Aluminium hydroxide

Chembox new
Name = Aluminium hydroxide
ImageFile = Gibbsite-crystal-3D-balls.png ImageFile2 = Hydroxid hlinitý.png IUPACName = Aluminium(III) hydroxide
OtherNames = Aluminium hydroxide,
Hydrated alumina
Section1 = Chembox Identifiers
CASNo = 21645-51-2
RTECS =
Section2 = Chembox Properties
Formula = Al(OH)₃
MolarMass = 78.00344 g/mol
Appearance = White amorphous powder.
Density = 2.4 g/cm³, solid.
Solubility = Insoluble.
MeltingPt = 300°C
BoilingPt =
pKa =
Section7 = Chembox Hazards
EUClass = Irritant (I)
RPhrases = R36, R37, R38
SPhrases = S26, S36
FlashPt = Non-flammable.
ExternalMSDS = [http://physchem.ox.ac.uk/MSDS/AL/aluminium_hydroxide.html External MSDS]
Section8 = Chembox Related
OtherAnions = None.
OtherCpds = Sodium oxide,
aluminium oxide hydroxide.

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 insoluble 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. [A. F. Wells, "Structural Inorganic Chemistry", 4th. edition 1975, Oxford University Press.]

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 carbonate, which has 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"), 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. [cite journal | author = Petrovsky N, Aguilar JC. | title = Vaccine adjuvants: current state and future trends | journal = Immunol Cell Biol. | year = 2004 | volume = 82 | issue = 5 | pages = 488–96 | doi = 10.1111/j.0818-9641.2004.01272.x]

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. [cite journal |author=Perl DP, Moalem S |title=Aluminum and Alzheimer's disease, a personal perspective after 25 years |journal=J. Alzheimers Dis. |volume=9 |issue=3 Suppl |pages=291–300 |year=2006 |pmid=17004365 |doi= |url=http://iospress.metapress.com/openurl.asp?genre=article&issn=1387-2877&volume=9&issue=3&spage=291] [cite journal |author=Kawahara M |title=Effects of aluminum on the nervous system and its possible link with neurodegenerative diseases |journal=J. Alzheimers Dis. |volume=8 |issue=2 |pages=171–82; discussion 209–15 |year=2005 |month=November |pmid=16308486 |doi= |url=http://iospress.metapress.com/openurl.asp?genre=article&issn=1387-2877&volume=8&issue=2&spage=171] 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. [cite journal |author=Carpenter DO |title=Effects of metals on the nervous system of humans and animals |journal=Int J Occup Med Environ Health |volume=14 |issue=3 |pages=209–18 |year=2001 |pmid=11764847 |doi= |url=] However, this hypothesis is controversial.

References

*

External links

* [http://www.ilo.org/public/english/protection/safework/cis/products/icsc/dtasht/_icsc03/icsc0373.htm International Chemical Safety Card 0373]