Vanadates

It seems certain that the free acids corresponding to these salts do not exist in the solid state, and that, with the possible exception of hexavanadic acid, mentioned below, they are also incapable of existing in solution, although salts of all the acids are known. The most stable class of salts is the metavanadates, the next in order of stability being the pyrovanadates, while the orthovanadates are few in number and undergo rapid hydrolysis even in the cold, to give the pyro-salts:

2Na₃VO₄ + H₂O ⇔ Na₄V₂O₇ + 2NaOH;

the pyro-salt is converted into the meta-salt on boiling the solution:

Na₄V₂O₇ + H₂O ⇔ 2NaVO₃ + 2NaOH.

These reactions are reversible, and removal of the caustic alkali by addition of acids effects the immediate conversion of ortho- or pyro-salts into the meta-salts. On the other hand, the presence of a large excess of caustic alkali favours the formation of the ortho- and pyro-salts. The order of stability is the reverse of that which applies to the ortho-, pyro-, and meta-phosphates. Orthophosphates are prepared from the other two classes either by boiling or by addition of weak acids.

Metavanadates of the alkalis are white or colourless, and give colourless aqueous solutions which rapidly become yellow, and, on addition of acids, red or orange. These coloured solutions contain polyvanadates, the formation of which is comparable to that of the polychromates and other salts formed by condensation of weakly acid oxides of metals, e.g. molybdates and borates. Thus, under definite conditions of temperature and concentration, potassium metavanadate is converted into the acid salt 2K₂O.3V₂O₅, in accordance with the equation:

6KVO₃ + 2HCl = 2K₂O.3V₂O₅ + 2KCl + H₂O.
White. → Red.

Cf 2K₂CrO₄ + 2HCl = K₂O.2CrO₃ + 2KCl + H₂O.
Yellow. → Red.

Polyvanadates produced in this manner are much more numerous than the polychromates, and have the general composition R^•₂O.nV₂O₅, where n is greater than one. Solutions of polyvanadates contain equilibrium mixtures the compositions of which vary considerably with the acidity, the temperature, and the concentration of the solution. Thus, by acidifying a solution of lithium metavanadate with acetic acid, crystals having the composition 3Li₂O.4V₂O₅.12H₂O are obtained, the mother-liquor from which, on being boiled, gives the compound 3Li₂O.5V₂O₅.14H₂O. Again, by treating a solution of potassium metavanadate with acetic acid under different conditions of temperature and concentration, a series of salts has been obtained in which n = 3/2, 5/3, 2, 5/2, and 3. In several salts the molecular proportions of vanadium pentoxide and basic oxide are not so simple, for example in the compound 22K₂O.24V₂O₅.7H₂O, and it is noteworthy that in several instances efforts to repeat the preparation of one particular compound have failed under apparently identical conditions. The acid vanadates on the whole have not been accurately investigated by physico-chemical methods, and their composition is sufficiently varied to suggest that, as in the case of the double vanadates and heteropoly-acids, they may be isomorphous mixtures of simple substances in varying proportions, despite the facts (1) that they are easily crystallisable compounds, and (2) that from their analytical data definite formula can be written down for them.

The acids corresponding to these various salts have not been isolated, but according to Dlillberg many of the poly- vanadates can be looked upon as being derived from hexavanadic acid, H₄V₆O₁₇ or 2H₂O.3V₂O₅, the existence of which in solution is indicated by the changes in electrical conductivity that ensue when alkali vanadates are gradually neutralised. When sodium orthovanadate, Na₃VO₄, is treated with increasing quantities of hydrochloric acid, it is found that the ions finally present in solution are Na^• and [HV₆O₁₇]'''. The fourth hydrogen atom in hexavanadic acid is not easily replaceable by a metal. The change takes place in the following sequence, commencing with the VO₄''' ion supplied by solution of the orthovanadate:

1. 2VO₄''' + 2H^• → V₂O₇'''' + 2H₂O (pyrovanadate).
2. 3V₂O₇'''' + 6H^• → V₃O₉''' + 3H₂O (metavanadate).
3. V₃O₉''' + H^• → HV₆O₁₇''' + H₂O (hexavanadate).

Hexavanadic acid is also stated to be formed in solution by the decomposition of pervanadic acid, which is produced when vanadium pentoxide is treated with hydrogen peroxide, but more recently the properties of the solution have been attributed to the formation of peroxyorthovanadic acid, HVO₄.H₂O. According to Dullberg the compound which has the composition Na₂O.2V₂O₅.9H₂O should be formulated as the trisodium salt of hexavanadic acid, Na₃HV₆O₁₇.13H₂O; similarly the compound which has the composition Na₂O.3V₂O₅.3H₂O should be formulated as the disodium salt of hexavanadic acid, Na₂H₂V₆O₁₇.2H₂O. It is of some interest to note that many of the heteropoly-acid compounds which contain vanadium can also be written as being derived from hexavanadic acid, although this theory of their constitution is not now held. The hexavanadic acid theory does not exclude the possibility of the existence of other more highly condensed acids. Many of the alkali poly vanadates can be prepared in two crystalline forms: (a) Orange, transparent crystals, and (b) golden, scaly masses, with a metallic lustre. It is suggested that the latter are derived from the more highly condensed acids.

All the vanadates are powerful oxidising agents and undergo reduction in acid solution in the manner already described for vanadium pentoxide. The alkali vanadates are usually easily soluble in water, and are white or pale yellow, crystalline compounds, and frequently dimorphous. They are insoluble in alcohol, which is often employed to throw them out from solution. When ammonia gas is passed over the vanadates of the heavy metals, or when the latter are digested in liquid ammonia, slow absorption ensues with formation of hexammine addition compounds; usually six molecules of ammonia are taken up for each atom of metal present.