Aluminium hydroxide

Aluminium hydroxide
IUPAC name	Aluminium hydroxide
Other names	Aluminium(III) hydroxide, Hydrated alumina
Identifiers
CAS number	[21645-51-2]
RTECS number	BD0940000
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
Solubility in water	0.0001 g/100 mL (20 °C)
Solubility	soluble in acids, alkalies, HCl, HSO4
Acidity (pKa)	>7
Hazards
MSDS	External MSDS
EU classification	Irritant (I) Xi
R-phrases	R36, R37, R38
S-phrases	S26, S36
NFPA 704	0 1 0
Flash point	Non-flammable.
Related compounds
Other anions	None.
Related compounds	Sodium oxide, aluminium oxide hydroxide.
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, is the most stable form of aluminium in normal conditions. It is found in nature as the mineral gibbsite (also known as hydrargillite) and its three, much more rare, 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. Heat-dried aluminium hydroxide powder is known as activated alumina and is used in gas purification, as a catalyst support and an abrasive.

Production

Aluminium hydroxide is an intermediate product in the Bayer process. In this process, bauxite is dissolved in hot sodium hydroxide solution, and insolubilities are filtered off. On cooling, aluminium hydroxide precipitates. The aluminium hydroxide is further calcined to give alumina, which may be smelted in the Hall-Héroult process in order to produce aluminium.

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.^[1]

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

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 contributes to induction of a good antibody (Th2) response. However, it has little capacity to stimulate cellular (Th1) immune responses, important for protection against many pathogens.^[2]

Because the brain lesions found in Alzheimer's disease contain aluminium, there is concern that consumption of excess aluminium compounds may cause or contribute to the development of this and other neurodegenerative diseases.^[3][4]

In addition, elevated aluminium levels in blood, resulting from kidney dialysis with well water containing high aluminium, result in dementia that is similar to but probably different from that of Alzheimer's disease.^[5]. 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.^[6]

References

• Galbraith, A; Bullock, S; Manias, E. Hunt, B. & Richards, A. (1999). Fundamentals of pharmacology: a text for nurses and health professionals. Harlow: Pearson. pp. 482. 

1. ^ A. F. Wells, Structural Inorganic Chemistry, 4th. edition 1975, Oxford University Press.
2. ^ Petrovsky N, Aguilar JC. (2004). "Vaccine adjuvants: current state and future trends". Immunol Cell Biol. 82 (5): 488–96. doi:10.1111/j.0818-9641.2004.01272.x. 
3. ^ Perl DP, Moalem S (2006). "Aluminum and Alzheimer's disease, a personal perspective after 25 years". J. Alzheimers Dis. 9 (3 Suppl): 291–300. PMID 17004365. http://iospress.metapress.com/openurl.asp?genre=article&issn=1387-2877&volume=9&issue=3&spage=291. 
4. ^ Kawahara M (November 2005). "Effects of aluminum on the nervous system and its possible link with neurodegenerative diseases". J. Alzheimers Dis. 8 (2): 171–82; discussion 209–15. PMID 16308486. http://iospress.metapress.com/openurl.asp?genre=article&issn=1387-2877&volume=8&issue=2&spage=171. 
5. ^ Carpenter DO (2001). "Effects of metals on the nervous system of humans and animals". Int J Occup Med Environ Health 14 (3): 209–18. PMID 11764847. 
6. ^ Petrik MS, Wong MC, Tabata RC, Garry RF, Shaw CA. (2007). "Aluminum adjuvant linked to Gulf War illness induces motor neuron death in mice.". Neuromolecular Med. 9 (9): 83–100. doi:10.1385/NMM:9:1:83. 

External links

• International Chemical Safety Card 0373