Tungsten Trioxide and Tungsten Dioxide

There are four stable tungsten oxides: yellow tungsten oxide (WO₃), blue tungsten oxide (WO_2.90), violet tungsten oxide (WO_2.72) and tungsten dioxide (WO₂).

Tungsten trioxide is lemon yellow crystalline powder, its density is 7.2 ~ 7.4g / cm3, its melting point is about 1470 ℃, boiling point is 1700 ~ 2000 ℃. Tungsten trioxide becomes 202.8 cal / mol at 800 ℃ sublimation. Tungsten anhydride can slightly soluble in water (0.2g / L), insoluble in all inorganic acids except hydrofluoric acid. Acid soluble of caustic solution (NaOH or KOH) and ammonia can form tungstate [NaWO₄, K₂WO₄ and (NH₄) 2WO₄]. In ammonia solution of tungsten trioxide dissolves slowly and dissolves more slowly at a high temperature. At room temperature, even a small amount of organic matter will be able to restore it, and change its color. But in the air, heating time and restore the original color at 700 ~ 900 ℃, the tungsten trioxide can easily restore in hydrogen.

Tungsten dioxide is brown powder, its density is 10.9 ~ 11.1g / cm3, boiling point is about 1700 ℃, generate heat is 134 kcal / mol. The reduction of tungsten trioxide becomes tungsten dioxide with the hydrogen at 575 ~ 600 ℃. Tungsten dioxide is rapidly oxidized tungsten trioxide in the air and heated to blue in the oxide at 500 ℃., tungsten carbon dioxide can be reduced to metallic tungsten at 1020 ℃.

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