Aluminum hydroxide structure, properties, uses, risks

1958
David Holt

The aluminum hydroxide is an inorganic compound whose chemical formula is Al (OH)3. Unlike other metal hydroxides, it is an amphoteric one, capable of reacting or behaving like an acid or a base, depending on the medium. It is a white solid quite insoluble in water, so it finds use as a component of antacids.

Like Mg (OH)two or brucite, with which it shares certain chemical and physical characteristics, in pure form it looks like an amorphous and dull solid; but when it crystallizes with some impurities, it acquires crystalline forms as if they were pearls. Among these minerals, natural sources of Al (OH)3, the gibbsite is found.

Special gibbsite crystal. Source: Rob Lavinsky, iRocks.com - CC-BY-SA-3.0 [CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0)]

Besides gibbsite there are also the minerals bayerite, nordstrandite and doleyite, forming the four polymorphs of aluminum hydroxide. Structurally they are very similar to each other, hardly differing in the way in which the layers or sheets of ions are positioned or coupled, as well as the type of impurities contained.

By controlling the pH and synthesis parameters, any of these polymorphs can be prepared. Also, some chemical species of interest can be intercalated between its layers, so that intercalation materials or compounds are created. This represents the use of a more technological approach for Al (OH)3. Its other uses are as antacids.

On the other hand, it is used as a raw material to obtain alumina, and its nanoparticles have been used as catalytic support..

Article index

  • 1 Structure
    • 1.1 Formula and octahedron
    • 1.2 Polymorphs
  • 2 Properties
    • 2.1 Physical appearance
    • 2.2 Molar mass
    • 2.3 Density
    • 2.4 Melting point
    • 2.5 Solubility in water
    • 2.6 Solubility product
    • 2.7 Amphotericism
  • 3 Nomenclature
  • 4 Uses
    • 4.1 Raw material
    • 4.2 Catalytic supports
    • 4.3 Intercalation compounds
    • 4.4 Fire retardant
    • 4.5 Medicinal
    • 4.6 Adsorbent
  • 5 Risks
  • 6 References

Structure

Formula and octahedron

The chemical formula Al (OH)3 indicates at once that the relation Al3+: OH- is 1: 3; that is, there are three OH anions- for each Al cation3+, which is the same as saying that a third of its ions correspond to aluminum. Thus, the Al3+ and OH- interact electrostatically until their attraction-repulsions define a hexagonal crystal.

However, the Al3+ not necessarily surrounded by three OH- but six; therefore, we speak of a coordination octahedron, Al (OH)6, in which there are six Al-O interactions. Each octahedron represents a unit with which the crystal is built, and a number of them adopt triclinic or monoclinic structures.

The bottom image partially represents the Al (OH) octahedra6, since only four interactions are observed for the Al3+ (light brown spheres).

Hexagonal crystal of gibbsite, an aluminum hydroxide mineral. Source: Benjah-bmm27 [Public domain].

If this structure is carefully observed, which corresponds to that of the mineral gibbsite, it can be seen that the white spheres make up the "faces" or surfaces of the ion layers; these are, the hydrogen atoms of the OH ions-.

Note also that there is a layer A and another B (spatially they are not identical), joined together by hydrogen bonds..

Polymorphs

Layers A and B are not always coupled in the same way, just as their physical environments or host ions (salts) can change. Consequently, Al (OH) crystals3 vary in four mineralogical or, in this case, polymorphic forms.

Aluminum hydroxide is then said to have up to four polymorphs: gibbsite or hydrargillite (monoclinic), bayerite (monoclinic), doyleite (triclinic) and nordstrandite (triclinic). Of these polymorphs, gibbsite is the most stable and abundant; the others are classified as rare minerals.

If the crystals were observed under a microscope, it would be seen that their geometry is hexagonal (although somewhat irregular). The pH plays an important role in the growth of such crystals and on the resulting structure; that is, given a pH, a polymorph or another can be formed.

For example, if the medium where Al (OH) precipitates3 it has a pH lower than 5.8 gibbsite is formed; while if the pH is higher than this value, bayerite is formed.

In more basic media, nordstrandite and doyleite crystals tend to form. Thus, being the most abundant gibbsite, it is a fact that reflects the acidity of its weathered environments..

Properties

Physical appearance

White solid that can come in different formats: granular or powder, and amorphous in appearance.

Molar mass

78.00 g / mol

Density

2.42 g / mL

Melting point

300 ° C. It has no boiling point because the hydroxide loses water to transform into alumina or aluminum oxide, AltwoOR3.

Water solubility

1 10-4 g / 100 mL. However, its solubility increases with the addition of acids (H3OR+) or alkalis (OH-).

Solubility product

Ksp = 3 10−34

This very small value means that only a tiny portion dissolves in water:

Al (OH)3(s)  <=>  To the3+(aq) + 3OH-(ac)

And in fact this negligible solubility makes it a good acidity neutralizer, since it does not basify the gastric environment too much because it does not release almost OH ions.-.

Amphotericism

El Al (OH)3 it is characterized by its amphoteric character; that is, it can react or behave as if it were an acid or a base.

