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Pyrovanadates, R4V2O7

Pyrovanadates, R4V2O7 or 2R2O.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

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.

Calcium Pyrovanadate, 2Ca2V2O7

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.

Copper Pyrovanadate, Cu2V2O7

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.

Lead Pyrovanadate, Pb2V2O7

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.

Lithium Pyrovanadate, Li4V2O7

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

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

Potassium Pyrovanadate, K4V2O7

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

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.

Sodium Pyrovanadate, Na4V2O7

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

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,H2O

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

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.

Basic Metavanadates

Several vanadates are known the compositions of which are intermediate between that of the pyro vanadates, 2R2O.V2O5, and that of the metavanadates, R2O.V2O5. They can be looked upon as basic metavanadates. The following have been described:

Basic Lithium Metavanadate, 3Li2O,2V2O5,15H2O

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

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

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

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

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

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

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

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