Aluminium hydroxide

Aluminium hydroxide, Al(OH)3, is found in nature as the mineral gibbsite (also known as hydrargillite) and its three much rarer polymorphs: bayerite, doyleite, and nordstrandite. Aluminium hydroxide is amphoteric in nature, i.e., it has both basic and acidic properties. Closely related are aluminium oxide hydroxide, AlO(OH), and aluminium oxide or alumina (Al2O3), the latter of which is also amphoteric. These compounds together are the major components of the aluminium ore bauxite.

Aluminium hydroxide
Names
Preferred IUPAC name
Aluminium hydroxide
Systematic IUPAC name
Aluminium(3+) trioxidanide
Other names
Aluminic acid

Aluminic hydroxide
Aluminium(III) hydroxide
Aluminium hydroxide
Aluminum trihydroxide
Hydrated alumina

Orthoaluminic acid
Identifiers
3D model (JSmol)
ChEBI
ChEMBL
ChemSpider
DrugBank
ECHA InfoCard 100.040.433
KEGG
RTECS number
  • BD0940000
UNII
Properties[1][2]
Al(OH)3
Molar mass 78.00 g/mol
Appearance White amorphous powder
Density 2.42 g/cm3, solid
Melting point 300 °C (572 °F; 573 K)
0.0001 g/100 mL
3×10−34
Solubility soluble in acids and alkalis
Acidity (pKa) >7
Isoelectric point 7.7
Thermochemistry[3]
Std enthalpy of
formation (ΔfH⦵298)
 −1277 kJ·mol−1
Pharmacology[4]
A02AB01 (WHO)
  • US: B (No risk in non-human studies)
    Hazards
    Safety data sheet External MSDS
    GHS pictograms
    GHS hazard statements
    		 H319, H335
    P264, P261, P280, P271, P312, P304+340, P305+351+338, P337+313
    NFPA 704 (fire diamond)
    Flammability code 0: Will not burn. E.g. waterHealth code 1: Exposure would cause irritation but only minor residual injury. E.g. turpentineReactivity code 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
    0
    1
    0
    Flash point Non-flammable
    Lethal dose or concentration (LD, LC):
    >5000 mg/kg (rat, oral)
    Related compounds
    Other anions
    		 None
    Related compounds
    		 Sodium oxide,
    aluminium oxide hydroxide
    Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
    N verify (what is YN ?)
    Infobox references

    Nomenclature

    The naming for the different forms of aluminium hydroxide is ambiguous and there is no universal standard. All four polymorphs have a chemical composition of aluminium trihydroxide (one aluminium atom attached to three hydroxide groups).[5]

    Gibbsite is also known as hydrargillite, named after the Greek words for water (hydra) and clay (argylles). The first compound named hydrargillite was thought to be aluminium hydroxide, but was later found to be aluminium phosphate; despite this, both gibbsite and hydrargillite are used to refer to the same polymorphism of aluminium hydroxide, with gibbsite used most commonly in the United States and hydrargillite used more often in Europe. In 1930, it was referred to as α-alumina trihydrate to contrast it with bayerite, which was called β-alumina trihydrate (the alpha and beta designations were used to differentiate the more- and less-common forms respectively). In 1957, a symposium on alumina nomenclature attempted to develop a universal standard, resulting in gibbsite being designated γ-Al(OH)3, bayerite becoming α-Al(OH)3, and nordstrandite being designated Al(OH)3. Based on their crystallographic properties, a suggested nomenclature and designation is for gibbsite to be α-Al(OH)3, bayerite to be designated β-Al(OH)3, and both nordstrandite and doyleite are designated Al(OH)3. Under this designation, the α and β prefixes refer to hexagonal, close-packed structures and altered or dehydrated polymorphisms respectively, with no differentiation between nordstrandite and doyleite.[5]

    Properties

    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.[6][7]

    Aluminium hydroxide is amphoteric. In acid, it acts as a Brønsted–Lowry base by picking up hydrogen ions and neutralizes the acid, yielding a salt:[8]

    3 HCl + Al(OH)3 → AlCl3 + 3 H2O

    In bases, it acts a Lewis acid by taking an electron pair from the hydroxide ions:[8]

    Al(OH)3 + OH− → Al(OH)4−

    Polymorphism

    Four polymorphs of aluminium hydroxide exist, all based on the common combination of one aluminium atom and three hydroxide molecules into different crystalline arrangements that determine the appearance and properties of the compound. The four combinations are:[5]

    All polymorphs are composed of layers of octahedral aluminium hydroxide units with the aluminium atom in the centre and the hydroxyl groups on the sides, with hydrogen bonds holding the layers together. The polymorphs vary in how the layers stack together, with the arrangements of the molecules and layers determined by the acidity, presence of ions (including salt) and the substrate it forms on. Under most conditions, gibbsite is the most chemically stable form of aluminium hydroxide. All forms of Al(OH)3 crystals are hexagonal.[5]

    Production

    Virtually all the aluminium hydroxide used commercially is manufactured by the Bayer process[9] which involves dissolving bauxite in sodium hydroxide at temperatures up to 270 °C (518 °F). The waste solid, bauxite tailings, is removed and aluminium hydroxide is precipitated from the remaining solution of sodium aluminate. This aluminium hydroxide can be converted to aluminium oxide or alumina by calcination.