For example, it reacts with H ions3OR+ (if the medium is aqueous) to form the complex aqueous [Al (OHtwo)6]3+; which, in turn, is hydrolyzed to acidify the medium, therefore Al3+ an acid ion:

Al (OH)3(s) + 3H3OR+(ac) => [Al (OHtwo)6]3+(ac)

[Al (OHtwo)6]3+(ac) + HtwoO (l)   <=>  [Al (OHtwo)5(OH)]two+(ac) + H3OR+(ac)

When this happens, it is said that the Al (OH)3 behaves like a base, since it reacts with the H3OR+. On the other hand, it can react with the OH-, behaving like an acid:

Al (OH)3(s) + OH-(ac) => Al (OH)4-(ac)

In this reaction the white precipitate of Al (OH)3 dissolves in the presence of excess OH ions-; fact that does not happen the same with other hydroxides, such as magnesium, Mg (OH)two.

El Al (OH)4-, aluminate ion, can be more appropriately expressed as: [Al (OHtwo)two(OH)4]-, highlighting the coordination number of 6 for the Al cation3+ (the octahedron).

This ion can continue to react with more OH- until completing the coordination octahedron: [Al (OH)6]3-, called hexahydroxoaluminate ion.

Nomenclature

The name 'aluminum hydroxide', with which more reference has been made to this compound, corresponds to that governed by the stock nomenclature. The (III) is omitted at the end of it, since the oxidation state of aluminum is +3 in all its compounds.

The other two possible names to refer to Al (OH)3 They are: aluminum trihydroxide, according to the systematic nomenclature and the use of the Greek numerator prefixes; and aluminum hydroxide, ending with the suffix -ico for having a single oxidation state.

Although in the chemical field the nomenclature of Al (OH)3 does not represent any challenge or confusion, outside of it it tends to be mixed with ambiguities.

For example, the mineral gibbsite is one of the natural polymorphs of Al (OH)3, which they also name as γ-Al (OH)3 or α-Al (OH)3. However, α-Al (OH)3 may also correspond to the mineral bayerite, or β-Al (OH)3, according to crystallographic nomenclature. Meanwhile, the polymorphs nordstrandite and doyleite are usually designated simply as Al (OH)3.

The following list clearly summarizes what has just been explained:

-Gibbsite: (γ or α) -Al (OH)3

-Bayerite: (α or β) -Al (OH)3

-Nordstrandite: Al (OH)3

-Doyleite: Al (OH)3

Applications

Raw material

The immediate use for aluminum hydroxide is as a raw material for the production of alumina or other compounds, inorganic or organic, of aluminum; for example: AlCl3, By not3)3, AlF3 or NaAl (OH)4.

Catalytic supports

Nanoparticles of Al (OH)3 they can act as catalytic supports; that is, the catalyst joins them to remain fixed on their surface, where chemical reactions are accelerated.

Intercalation compounds

In the section on structures it was explained that Al (OH)3 It consists of layers or sheets A and B, coupled to define a crystal. Inside it, there are small octahedral spaces or holes that can be occupied by other ions, metallic or organic, or neutral molecules..

When Al (OH) crystals are synthesized3 With these structural modifications, an intercalation compound is said to be being prepared; that is, they insert or insert chemical species between sheets A and B. In doing so, new materials made from this hydroxide emerge.

Fire retardant

El Al (OH)3 it is a good fire retardant that finds application as a filler material for many polymeric matrices. This is because it absorbs heat to release water vapor, just as Mg (OH) does.two or the brucita.

Medicinal

El Al (OH)3 It is also a neutralizer of acidity, reacting with the HCl in gastric secretions; again, similarly to Mg (OH)two milk of magnesia.

Both hydroxides can in fact be mixed in different antacids, used to alleviate the symptoms of people suffering from gastritis or stomach ulcers..

Adsorbent

When heated below its melting point, aluminum hydroxide transforms into activated alumina (as well as activated carbon). This solid is used as an adsorbent for undesirable molecules, whether dyes, impurities, or polluting gases..

Risks

The risks that aluminum hydroxide can pose are not due to it as a solid, but as a medicine. It does not need any protocol or regulations to store it, since it does not react vigorously with oxidizing agents, and it is not flammable..

When ingested in antacids available in pharmacies, undesirable side effects can occur, such as constipation and inhibition of phosphate in the intestines. Likewise, and although there are no studies to prove it, it has been associated with neurological disorders such as Alzheimer's disease.

References

  1. Shiver & Atkins. (2008). Inorganic chemistry. (Fourth edition). Mc Graw Hill.
  2. Wikipedia. (2019). Aluminum hydroxide. Recovered from: en.wikipedia.org
  3. National Center for Biotechnology Information. (2019). Aluminum hydroxide. PubChem Database. CID = 10176082. Recovered from: pubchem.ncbi.nlm.nih.gov
  4. Danielle Reid. (2019). Aluminum Hydroxide: Formula & Side Effects. Study. Recovered from: study.com
  5. Robert Schoen & Charles E. Roberson. (1970). Structures of aluminum hydroxide and geochemical implications. The American Mineralogist, Vol 55.
  6. Vitaly P. Isupov & col. (2000). Synthesis, Structure, Properties, and Application of Aluminum Hydroxide Intercalation Compounds. Chemistry for Sustainable Development 8,121-127.
  7. Drugs. (March 24, 2019). Aluminum hydroxide Side Effects. Recovered from: drugs.com

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