Aluminium hydroxide

Aluminium hydroxide
File:Gibbsite-crystal-3D-balls.png
File:Hydroxid hlinitý.PNG
IUPAC name Aluminium hydroxide
Other names Aluminium(III) hydroxide, Hydrated alumina
style="background: #F8EABA; text-align: center;" colspan="2" | Identifiers
CAS number	21645-51-2 Y
ChemSpider	8351587
RTECS number	BD0940000
SMILES
InChI
InChI key	WNROFYMDJYEPJX-DFZHHIFOAJ
style="background: #F8EABA; text-align: center;" colspan="2" | Properties
Molecular formula	Al(OH)3
Molar mass	78.00 g/mol
Appearance	White amorphous powder
Density	2.42 g/cm³, solid
Melting point	300 °C, 573 K, 572 °F
Solubility in water	0.0001 g/100 mL (20 °C)
Solubility	soluble in acids, alkalis, HCl, H2SO4
Acidity (pKa)	>7
style="background: #F8EABA; text-align: center;" colspan="2" | Hazards
MSDS	External MSDS
EU classification	Irritant (I) Xi
R-phrases	R36 R37 R38
S-phrases	S26 S36
NFPA 704	75px 0 1 0
Flash point	Non-flammable
style="background: #F8EABA; text-align: center;" colspan="2" | Related compounds
Other anions	None
Related compounds	Sodium oxide, aluminium oxide hydroxide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Aluminium hydroxide, Al(OH)₃, ATH, sometimes erroneously called Hydrate of alumina, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three, much more rare forms, polymorphs: bayerite, doyleite and nordstrandite. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide, Al₂O₃, differing only by loss of water. These compounds together are the major components of the aluminium ore bauxite. Freshly precipitated aluminium hydroxide forms gels, which is the basis for application of aluminium salts as flocculants in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be dehydrated (e.g., with the utility of water-miscible non-aqueous solvents like ethanol) to form an amorphous aluminium hydroxide powder, which is readily soluble in acids. Aluminium hydroxide powder which has been heated to an elevated temperature under carefully controlled conditions is known as activated alumina and is used as a desiccant, an adsorbent, in gas purification, as a Claus catalyst support, water purification, and an adsorbent for the catalyst during the manufacture of polyethylene by the Sclairtech process.

Chemistry

Gibbsite has a typical metal hydroxide structure with hydrogen bonds. It is built up of double layers of hydroxyl groups with aluminium ions occupying two-thirds of the octahedral holes between the two layers.

Aluminium hydroxide is amphoteric. It dissolves in acid, forming Al(H₂O)₆³⁺ (hexaaquaaluminate) or its hydrolysis products. It also dissolves in strong alkali, forming Al(OH)₄^- (tetrahydroxoaluminate).

Production

Virtually all the aluminium hydroxide used commercially is manufactured by the Bayer process which involves dissolving bauxite in sodium hydroxide as the first step. It is marketed as Alu-Tabs.

Uses

Annual production is some 100 million tonnes, over 90% of which is converted to aluminium oxide (alumina) which is used in the manufacture of aluminium metal.

The major other uses of aluminium hydroxide is as a feedstock for the manufacture of other aluminium compounds: specialty calcined aluminas, aluminium sulfate, polyaluminium chloride, aluminium chloride, zeolites, sodium aluminate, activated alumina, aluminium nitrate.

Use as a fire retardant

Aluminium hydroxide also finds use as a fire retardant filler for polymer applications in a similar way to magnesium hydroxide and hydromagnesite. It decomposes at about 180 °C, absorbing a considerable amount of heat in the process and giving off water vapour. In addition to behaving as a fire retardant, it is very effective as a smoke suppressant in a wide range of polymers, most especially in polyesters, acrylics, ethylene vinyl acetate, epoxies, PVC, rubber.

Pharmacology

Pharmacologically, this compound is used as an antacid under names such as Alu-Cap, Aludrox or Pepsamar. The hydroxide reacts with excess acid in the stomach, reducing its acidity. This decrease of acidity of the contents of the stomach may in turn help to relieve the symptoms of ulcers, heartburn or dyspepsia. It can also cause constipation and is therefore often used with magnesium hydroxide or magnesium carbonate, which have counterbalancing laxative effects. This compound is also used to control phosphate (phosphorus) levels in the blood of people suffering from kidney failure.

Aluminium hydroxide is included as an adjuvant in some vaccines (e.g., Alhydrogel, Anthrax Vaccine), since it appears to contribute to induction of a good antibody (Th2) response. Its pharmacological action is not known. However, it has little capacity to stimulate cellular (Th1) immune responses, important for protection against many pathogens.

Because the brain lesions found in Alzheimer's disease sometimes contain traces of aluminium, there is concern that consumption of excess aluminium compounds may cause or contribute to the development of this and other neurodegenerative diseases. However, multiple epidemiological studies have found no connection between exposure to aluminium and neurological disorders.

In addition, elevated aluminium levels in blood, resulting from kidney dialysis with well water containing high aluminium, may result in dementia that is similar to but probably different from that of Alzheimer's disease.. However, this hypothesis is controversial.

In 2007, tests with mice of the anthrax vaccine using aluminium hydroxide adjuvant were reported as resulting in adverse neuropathy symptoms.

External links

• International Chemical Safety Card 0373

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