Identifying Tungsten Minerals

Reduction Test in Hydrochloric Acid. Treat a tungsten mineral with hydrochloric acid. If it is decomposed by the acid a yellow precipitate of tungsten oxide, WO₃, will result. Acid to the acid a few grains of metallic tin and boil. The hydrogen set free by the action of the hydrochloric acid on the tin serves as a reducing agent and converts the yellow WO₃ to a blue precipitate which is a mixture of the two oxides WO₃ and WO_2. On continued reduction the oxide becomes all WO₂ and is brown in color. The test is similar to the one for niobium, but is to be distinguished from that, since the blue color in the tungsten test does not disappear on dilution of the solution; and further, it turns to brown on continued reduction. If the tungsten mineral is not attacked by hydrochloric acid, its powder must first be thoroughly fused with sodium carbonate. The resulting mass is powdered and digested with water, which will dissolve the sodium unstated formed during the fusion. After filtering the reduction test is made as described above.