Tungsten Trioxide
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. 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 ¡ú 2W + 3CO2
WO3 + 3H2 + heat ¡ú W + 3H2O
Tungsten(VI) oxide occurs naturally in form of hydrates, which include minerals: tungstite WO3¡¤H2O, meymacite WO3¡¤2H2O and hydrotungstite (of same composition as meymacite, however sometimes written as H2WO4). These minerals are rare to very rare secondary tungsten minerals.
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.
In 1841, a chemist named Robert Oxland gave the first procedures for preparing tungsten trioxide and sodium tungstate. He was granted patents for his work soon after, and is considered to be the founder of systematic tungsten chemistry.
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.
CaWO4 + 2HCl ¡ú CaCl2 + H2WO4
H2WO4 + heat ¡ú H2O + WO3
Another common way to synthesize WO3 is by calcination of ammonium paratungstate (APT) under oxidizing conditions:
(NH4)10[H2W12O42]•4H2O ¡ú 12 WO3 + 10NH3 + 11H2O
Structure
The crystal structure of tungsten trioxide is temperature dependent. It is tetragonal at temperatures above 740 ¡ãC, orthorhombic from 330 to 740 ¡ãC, monoclinic from 17 to 330 ¡ãC, and triclinic from -50 to 17 ¡ãC. The most common structure of WO3 is monoclinic with space group P21/n.
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. 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. 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 screen phosphors and also for fireproofing fabrics. Due to its rich yellow color, WO3 is also used as a pigment in ceramics and paints.
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. This allows the user to tint their windows, changing the amount of heat or light passing through. Another new use for tungsten is in Dense Inert Metal Explosives, the first notable use of these devices was during the 2008¨C2009 Israel¨CGaza conflict.
COLLETED BY YAMA FROM WWW.TUNGSTEN-CARBIDE.COM.CN
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