstates, so that while a freshly prepared solution is neutral to phenolphthalein, it gradually becomes acid on standing - more rapidly on boiling. For this reason the electrical conductivities of the solutions slowly increase at ordinary temperatures.

According to Hallopeau the free paratungstic acid is formed in dilute solution when the barium salt is treated with dilute sulphuric acid. Concentration of the solution, even in vacuo, causes decomposition, and on prolonged boiling, metatungstic acid is formed. Alkalies neutralise the solution, yielding paratungstates. There is, however, no proof that this solution contains any special modification of tungstic acid.

In the following pages a description is given of the individual normal and acid salts of tungstic acid, including the paratungstates.