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Element Index (A)
Actinium
Aluminium
Americium
Ammonium
Antimony
Arsenic
Chemical's truth
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Combinatorial chemistry is the study of techniques capable of rapidly synthesizing and generating millions of related compounds. |
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Aluminium oxide
(corundum, alumina, aluminium sesquioxide)
CAS no: 1344-28-1 Formula: Al2O3 Molecular weight: 101.961
Physical Data
Appearance: White powder with hexagonal crystalline structure.
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Melting point: 2053°C
Boiling point: ~3000°C
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Density: 3.97 g/cm3 (alpha)
Solubility: -
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Thermodynamic Data
Enthalpy of formation: -1675.7 kJ/mol
Gibbs energy of formation: -1582.3 kJ/mol-
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Entropy: 50.9 J/mol K
Heat capacity: 79.0 J/mol K
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Production and preparation
Aluminium oxide occurs in nature as the mineral corundum or emery. The most common ore for aluminium is bauxite, which contains about 50% of the oxide.
The reddish, clay-like ore was named after the French village, Les Baux, where it was first discovered by P. Berthier in 1821.
It is produced industrially by heating aluminium hydroxide at high temperature (~1200°C). Heating at a lower temperature (500-800°C) will give the less stable gamma form of the oxide.
Other forms such as beta and delta also exist with the inclusion of alkali metal ions.
In laboratory, the oxide can be prepared by adding a suitable
amount of sodium hydroxide into a soultion of Al(III) salt, such as aluminium chlroide. The white hyrodxide precepitate is then heated in an oven to produce the
oxide.
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Behavior and Chemical Properties
Aluminium oxide is very hard, with Mohr scale of 9 and inert to chemical attacks and a good electrical insulator. Aluminium oxide can exist in several crystal habits, of which the alpha form is the most stable with hexagonal arrangement. The gamma form has a slightly
lower density and readily converts to the alpha form on heating.
It is resistant to most acids, except hydrochloric acid and also form complexes with OH- from alkaline
solutions. Since the oxide can react with both acid and alkali, it is also called an amphoteric oxide. With acid:
With sodium hydroxide, a white precipitate of aluminium hydorxide is initially formed but redissolves in excess sodium hydroxide to give a clear complex sodium aluminate solution:
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History and Uses
Because of its chemical inertness, hardness and high melting point, aluminium oxide is used in abrasive applications (including tooth paste), refractories and ceramics.
The oxide also forms a protective layers in telescope mirror coatings. The oxide is also the major component in the electrolytic production (the Hall-Héroult process) of the aluminium metal.
Larger, natural crystals when doped with impurities are used in jewelry. For instance, red ruby, which contains chromium, Cr(III) and sapphire, which contains traces of titanium and iron that gives the blue coloration.
On the other hand, some synthetic ruby and sapphire have also been produced and are used in laser applications. Pure, powdered aluminium oxide is used in making chromatography plates and columns for analytical uses.
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Hazard, Storage and Handling
Store in a cool, dry place, away from incompatible substances such as hydrochloric acid, alkalis, ethylene oxide, vinyl acetate, halocarbons.
May be harmful in inhalation or ingestion. Dusts may cause eye and respiratory tract irritation. Wear goggles and breathing protection in dusty environment.
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(Last update: February 2005) |
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