- Structure of aluminum carbonate
- Aluminum ammonium hydroxide carbonate
- Properties
- Molar mass
- Instability
- Dry
- Wet
- Physical
- Applications
- References
The aluminum carbonate is an inorganic salt having the chemical formula A the 2 (CO 3) 3. It is a practically non-existent metallic carbonate, given its high instability under normal conditions.
Among the reasons for its instability we can mention the weak electrostatic interactions between the Al 3+ and CO 3 2- ions, which in theory should be very strong due to the magnitudes of their charges.
Aluminum carbonate formula. Source: Gabriel Bolívar.
Salt faces no drawbacks on paper when writing the chemical equations of its reactions; but in practice it works against her.
Despite what has been said, aluminum carbonate can occur in the company of other ions, such as the mineral dawsonite. Likewise, there is a derivative in which it interacts with aqueous ammonia. The rest is considered a mixture between Al (OH) 3 and H 2 CO 3; which is equal to an effervescent solution with a white precipitate.
This mixture has medicinal uses. However, the pure, isolable and manipulable salt of Al 2 (CO 3) 3 has no known possible applications; at least not under enormous pressure or extreme conditions.
Structure of aluminum carbonate
The crystal structure for this salt is unknown, because it is so unstable that it cannot be characterized. From its formula Al 2 (CO 3) 3, however, it is known that the ratio of the Al 3+ and CO 3 2- ions is 2: 3; In other words, for every two Al 2+ cations there must be three CO 3 2- anions interacting electrostatically with them.
The problem is that both ions are very unequal in size; Al 3+ is very small while CO 3 2- is bulky. This difference by itself already affects the lattice stability of the crystal lattice, whose ions would interact "awkwardly" if this salt could be isolated in the solid state.
In addition to this aspect, Al 3+ is a highly polarizing cation, a property that deforms the electronic cloud of CO 3 2-. It is as if you want to force it to bond covalently, even though the anion cannot.
Consequently, the ionic interactions between Al 3+ and CO 3 2- tend towards covalence; another factor that adds to the instability of Al 2 (CO 3) 3.
Aluminum ammonium hydroxide carbonate
The chaotic relationship between Al 3+ and CO 3 2- softens in appearance when there are other ions present in the crystal; such as NH 4 + and OH -, coming from a solution of ammonia. This quartet of ions, Al 3+, CO 3 2-, NH 4 + and OH -, do manage to define stable crystals, even capable of adopting different morphologies.
Another example similar to this is observed in the mineral dawsonite and its orthorhombic crystals, NaAlCO 3 (OH) 2, where Na + replaces NH 4 +. In these salts their ionic bonds are strong enough so that the water does not promote the release of CO 2; or at least, not abruptly.
Although NH 4 Al (OH) 2 CO 3 (AACC, for its acronym in English), nor NaAlCO 3 (OH) 2 represent aluminum carbonate, they can be considered as basic derivatives thereof.
Properties
Molar mass
233.98 g / mol.
Instability
In the previous section, it was explained from a molecular perspective why Al 2 (CO 3) 3 is unstable. But what transformation does it undergo? There are two situations to consider: one dry, and the other "wet."
Dry
In the dry situation, the anion CO 3 2- reverts to CO 2 by the following decomposition:
Al 2 (CO 3) 3 => Al 2 O 3 + 3CO 2
Which makes sense if this is synthesized subjected to alumina to high pressures of CO 2; that is, the reverse reaction:
Al 2 O 3 + 3CO 2 => Al 2 (CO 3) 3
Therefore, to prevent Al 2 (CO 3) 3 from decomposing, the salt would have to be subjected to high pressure (using N 2, for example). In this way the formation of CO 2 would not be thermodynamically favored.
Wet
While in the wet situation, CO 3 2- undergoes hydrolysis, which generates small amounts of OH -; but enough to precipitate the aluminum hydroxide, Al (OH) 3:
CO 3 2- + H 2 O <=> HCO 3 - + OH -
Al 3+ + 3OH - <=> Al (OH) 3
And on the other hand, Al 3+ is also hydrolyzed:
Al 3+ + H 2 O <=> Al (OH) 2 2+ + H +
Although the Al 3+ would actually first hydrate to form the Al (H 2 O) 6 3+ complex, which is hydrolyzed to give 2+ and H 3 O +. Then H 3 O (or H +) protonates CO 3 2- to H 2 CO 3, which decomposes to CO 2 and H 2 O:
CO 3 2- + 2H + => H 2 CO 3
H 2 CO 3 <=> CO 2 + H 2 O
Note that in the end Al 3+ behaves as an acid (releases H +) and a base (releases OH - with the solubility equilibrium of Al (OH) 3); that is, it exhibits amphotericism.
Physical
If it can be isolated, this salt is likely to be white, like many other aluminum salts. Also, due to the difference between the ionic radii of Al 3+ and CO 3 2-, it would surely have very low melting or boiling points compared to other ionic compounds.
And regarding its solubility, it would be infinitely soluble in water. Furthermore, it would be a hygroscopic and deliquescent solid. However, these are just guesswork. Other properties would have to be estimated with computer models subjected to high pressures.
Applications
The known applications of aluminum carbonate are medical. It was used as a mild astringent and as a drug to treat gastric ulcer and inflammation. It has also been used to prevent urinary stone formation in humans.
It has been used to control an increase in the body's phosphate content and also to treat the symptoms of heartburn, acid indigestion, and stomach ulcers.
References
- XueHui L., Zhe T., YongMing C., RuiYu Z. & Chenguang L. (2012). Hydrothermal Synthesis of Ammonium Aluminum Carbonate Hydroxide (AACH) Nanoplatelets and Nanofibers pH-Controlled Morphologies. Atlantis Press.
- Robin Lafficher, Mathieu Digne, Fabien Salvatori, Malika Boualleg, Didier Colson, Francois Puel (2017) Ammonium aluminum carbonate hydroxide NH4Al (OH) 2CO3 as an alternative route for alumina preparation: comparison with the classical boehmite precursor. Powder Technology, 320, 565-573, DOI: 10.1016 / j.powtec.2017.07.0080
- National Center for Biotechnology Information. (2019). Aluminum carbonate. PubChem Database., CID = 10353966. Recovered from: pubchem.ncbi.nlm.nih.gov
- Wikipedia. (2019). Aluminum carbonate. Recovered from: en.wikipedia.org
- Aluminumsulfate. (2019). Aluminum Carbonate. Recovered from: aluminumsulfate.net