Metatungstic Acid, H₂W₄O₁₃

It has already been mentioned that the metatungstates possess characteristic properties which are quite distinct from those of the derivatives of ordinary tungstic acid, and that metatungstic acid is definitely known to exist. The ammonium salt was first obtained by Margueritte, who formulated it as (NH₄)₂O.3WO₃.5H₂O. Laurent first suggested the name "metatungstate" and gave the composition of the ammonium salt as 5(NH₄)₂O.18WO₃.31H₂O. That the ratio of base to acid present was 1R^•₂O:4WO₃ was first recognised by Lotz, and a large number of salts were prepared and examined by Scheibler and Marignac. The former observed that all metatungstates are dehydrated with difficulty, and that the residue after complete expulsion of water at a high temperature no longer possesses the properties of a metatungstate and is insoluble in water.

The dehydration of metatungstates was investigated by Friedheim, who found that the potassium and ammonium salts lost all but 1 molecular proportion of water at 100° C., whilst the sodium, barium, manganese, cadmium, and lead salts retained 3 molecular proportions at 100° C., and with the exception of the lead salt still contained 1 molecular proportion up to 220° C. He therefore concluded that part of the water content was water of constitution. However, much difficulty was encountered in determining the actual constitution of the acid and its salts owing to the uncertainty of dehydration experiments at 100° to 120° C. as a means of determining water content, and because of the difficulty of distinguishing between water of crystallisation and water of constitution.

Further light was brought to the subject, however, by Copaux, who observed that true isomorphism existed between metatungstic acid and the complex heteropolyacids, boro-, phospho-, and silicotungstic acids; and also between certain salts of these acids, for example, the ammonium, potassium, and barium salts. He therefore concluded that these compounds should have similar constitutions, although it was recognised that compounds of high molecular weight occasionally exhibit isomorphism with others which are not of similar constitution. He prepared a new hydrated potassium metatungstate, of composition K₂O.4WO₃.6.5H₂O, which was isomorphous with the corresponding borotungstate and silicotungstate, and by increasing the metatungstate formula sixfold, he formulated the three compounds thus:

6K₂O.(3H₂O.24WO₃).36H₂O,
5K₂O.(B₂O₃.24WO₃).36H₂O,
4K₂O.(2SiO₂.24WO₃).36H₂O.

Similarly he obtained analogous formulae for the barium salts:

6BaO.(3H₂O.24WO₃).54H₂O,
5BaO.(B₂O₃.24WO₃).54H₂O.

It is to be noticed that the molecular proportion of the base is not the same in the isomorphous compounds, but it is striking that the taking of 3H₂O as water of constitution should result in the amounts of water of crystallisation being the same. The idea receives support from the fact that the properties of the metatungstates are remarkably similar to those of the boro-, silico-, and phospho-tungstates; for example, the salts and their acids are prepared by analogous methods, are readily soluble in water, and are easily crystallised. The free acids were formulated thus:

6H₂O. (3H₂O.24WO₃).48H₂O,
5H₂O.(B₂O₃.24WO₃).48H₂O,
4H₂O.(2SiO₂.24WO₃).48H₉O, 3H₂O.(P₂O₅.24WO₃).48H₂O,

so that metatungstic acid appeared to be a heteropolyacid in which water functioned in the place of an acidic oxide.

But, as Rosenheim pointed out, the ratio of fixed H₂O to WO₃ equals 1:4 at least, and probably 3:4, whereas in the given formula the ratio is only 1:8, and this author suggested that a more satisfactory formula might be obtained by regarding the acid as a co-ordinative aquo-compound - in accordance with Werner's views - for example, H₂[WO.(WO₄)₃.(H₂O)₃] or H₂[WO.(WO₄)₃.H₂O]. After further investigation 2 Copaux suggested a new formula based on Miolati's views according to which the heteropolyacids are derived from co-ordinative saturated acids by replacing oxygen atoms by means of complex anions. Thus the boro-, silico- and phospho-tungstic acids are formulated:

H₉[B(W₂O₇)₆]; H₈[Si(W₂O₇)₆]; H₇[P(W₂O₇)₆],

being derived from the hypothetical acids,

H₉[B^•••O₆]; H₈[Si^••••O₆]; H₇[P^{•••••}O₆],

the basicity of which is determined by the difference between the valency of the central element and the combined valency of the surrounding six groups. Similarly, the hypothetical acid corresponding to co- ordinative saturated water is

H₁₀[H₂O₆],

and by substitution the formula for metatungstic acid becomes

H₁₀[H₂(W₂O₇)₆].