    The residue or bauxite tailings, which is mostly iron oxide, is highly caustic due to residual sodium hydroxide. It was historically stored in lagoons; this led to the Ajka alumina plant accident in 2010 in Hungary, where a dam bursting led to the drowning of nine people. An additional 122 sought treatment for chemical burns. The mud contaminated 40 square kilometres (15 sq mi) of land and reached the Danube. While the mud was considered non-toxic due to low levels of heavy metals, the associated slurry had a pH of 13.[10]

    Uses

    One of the major uses of aluminium hydroxide is as a feedstock for the manufacture of other aluminium compounds: speciality calcined aluminas, aluminium sulfate, polyaluminium chloride, aluminium chloride, zeolites, sodium aluminate, activated alumina, and aluminium nitrate.[7]

    Freshly precipitated aluminium hydroxide forms gels, which are the basis for the application of aluminium salts as flocculants in water purification. This gel crystallizes with time. Aluminium hydroxide gels can be dehydrated (e.g. using 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, as an adsorbent in gas purification, as a Claus catalyst support for water purification, and as an adsorbent for the catalyst during the manufacture of polyethylene by the Sclairtech process.

    Fire retardant

    Aluminium hydroxide also finds use as a fire retardant filler for polymer applications in a similar way to magnesium hydroxide and mixtures of huntite and hydromagnesite.[11][12][13][14][15] It decomposes at about 180 °C (356 °F), 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 and rubber.[16]

    Pharmaceutical

    Under the generic name "algeldrate", aluminium hydroxide is used as an antacid in humans and animals (mainly cats and dogs). It is preferred over other alternatives such as sodium bicarbonate because Al(OH)3, being insoluble, does not increase the pH of stomach above 7 and hence, does not trigger secretion of excess acid by the stomach. Brand names include Alu-Cap, Aludrox, Gaviscon or Pepsamar. It reacts with excess acid in the stomach, reducing the acidity of the stomach content,[17][18] which may relieve the symptoms of ulcers, heartburn or dyspepsia. Such products can cause constipation, because the aluminium ions inhibit the contractions of smooth muscle cells in the gastrointestinal tract, slowing peristalsis and lengthening the time needed for stool to pass through the colon.[19] Some such products are formulated to minimize such effects through the inclusion of equal concentrations of magnesium hydroxide or magnesium carbonate, which have counterbalancing laxative effects.[20]

    This compound is also used to control hyperphosphatemia (elevated phosphate, or phosphorus, levels in the blood) in people and animals suffering from kidney failure. Normally, the kidneys filter excess phosphate out from the blood, but kidney failure can cause phosphate to accumulate. The aluminium salt, when ingested, binds to phosphate in the intestines and reduce the amount of phosphorus that can be absorbed.[21][22]

    Precipitated aluminium hydroxide is included as an adjuvant in some vaccines (e.g. anthrax vaccine). One of the well-known brands of aluminium hydroxide adjuvant is Alhydrogel, made by Brenntag Biosector.[23] Since it absorbs protein well, it also functions to stabilize vaccines by preventing the proteins in the vaccine from precipitating or sticking to the walls of the container during storage. Aluminium hydroxide is sometimes called "alum", a term generally reserved for one of several sulfates.

    Vaccine formulations containing aluminium hydroxide stimulate the immune system by inducing the release of uric acid, an immunological danger signal. This strongly attracts certain types of monocytes which differentiate into dendritic cells. The dendritic cells pick up the antigen, carry it to lymph nodes, and stimulate T cells and B cells.[24] It appears to contribute to induction of a good Th2 response, so is useful for immunizing against pathogens that are blocked by antibodies. However, it has little capacity to stimulate cellular (Th1) immune responses, important for protection against many pathogens,[25] nor is it useful when the antigen is peptide-based.[26]

    Potential adverse effects

    In the 1960s and 1970s it was speculated that aluminium was related to various neurological disorders, including Alzheimer's disease.[27][28] Since then, multiple epidemiological studies have found no connection between exposure to environmental or swallowed aluminium and neurological disorders, though injected aluminium was not looked at in these studies. [29][30][31]

