Pyrovanadates, R^•₄V₂O₇

Pyrovanadates, R^•₄V₂O₇ or 2R^•₂O.V₂O₅. The alkali pyrovanadates are prepared by dissolving the equivalent quantity of vanadium pentoxide in solutions of alkalis, or by the spontaneous decomposition in solution of the alkali orthovanadates. Pyrovanadates of other metals are obtained by fusing vanadium pentoxide with the salts or hydroxides of the metals in molecular proportions, or, when they are sufficiently insoluble, by double decomposition between an alkali pyrovanadate and a salt of the metal required.

Pyrovanadates are more stable than orthovanadates, but, in consequence of the weakly acid character of pyrovanadic acid, they undergo easy conversion into the metavanadates. Sodium pyrovanadate in solution is thus converted by carbon dioxide into the metavanadate:

Na₄V₂O₇ + CO₂ = 2NaVO₃ + Na₂CO₃. Ammonium pyrovanadate does not appear to exist; addition of ammonium chloride to a solution of a pyrovanadate precipitates ammonium metavanadate:

4NH₄Cl + Na₄V₂O₇ = 2NH₄VO₃ + 4NaCl + 2NH₃ + H₂O.

These reactions suggest that sodium pyrovanadate undergoes partial hydrolysis in solution:

Na₄V₂O₇ + H₂O ⇔ 2NaVO₃ + 2NaOH.

The introduction of carbon dioxide into the solution removes the caustic soda (or, from the point of view of the ionic theory, the OH' ions), so that the equilibrium is disturbed and the reaction then proceeds completely from left to right. Similarly, addition of ionised ammonium chloride suppresses the concentration of the OH' ions already present in solution; more of the pyrovanadate therefore undergoes hydrolysis, in order that the equilibrium concentrations of ions shall be maintained, until all the pyrovanadate is converted into metavanadate.

The weakly acid nature of pyrovanadic acid is also shown by the fact that solutions of pyrovanadates react alkaline to phenolphthalein. The solutions have been shown to contain the colourless (V₂O₇)'''' ion. Silver pyrovanadate gives esters of pyrovanadic acid only with the higher alkyl halides, e.g. amyl pyrovanadate, (C₅H₁₁)₄V₂O₇. The following pyrovanadates have been prepared:

Barium Pyrovanadate, Ba₂V₂O₇, is precipitated on addition of barium chloride to a solution of sodium pyrovanadate or of other vanadates in the presence of ammonia. It has more recently been prepared by the action of barium peroxide on vanadium pentoxide. It is a white, amorphous powder which melts above 863° C.

5H₂O.Calcium Pyrovanadate, 2Ca₂V₂O₇.5H₂O, is formed by adding calcium chloride to a solution of sodium pyrovanadate and drying the precipitate at 100° C. The dihydrate, Ca₂V₂O₇.2H₂O, has been obtained in transparent needles by adding calcium chloride to ammonium metavanadate solution and then excess of ammonium hydroxide.

3H₂O.Copper Pyrovanadate, Cu₂V₂O₇.3H₂O, has been obtained in greenish- yellow, transparent, rhombohedral plates by the action of copper sulphate on ammonium metavanadate. The anhydrous salt has also been prepared by saturating a solution of vanadium pentoxide and a copper salt with ammonia. According to Radau, addition of copper sulphate to solutions of pyrovanadates gives rise to precipitates the composition of which approximates to the formula 8CuO.3V₂O₅.

This salt is of interest in that it is the artificial form of descloizite, one of the important natural ores of vanadium. It is obtained by boiling mixed solutions of lead nitrate and ammonium metavanadate in the presence of acetic acid. A pale yellow basic pyrovanadate of lead, 2Pb₂V₂O₇.PbO, is obtained by the addition of lead acetate to a solution of a vanadate. The composition of the compound produced in this manner is, however, not constant.

6H₂O.When a solution of sodium metavanadate is made strongly alkaline with lithium hydroxide and concentrated in vacuo, white, silky needles of the Hexahydrate are obtained, which leave a white, nacreous mass of the anhydrous salt, Li₄V₂O₇, on being heated. The tetrahydrate, Li₄V₂O₇.4H₂O, has been obtained by fusing vanadium pentoxide with lithium nitrate and extracting the melt with water, in which it is readily soluble.

Manganese Pyrovanadate, Mn₂V₂O₇, is obtained in large, brilliant brown needles by fusing together a mixture of vanadium pentoxide, sodium bromide and manganese bromide.

