3-Tungstophosphates

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.