Tungsten(VI) oxide

Tungsten(VI) oxide, also known as tungsten trioxide or tungstic anhydride, WO₃, is a chemical compound containing oxygen and the transition metal tungsten. It is obtained as an intermediate in the recovery of tungsten from its minerals.^[1] Tungsten ores are treated with alkalis to produce WO₃. Further reaction with carbon or hydrogen gas reduces tungsten trioxide to the pure metal.

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History

Tungsten has a rich history dating back to its discovery during the 18th century. Peter Woulfe was the first to recognize a new element in the naturally occurring mineral, wolframite. Tungsten was originally known as wolfram, explaining the choice of "W" for its elemental symbol. Swedish chemist Carl Wilhelm Scheele contributed to its discovery as well with his studies on the mineral scheelite^[1].

In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate^[2]. He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry^[2].

Preparation

Tungsten trioxide can be prepared in several different ways. CaWO₄, or scheelite, is allowed to react with HCl to produce tungstic acid, which decomposes to WO₃ and water at high temperatures^[1].

Another common way to synthesize WO₃ is by calcination of ammonium paratungstate (APT) under oxidizing conditions^[2]:

Structure

The crystal structure of tungsten trioxide is temperature dependent. It is tetragonal at temperatures above 740 °C, orthorhombic from 330 to 740 °C, monoclinic from 17 to 330 °C, and triclinic from -50 to 17 °C. The most common structure of WO₃ is monoclinic with space group P2₁/n^[2].

Chemical Properties

As mentioned above, tungsten trioxide can be synthesized by calcination of APT. Depending upon reaction conditions such as rate and temperature, the chemical properties of WO₃ vary. For instance, at low temperatures the tungsten trioxide produced is highly reactive and dissolves easily in H₂O^[2]. At higher temperatures, it does not dissolve in water and is characterized by coarse grains. If the calcination reaction is performed under reducing instead of oxidizing conditions, an entirely different product called tungsten blue oxide (WO_3-x) results^[3]. This compound is a mixture of different constituents including tungsten trioxide, ammonium and WO₂.

Uses

Tungsten trioxide is used for many purposes in everyday life. It is frequently used in industry to manufacture tungstates for x-ray screens and also for fireproofing fabrics^[4]. Due to its rich yellow color, WO₃ is also used as a pigment in ceramics and paints^[1].

In recent years, tungsten trioxide has been employed in the production of electrochromic windows, or smart windows. These windows are electrically switchable glass that change light transmission properties with an applied voltage^[5]. Basically, this allows the user to tint their windows and change the amount of heat or light passing through.

References

1. ^ ^{a b c d} Patnaik, Pradyot. Handbook of Inorganic Chemicals. New York: McGraw-Hill, 2003.
2. ^ ^{a b c d e} Lassner, Erik and Wolf-Dieter Schubert. Tungsten: Properties, Chemistry, Technology of the Element, Alloys, and Chemical Compounds. New York: Kluwer Academic, 1999.
3. ^ "Tungsten Oxides & Acids" International Tungsten Industry Association. 2003.
4. ^ "Tungsten trioxide." The Merck Index Vol 14, 2006.
5. ^ Lee, W.J.; Fang, Y.K.; Ho, J.; Hsieh, W.T.; Ting, S.F. J. Electron. Mater. 2000, 29 (2), 183.