4H₂O.Potassium Pyrovanadate, K₄V₂O₇.4H₂O, separates out in colourless, transparent crystals when one molecular proportion of vanadium pentoxide is dissolved in two molecular proportions of potassium hydroxide. These lose water when heated, melt, and leave a crystalline mass of the anhydrous salt K₄V₂O₇, which is deliquescent and melts at 910° C. A white, crystalline, readily soluble trihydrate, K₄V₂O₇.3H₂O, has also been prepared.

Silver Pyrovanadate, Ag₄V₂O₇, is obtained as a dense yellow precipitate by the addition of neutral silver nitrate to sodium pyrovanadate solution. It has also been obtained as brilliant yellow, transparent plates. M.pt. 383° C.

18H₂O.Sodium Pyrovanadate, Na₄V₂O₇.18H₂O, is prepared by fusing vanadium pentoxide (1 mol.) with sodium carbonate (2 mols.) and extracting the melt with water. Alternatively, the vanadium pentoxide is dissolved in caustic soda solution, using the same molecular proportions. It forms long, six-sided plates, or pearly, glistening needles which are efflorescent and readily soluble in water. At 100° C. it loses seventeen molecules of water, the last molecule being evolved at 140° C. The anhydrous salt melts at 654° or 632° C. By partial dehydration, and crystallisation from a mixture of alcohol and water, an octahydrate, Na₄V₂O₇.8H₂O, has been obtained.

Thallium Pyrovanadatei Tl₄V₂O₇, is precipitated as a light yellow powder by the addition of thallium sulphate to a cold, saturated solution of sodium orthovanadate. It also results on fusing a mixture of vanadium pentoxide and thallium carbonate. It melts at 454° or 416° C., and is soluble in about 5000 parts of water at 14° C.

Thorium Pyrovanadate, Th₂V₂O₇.6H₂O, is greenish-yellow, and is obtained by the action of thorium chloride in dilute solution on ammonium metavanadate.

Zinc Pyrovanadate, Zn₂V₂O₇, forms orange-red prisms when vanadium pentoxide is fused with a mixture of sodium bromide and zinc bromide. It is appreciably soluble in water.

Several vanadates are known the compositions of which are intermediate between that of the pyro vanadates, 2R^•₂O.V₂O₅, and that of the metavanadates, R^•₂O.V₂O₅. They can be looked upon as basic metavanadates. The following have been described:

Basic Lithium Metavanadate, 3Li₂O.2V₂O₅.15H₂O or Li₂O.4LiVO₃.15H₂O, is obtained by crystallising lithium orthovanadate from dilute nitric acid.

Basic Potassium Metavanadate, 5K₂O.4V₂O₅.7H₂O or K₂O.8KVO₃.7H₂O, separates in white crystals when a solution of potassium pyrovanadate, K₄V₂O₇, is acidified with acetic acid and concentrated.

Basic Sodium Metavanadate, 3Na₂O.2V₂O₅.2H₂O or Na₂O.4NaVO₃.2H₂O, and the hexahydrate, Na₂O.4NaVO₃.6H₂O, have both been prepared by fusing sodium carbonate (3 mols.) with vanadium pentoxide (2 mols. ). They are not readily soluble in water.

Basic Thallium Metavanadates, 3Tl₂O.2V₂O₅ or Tl₂O.4TlVO₃, is obtained as a sparingly soluble, yellow powder by the action of thallium sulphate on the corresponding sodium compound.

6Tl₂O.SV₂O₅ or Tl₂O.10TlVO₃ is prepared by the action of thallium sulphate on sodium pyrovanadate, Na₄V₂O₇, in the presence of excess of vanadium pentoxide. It dissolves in 9372 parts of water at 11° C. and in 3366 parts of water at 100° C.

Basic Silver Metavanadate, 3Ag₂O.2V₂O₅ or Ag₂O.4AgVO₃, is prepared by the action of silver nitrate on the corresponding sodium salt. It is a dark yellow compound, almost insoluble in water.

Basic Lead Metavanadate, 3PbO.2V₂O₅.2H₂O or PbO.2Pb(VO₃)₂. 2H₂O, is obtained as a yellow powder by the action of lead nitrate on an acid manganese vanadate.

Basic Strontium Metavanadate, 3SrO.2V₂O₅.2H₂O or SrO.2Sr(VO₃)₂.2H₂O, has also been prepared.