3-Tungstophosphates, (P2O5:WO3 = 1:6). - From a concentrated solution containing sodium tungstate and phosphate, made faintly acid with acetic acid and allowed to stand for a few days, a mass of crystals, containing two salts, separates. On washing with water, the sodium salt of this series, 3Na2O.P2O5.6WO3.aq., dissolves, leaving a very sparingly soluble salt, the composition of which appears to be 14Na3O.5P2O5.19WO3.aq., or 29Na2O.10P2O5.39WO3.aq.
The soluble sodium 3-tungstophosphate may be obtained pure by repeated recrystallisation, and yields aggregates of white striated prisms of composition 3Na2O.3H2O.P2O5.6WO3 + 13H2O. As this formula indicates, 3 molecules of water appear to be firmly held as water of constitution. The corresponding potassium and ammonium salts, 3K2O.3H2O.P2O5.6WO3 + 11H2O and 3(NH4)2O.3H2O.P2O5.6WO3 + 6H2O, are prepared by adding potassium or ammonium chloride to a solution of the sodium salt, than which they are much less soluble. The former yields brilliant prismatic needles, the latter short stout prisms. The free acid has not been prepared. |