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12-Tungstophosphoric Acid, H7[P(W2O7)6]

12-Tungstophosphoric Acid, H7[P(W2O7)6].xH2O, ordinary phosphotungstic acid, (P2O5:WO3 = 1:24), is analogous to 12-molybdo-phosphoric acid and similar to metatungstic acid. It may be prepared by dissolving 500 grams of pure sodium tungstate and 250 grams of sodium phosphate crystals in 500 c.c. of water and evaporating until a skin forms on the surface; 750 c.c. of hydrochloric acid are then added and the mixture again evaporated and allowed to cool, when the acid may be shaken out with ether after evaporating the ether and crystallising from water, the acid is obtained in large octahedral crystals of composition H7[P(W2O7)6].28H2O. This hydrate is not stable at ordinary temperatures and readily changes into a 22-hydrate, which crystallises in rhombohedra from solutions maintained at 50° C. The 28-hydrate may be kept in a stable condition at ordinary temperatures by the addition of 1 per cent, of the stable 28-hydrate of 12-molybdophosphoric acid. If concentrated nitric acid is added to a solution of the acid, small prismatic crystals of a 19-hydrate are precipitated. The acid melts at about 90° C. It is very soluble in water, yielding dense solutions. The equivalent conductivities of solutions containing 1/7 molecule in v litres at 25° C. have been found to be:

v =32641282565121024
Λ =168.9181.0198.4220.3249.3274.5

The hydrogen-ion concentration in a 0.005 N solution is 4.1×10-3. The formula for the acid given above suggests that its basicity should be 7, and the neutralisation curves of solutions with sodium hydroxide show that it is at least hexabasic. However, only acid salts of the metals have been prepared, but the normal guanidinium salt, (CN3H6)7 [P(W2O7)6].12H2O, has been obtained by the addition of guanidinium carbonate to the free acid.

The alkali salts of this acid may be prepared:
  1. By the addition of the alkali hydroxide or carbonate to a solution of the free acid;
  2. By saturation of a boiling solution of alkali phosphate with tungstic anhydride; or
  3. By the addition of a mineral acid, preferably hydrochloric acid, to a solution of alkali tungstate containing a small quantity of alkali phosphate.
The salts obtained correspond to the general formula 3R2O.P2O5.24WO3.xH2O (R = NH4, K, Na). The ammonium and potassium salts are white, crystalline, almost insoluble powders; the sodium salt yields large transparent crystals which become opaque on exposure to air and are readily soluble in water. The latter salt forms two hydrates, the one stable at ordinary temperatures having the composition Na3H4[P(W2O7)6].13H2O, whilst at lower temperatures a hydrate containing 19H2O is the stable form. That these salts are acid salts and not normal is confirmed by equivalent conductivity measurements. The corresponding salts of barium, copper, and silver have been prepared, as also have the more strongly acid salts, 2BaO.P2O5.24WO3.59H2O, BaO.P2O5.24WO3.59H2O, and Ag2O,P2O5.24WO3.60H2O; the silver salts are insoluble in water. In some of the earlier literature salts of this series are described under formulae of the type xR2O.P2O5. 20WO3.yH2O. Phosphotungstic acid and the phosphotungstates are used as reagents for the precipitation of alkaloids and proteins, and react with many other organic compounds. The acid is also used as a clarifying agent in urine analysis. Potassium and ammonium salts may be detected by means of phosphotungstic acid, with which they give insoluble precipitates.

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