This is in accordance with the general properties and isomorphic relations of the acid and its salts, but such a formula indicates a deca- basic acid, whereas, with the exception of an ill-defined mercury salt, only hexabasic salts, M^•₆H₄[H₂(W₂O₇)₆], have been obtained. Even with such a weak base as dimethylpyrone, the salt produced is (C₇H₈O₂)₆.H₁₀[H₂(W₂O₇)₆]. This low basicity may be due to the instability of the anion, but at present needs explanation. However, the above formula appears to conform with our present state of knowledge and is the most satisfactory yet suggested.

Metatungstic Acid, H₂W₄O₁₃.xH₂O or H₁₀[H₂(W₂O₇)₆].aq., was first isolated by Scheibler by decomposing the barium salt with dilute sulphuric acid, filtering the mixture, and evaporating the filtrate in vacuo over sulphuric acid. It may also be prepared: (1) by decomposing lead metatungstate with hydrogen sulphide:

PbW₄O₁₃ + H₂S = H₂W₄O₁₃ + PbS;

the excess of hydrogen sulphide is displaced from the filtered solution by passing a stream of carbon dioxide, and then on concentration in vacuo small yellow octahedral crystals of metatungstic acid are deposited; (2) by the addition of ether and concentrated hydrochloric (or sulphuric) acid to a concentrated aqueous solution of ammonium metatungstate. The liquid separates into three layers, the lowest of which is yellow and contains the free acid together with ether and the mineral acid. The free metatungstic acid may be obtained by evaporation in a current of air; it separates as crystals of composition H₂W₄O₁₃.8H₂O and H₂W₄O₁₃.6H₂O.

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):

Temperature,° C.	Grams H2W4O13.9H2O in 100 Grams H2O.	Grams H2W4O13 in 100 Grams H2O.	Density.
0	41.5	35.43	1.602
22	88.6	75.65	2.524
43.5	111.9	95.54	3.650

The density of solutions of various concentrations may be deduced from the formula:

D_{17.5° C} = 1 + 0.00903p + 0.0000633p² + 0.00000141p³, where p = per cent. WO₃ present.

The acid is insoluble in ether except in presence of a mineral acid. With absolute alcohol it yields a colloidal solution.

The crystals of metatungstic acid are isomorphous with the hetero-polyacids SiO₂.12WO₃.26H₂O and P₂O₅.24WO₃.5½H₂O, and determinations of hydrogen-ion concentration in solutions of the three acids show them to be of about equal strength and somewhat stronger than phosphoric acid. Dilute solutions of metatungstic acid are quite stable in the cold, but more concentrated solutions gradually precipitate the white hydrate, H₂WO₄.H₂O. At 100° C., first white and then yellow tungstic acid separates, even from very dilute solutions.

When a solution of metatungstic acid is electrolysed, it is reduced to a blue solution, lower oxides apparently being formed.

The crystals of metatungstic acid effloresce readily in the air. They have a density of 3.93. On heating, the crystals lose water, becoming anhydrous at about 180° C.

The metatungstates, M^•₂W₄O₁₃.aq. or M^•₆H₄[H₂(W₂O₇)₆].aq., are, with the exception of the lead and mercurous salts, soluble in water. The general methods of preparing the alkali salts are described under potassium metatungstate; other metatungstates are usually obtained by the addition of the sulphate of the metal to a solution of barium metatungstate, or by neutralisation of metatungstic acid with the oxide or carbonate. Solutions of metatungstates possess a more bitter taste than those of ordinary tungstates. The salts crystallise only after prolonged evaporation in the cold, whilst from hot solutions gummy residues are obtained on concentration.

In acid solution, metatungstates precipitate alkaloids and other organic bases, while ether in presence of a mineral acid precipitates metatungstic acid.