How To Purify Tungstic Acid?
The most commonly used method to remove SiO2 and alkali metal and alkaline earth metal impurities in crude tungstic acid is ammonia purification method. When tungstic acid is dissolved in ammonia liquid to generate ammonium tungstate solution, the impurities SiO2, iron hydroxide, manganese hydroxide and calcium in the form of calcium tungstate all enter the insoluble residue.
H2WO4 + 2NH4OH = (NH4)2WO4 + 2H2O
FeCl3 + 3NH4OH = Fe(OH)3¯ + 3NH4Cl
MnCl2 + 2NH4OH = Mn(OH)2¯ + 2NH4Cl
CaCl2 + (NH4)2WO4 = CaWO4¯ + 2NH4Cl
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