White Tungstic Acid Preparation Method
White tungstic acid is white microcrystalline powder with significant activity, little light sensitivity, easily to be reduced. White tungstic acid can be used in the preparation of lots of isopolyacid and heteropoly acid, and can also be used for the manufacture of tungsten carbide, low valence tungsten cluster metal compounds ect..
White tungstic acid will get when the acidity of reacting solution becomes low, while when adding acid into sodium tungstate (Na2WO4), no matter the hydrochloric, sulfuric, nitric and perchloric or trichloroformic acid ect., there is always colloidal white tungstic acid generated. Which is the part reaction, after stirring hydrochloric acid will soon react with the rest tungstic and low polymerization tungstate ions to form a high level of polyacid ions. Since the soluble of polyphosphate and retaining in the solution, the produced white tungstic acid can be seen but can not be caught. While since the part reaction often occurs very fast with the high concentration, so the white tungstic acid is always being colloidal.
The preparation of white tungstic acid by reverse addition method, which is adding sodium tungstate solution into dilute nitric acid with stirring; for making the slow reaction. The white tungstic acid get by this method has the fine grain and can be filtered while the precipitation speed is not too fast, however the washing is not easy; for speeding up the precipitation and the product easily to be washed, a small amount of aluminum nitrate solution is supposed to be added for preventing gelatinization and speeding up precipitation condensation after the sodium tungstate is added into nitric acid completely. After placing for 3~4 hours, the product can be precipitated, and washed with water for some times, and finally washed with ethanol or acetone and drained (Noted: The washing with ethanol or acetone will maintain the tungstic acid in white).