Vanado-arsenates

These compounds are analogous to the molybdo- and tungsto-arsenates. They are derived from the hypothetical arseno-acid H₇[AsO₆]. The salts which have hitherto been prepared are all of low vanadium pentoxide content compared with the amount of arsenic pentoxide present, and correspond therefore to the luteo vanado-phosphates. Three series are known. The first has the general formula 2R''O.V₂O₅.As₂O₅.xH₂O or , where R'' may be Mg, Zn, Cu or Co. They form yellow crystals which are obtained by the addition of arsenic pentoxide to solutions of the respective vanadates, or by the action of the carbonate of the metal on the free vanado-arsenic acid (see below). Of the second series only the ammonium and potassium salts are known, viz. (NH₄)₂O.2V₂O₅.As₂O₅.5H₂O or , and K₂O.2V₂O₅.As₂O₅.5H₂O or . These also give rise to yellow crystals. The third series has the general formula 2R₂'O.2V₂O₅.3As₂O₅.xH₂O, where R₂' may be (NH₄)₂, Ca, Sr, Mg, Zn, Mn, Co, Ni. They result from the action of arsenic pentoxide on solutions of vanadates, and form red crystals which, however, are unstable, and are readily converted by water into compounds of the first series.

The free acid of the first series has been obtained in yellow plates by boiling vanadium pentoxide with a solution of arsenic acid. Its composition is V₂O₅.As₂O₅.10H₂O or . Recrystallisation of this hydrate from concentrated nitric acid gives a dihydrate, V₂O₅.As₂O₅.2H₂O, which can be looked upon as the acid . The anhydride, V₂O₅.As₂O₅, is obtained by heating the dihydrate to 440° C.

A number of compounds have been described which are analogous to the molybdo-vanadophosphates, and which contain arsenic for the nuclear atom of the complex anion. In many cases these compounds approximate to the general formula , where x + y = 6; x and y are not necessarily whole numbers, because of the tendency shown by these heteropoly-acid salts to form isomorphous mixtures of simple compounds. Ammonium, barium and thallium salts have been prepared.

These are comparable to the molybdo-vanadoarsenates, and can be represented generally as



where x + y = 6, although x and y are not necessarily whole numbers. Canneri found a definite relation to exist between the composition of the salt obtained and that of the solution from which it separates, whence it is inferred that a condition of equilibrium exists between various salts in solution or between their ions; this equilibrium for fixed concentrations is remarkably sensitive to changes in temperature or acidity. Among other salts three series of ammonium salts have been obtained, which approach to the following formulae:

; ; .

Barium and thallium salts have also been described, and some of the free acids have been isolated.

Canneri has succeeded in preparing a large number of mixed crystals of salts, and in some cases of the free acids, which belong to different series; for example, mixed crystals of tungsto-vanadophosphates and tungsto-vanadoarsenates, of tungsto-vanadophosphates and molybdo-vanadophosphates, of tungsto-vanadoar senates and molybdo-vanadoarsenates; and, finally, mixed crystals of members of all the four series mentioned have been obtained. The mixed crystals have the- same crystalline habit as that of their components, and complete isomorphism exists between any two series which mix in all proportions to form continuous series of mixed crystals containing four or five different oxygenated acid radicals. The crystallographic data of a number of ammonium tungsto-vanadoarsenates have also recently been independently determined, and it has been shown that the crystal form is unaffected when the arsenic is substituted by phosphorus, or when the (W₂O₇)'' radicals are partially substituted by (Mo₂O₇)''.

Several ammonium salts have been prepared, which are most probably isomorphous mixtures of arsenates and phosphates.

Vanado-tungstomolybdoarsenophosphates have been described; these are also isomorphous mixtures of simpler compounds.