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Atomistry » Tungsten » Compounds » 12-Tungstophosphoric Acid | ||||||||||||||||
Atomistry » Tungsten » Compounds » 12-Tungstophosphoric Acid » |
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:
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:
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