Tungstic acid is incompletely precipitated from solutions of tungstates by tannic acid. If, however, phenazone (2,3-dimethyl-l-phenyl-5-pyrazolone) is added to the cold solution after treatment with excess of tannic acid, precipitation is quantitative. This process effects a separation from aluminium, and also from iron, chromium, manganese, zinc, cobalt, and nickel if a double precipitation is used.
The solution of tungstate (200-250 mL) should contain not more than 0.15 g of WO3, and be faintly ammoniacal. Add 6-7 mL of concentrated sulphuric acid and 7-8g of ammonium sulphate, and heat to boiling. Treat with 6 mL of 10 per cent aqueous tannic acid solution, keep the mixture on the water bath for a few minutes, and allow to cool to room temperature. A flocculent dark-brown precipitate separates. When cold, stir in lOmL of a 10 per cent aqueous solution of phenazone. Filter the precipitate through a weighed silica, or porcelain, filtering crucible (Note 1), wash with the special wash liquid (Note 2), and ignite to constant weight at 800-900 °C. Weigh as WO3.
Notes. (1) The filtrate must be colourless. If it is yellow, insufficient phenazone has been added.
(2) The special wash liquid contains 1 mL concentrated sulphuric acid, 10 g ammonium sulphate, and 0.4 g phenazone in 200 mL of water.