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Tungsten(VI) oxide

Tungsten trioxide
IUPAC name Tungsten trioxide
Other names Tungstic anhydride, tungsten (VI) oxide,
tungstic oxide
CAS number 1314-35-8
RTECS number YO7760000
Molecular formula WO3
Molar mass 231.84 g/mol
Appearance Yellow powder
Density 7.16 g/cm3
Melting point

1473 °C

Boiling point

~1700 °C

Solubility in water Insoluble
Solubility in other solvents Alkaline solutions, slightly soluble
in acids
Crystal structure Monoclinic
Trigonal planar
MSDS External MSDS
Main hazards May cause eye/skin,
respiratory, and digestive tract irritation.
R-phrases R22-R36/37/38
S-phrases S26-S36
Flash point N/A
Related Compounds
Related compounds WO2, H2WO4 (tungstic acid)
Except where noted otherwise, data are given for
materials in their standard state
(at 25 °C, 100 kPa)

Infobox disclaimer and references

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.

2WO3 + 3C + heat → 2W + 3CO2
WO3 + 3H2 + heat → W + 3H2O



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].


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].

CaWO4 + 2HCl → CaCl2 + H2WO4
H2WO4 + heat → H2O + WO3

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

(NH4)10[H2W12O42]•4H2O → 12 WO3 + 10NH3 + 11H2O


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 (WO3-x) results[3]. This compound is a mixture of different constituents including tungsten trioxide, ammonium and WO2.


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.


  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.
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Tungsten(VI)_oxide". A list of authors is available in Wikipedia.
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