    References
    1. For solubility product: "Archived copy". Archived from the original on 15 June 2012. Retrieved 17 May 2012.CS1 maint: archived copy as title (link)
    2. For isoelectric point: Gayer, K. H.; Thompson, L. C.; Zajicek, O. T. (September 1958). "The solubility of aluminum hydroxide in acidic and basic media at 25 ?c". Canadian Journal of Chemistry. 36 (9): 1268–1271. doi:10.1139/v58-184. ISSN 0008-4042.
    3. Zumdahl, Steven S. (2009). Chemical Principles (6th ed.). Houghton Mifflin Company. ISBN 978-0-618-94690-7.
    4. Black, Ronald A.; Hill, D. Ashley (15 June 2003). "Over-the-Counter Medications in Pregnancy". American Family Physician. 67 (12): 2517–2524. ISSN 0002-838X. PMID 12825840. Retrieved 1 July 2017.
    5. Karamalidis, A. K.; Dzombak D. A. (2010). Surface Complexation Modeling: Gibbsite. John Wiley & Sons. pp. 15–17. ISBN 978-0-470-58768-3.
    6. Wells, A. F. (1975), Structural Inorganic Chemistry (4th ed.), Oxford: Clarendon PressCS1 maint: multiple names: authors list (link)
    7. Evans, K. A. (1993). "Properties and uses of aluminium oxides and aluminium hydroxides". In A. J. Downs (ed.). Chemistry of aluminium, gallium, indium, and thallium (1st ed.). London; New York: Blackie Academic & Professional. ISBN 9780751401035.
    8. Boundless (26 July 2016). "Basic and Amphoteric Hydroxides". Boundless Chemistry. Archived from the original on 22 August 2017. Retrieved 2 July 2017.
    9. Hind, AR; Bhargava SK; Grocott SC (1999). "The Surface Chemistry of Bayer Process Solids: A Review". Colloids Surf Physiochem Eng Aspects. 146 (1–3): 359–74. doi:10.1016/S0927-7757(98)00798-5.
    10. "Hungary Battles to Stem Torrent of Toxic Sludge". BBC News Website. 5 October 2010.
    11. Hollingbery, LA; Hull TR (2010). "The Fire Retardant Behaviour of Huntite and Hydromagnesite - A Review" (PDF). Polymer Degradation and Stability. 95 (12): 2213–2225. doi:10.1016/j.polymdegradstab.2010.08.019.
    12. Hollingbery, LA; Hull TR (2010). "The Thermal Decomposition of Huntite and Hydromagnesite - A Review" (PDF). Thermochimica Acta. 509 (1–2): 1–11. doi:10.1016/j.tca.2010.06.012.
    13. Hollingbery, LA; Hull TR (2012). "The Fire Retardant Effects of Huntite in Natural Mixtures with Hydromagnesite" (PDF). Polymer Degradation and Stability. 97 (4): 504–512. doi:10.1016/j.polymdegradstab.2012.01.024.
    14. Hollingbery, LA; Hull TR (2012). "The Thermal Decomposition of Natural Mixtures of Huntite and Hydromagnesite" (PDF). Thermochimica Acta. 528: 45–52. doi:10.1016/j.tca.2011.11.002.
    15. Hull, TR; Witkowski A; Hollingbery LA (2011). "Fire Retardant Action of Mineral Fillers" (PDF). Polymer Degradation and Stability. 96 (8): 1462–1469. doi:10.1016/j.polymdegradstab.2011.05.006.
    16. Huber Engineered Materials. "Huber Non-Halogen Fire Retardant Additives" (PDF). Retrieved 3 July 2017.
    17. Galbraith, A; Bullock, S; Manias, E; Hunt, B; Richards, A (1999). Fundamentals of pharmacology: a text for nurses and health professionals. Harlow: Pearson. p. 482.
    18. Papich, Mark G. (2007). "Aluminum Hydroxide and Aluminum Carbonate". Saunders Handbook of Veterinary Drugs (2nd ed.). St. Louis, Mo: Saunders/Elsevier. pp. 15–16. ISBN 9781416028888.
    19. Washington, Neena (2 August 1991). Antacids and Anti Reflux Agents. Boca Raton, FL: CRC Press. p. 10. ISBN 978-0-8493-5444-1.
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    21. Plumb, Donald C. (2011). "Aluminum Hydroxide". Plumb's Veterinary Drug Handbook (7th ed.). Stockholm, Wisconsin; Ames, Iowa: Wiley. pp. 36–37. ISBN 9780470959640.
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    27. "Alzheimer's Myth's". Alzheimer's Association. Retrieved 29 July 2012.
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    29. Rondeau V (2002). "A review of epidemiologic studies on aluminum and silica in relation to Alzheimer's disease and associated disorders". Rev Environ Health. 17 (2): 107–21. doi:10.1515/REVEH.2002.17.2.107. PMC 4764671. PMID 12222737.
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