Aluminum carbonate structure, properties, uses

The aluminum carbonate is an inorganic salt whose chemical formula is Al_two(CO₃)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 Al ions³⁺ and CO₃^two-, which in theory should be very strong due to the magnitudes of their charges. 

Salt does not face any inconvenience on paper when the chemical equations of its reactions are written; but in practice it turns against him.

Despite what has been said, aluminum carbonate can occur in the company of other ions, as occurs with the mineral dawsonite. Also, there is a derivative in which it interacts with aqueous ammonia. For the rest, it is considered a mixture between Al (OH)₃ and H_twoCO₃; which is equal to an effervescent solution with a white precipitate.

This mixture has medicinal uses. However, to the pure, isolable and manipulable salt of Al_two(CO₃)₃, no known possible applications; at least not under enormous pressure or extreme conditions.

Article index

• 1 Structure of aluminum carbonate
  • 1.1 Aluminum ammonium hydroxide carbonate
• 2 Properties
  • 2.1 Molar mass
  • 2.2 Instability
  • 2.3 Physical
• 3 Uses
• 4 References

Structure of aluminum carbonate

The crystal structure for this salt is unknown, because it is so unstable that it could not be characterized. From its formula Al_two(CO₃)₃, However, it is known that the proportion of Al ions³⁺ and CO₃^two- is 2: 3; that is, for every two Al cations^two+ there must be three CO anions₃^two- interacting electrostatically with them.

The problem is that both ions are very unequal in size; the Al³⁺ is very small while CO₃^two- it 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, the Al³⁺ it is a highly polarizing cation, a property that deforms the electronic cloud of CO₃^two-. It is as if you want to force it to bond covalently, even though the anion cannot..

Consequently, ionic interactions between Al³⁺ and CO₃^two- they tend towards covalence; another factor that adds to the instability of Al_two(CO₃)₃.

Aluminum ammonium hydroxide carbonate

The chaotic relationship between the Al³⁺ and CO₃^two- softens in appearance when other ions are present in the glass; such as NH₄⁺ and OH^-, from a solution of ammonia. This quartet of ions, Al³⁺, CO₃^two-, NH₄⁺ and OH^-, they 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, NaAlCO₃(OH)_two, where the Na⁺ replaces NH₄⁺. In these salts, their ionic bonds are strong enough that the water does not promote the release of CO._two; or at least not abruptly.

Although the NH₄Al (OH)_twoCO₃ (AACC, for its acronym in English), nor the NaAlCO₃(OH)_two 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_two(CO₃)₃ it is unstable. But what transformation does it undergo? Two situations must be considered: one dry, and the other "wet".

Dry

In the dry situation, the anion CO₃^two- reverts to CO_two using the following decomposition:

To the_two(CO₃)₃  => Al_twoOR₃ + 3CO_two

Which makes sense if it is synthesized under high CO pressure._two; that is, the reverse reaction:

To the_twoOR₃ + 3CO_two   => Al_two(CO₃)₃

Therefore, to prevent Al_two(CO₃)₃ the salt should be subjected to high pressure (using N_two, for example). In this way the formation of CO_two would not be thermodynamically favored.

Wet

While in the wet situation, the CO₃^two- undergoes hydrolysis, which generates small amounts of OH^-; but enough to precipitate the aluminum hydroxide, Al (OH)₃:

CO₃^two-     +    H_twoOR    <=>    HCO₃^-    +     Oh^-

To the³⁺    +    3OH^-    <=>    Al (OH)₃

And on the other hand, the Al³⁺ also hydrolyzes:

To the³⁺    +    H_twoOR    <=>    Al (OH)_two^two+   +   H⁺

Although the Al would actually hydrate first³⁺ to form the complex Al (H_twoOR)₆³⁺, which is hydrolyzed to give [Al (H_twoOR)₅OH]^two+ and H₃OR⁺. Then the H₃O (or H⁺) protone to CO₃^two- to H_twoCO₃, which decomposes to CO_two and H_twoOR:

CO₃^two-    +    2H⁺  => H_twoCO₃

H_twoCO₃   <=>  CO_two  +  H_twoOR

Note that in the end the Al³⁺ behaves like an acid (releases H⁺) and a base (releases OH^- with the solubility balance of Al (OH)₃); that is, it exhibits amphotericism.

Physical

If it can be isolated, this salt is likely to be white in color, like many other aluminum salts. Also, due to the difference between the ionic radii of Al³⁺ and CO₃^two-, 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 symptoms of heartburn, acid indigestion, and stomach ulcers..

References

1. 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.
2. 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
3. National Center for Biotechnology Information. (2019). Aluminum carbonate. PubChem Database., CID = 10353966. Recovered from: pubchem.ncbi.nlm.nih.gov
4. Wikipedia. (2019). Aluminum carbonate. Recovered from: en.wikipedia.org
5. Aluminumsulfate. (2019). Aluminum Carbonate. Recovered from: aluminumsulfate.net