Detection of Tungsten

The analysis of materials containing tungsten is comparatively simple owing to the ease with which the metal can be converted into soluble alkali tungstates, and to the fact that the most stable oxide, WO₃, is almost insoluble in all mineral acids with the exception of hydrofluoric and phosphoric acids. The element is usually met with in its ores, chiefly wolframite and scheelite; in alloys, especially ferro-tungsten and tungsten steels; as the metal - tungsten powder, tungsten electric light filaments; as alkali tungstates and tungstic anhydride, WO₃.

The finely powdered material is fused with excess of sodium carbonate or potassium hydrogen sulphate in a platinum crucible, or better, since there is serious loss of platinum in fusions of tungsten alloys, with potassium hydroxide in a silver or nickel crucible. The cooled mass is extracted with boiling water and filtered. On acidifying with concentrated hydrochloric acid, and reducing by means of metallic tin, a blue colour or precipitate indicates the presence of tungsten. If the reduction is continued for two days the colour becomes brown, due to the formation of tungsten dioxide. Other reducing agents, such as zinc, aluminium, or stannous chloride, may also be used, but the action of tin is most satisfactory and can easily be controlled.

The only other metals likely to give a blue colour are columbium, vanadium, molybdenum, and titanium. The colour due to columbium disappears on dilution. The colour due to vanadium may be obtained by means of tartaric acid, which gives no colour with tungsten. The colour formed on reduction of molybdenum is from violet to black, while that due to titanium is violet; these elements can easily be separated from the tungsten before the reduction test is made.

Solutions of alkali tungstates are precipitated by dilute mineral acids, yellow tungstic acid H₂WO₄ or the white hydrate H₂WO₄.H₂O being formed. The precipitate readily dissolves in alkalies, and on reduction yields the blue oxide.

A microchemical method for the identification of the tungstic acid precipitate consists in dissolving a portion in a few drops of concentrated ammonia and placing one drop on a slide; ammonium paratungstate soon crystallises as four-cornered plates on the edges of the drop, and as long needles in the centre.