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What is ammonium carbonate?

The ammonium carbonate is an inorganic salt nitrogen, ammoniacal specifically, the chemical formula (NH 4 ) 2 CO 3 . It is made by synthetic methods, among which the use of sublimation of a mixture of ammonium sulfate and calcium carbonate stands out: (NH 4 ) 2 SO 4 (s) + CaCO 3 (s) => (NH 4 ) 2 CO 3 (s) + CaSO 4 (s).

Generally, the ammonium and calcium carbonate salts are heated in a vessel to produce the ammonium carbonate. The industrial method that produces tons of this salt consists of passing carbon dioxide through an absorption column containing a solution of ammonia in water, followed by distillation.

Vapors containing ammonium, carbon dioxide and water condense to form ammonium carbonate crystals: 2NH 3 (g) + H 2 O (l) + CO 2 (g) → (NH 4 ) 2 CO 3 (s ). In the reaction, carbonic acid, H 2 CO 3 , is produced after dissolving carbon dioxide in water, and it is this acid that gives up its two protons, H + , to two ammonia molecules.

Properties of ammonium carbonate

Appearance of ammonium carbonate

It is a white, crystalline and colorless solid, with strong ammonia odors and flavors. It melts at 58ºC, decomposing into ammonia, water and carbon dioxide: exactly the previous chemical equation but in the opposite direction.

However, this decomposition occurs in two steps: first a molecule of NH 3 is released , producing ammonium bicarbonate (NH 4 HCO 3 ); and second, if the heating continues, the carbonate is disproportionate, releasing even more gaseous ammonia.

It is a solid very soluble in water and less soluble in alcohols. It forms hydrogen bonds with water, and when 5 grams is dissolved in 100 grams of water, it generates a basic solution with a pH around 8.6.

Its high affinity for water makes it a hygroscopic solid (absorbs moisture), and that is why it is difficult to find it in its anhydrous form. In fact, its monohydrated form, (NH 4 ) 2 CO 3 · H 2 O), is the most common of all and explains how salt carries ammonia gas, which causes odor.

In air it decomposes to generate ammonium bicarbonate and ammonium carbonate (NH 4 NH 2 CO 2 ).

Chemical structure

Ammonium carbonate
Model of the structure of ammonium carbonate; two cations of ammonium and one of carbonate

The upper image illustrates the chemical structure of ammonium carbonate. In the middle is the anion CO 2– , the flat triangle with a black center and red spheres; and on its two sides, the NH + ammonium cations with tetrahedral geometries.

The geometry of the ammonium ion is explained by the sp 3 hybridization of the nitrogen atom, arranging the hydrogen atoms (the white spheres) around it in the form of a tetrahedron. Interactions are established between the three ions by hydrogen bonds (H 3 N-H— O-CO 2– ).

Thanks to its geometry, a single CO 2– anion can form up to three hydrogen bonds; while NH + cations  may not be able to form their corresponding four hydrogen bonds due to electrostatic repulsions between their positive charges.

The result of all these interactions is the crystallization of an orthorhombic system. Why is it so hygroscopic and soluble in water? The answer is in the same paragraph above: hydrogen bonds.

These interactions are responsible for the rapid absorption of water from the anhydrous salt to form (NH 4 ) 2 CO 3 · H 2 O). This results in changes in the spatial arrangement of the ions, and consequently, in the crystal structure.

Structural curiosities

As simple as (NH 4 ) 2 CO 3 looks , it is so sensitive to countless transformations that its structure is a mystery subject to the true composition of the solid. This structure also varies according to the pressures that affect the crystals.

Some authors have found that ions are arranged as hydrogen-bonded coplanar chains (that is, a chain with a sequence NH + —CO 2– -…) in which water molecules probably serve as linkers to others. chains.

Furthermore, transcending the terrestrial sky, what are these crystals like in space or interstellar conditions? What are their compositions in terms of the stabilities of the carbonate species? There are studies that confirm the great stability of these crystals trapped in planetary ice masses and comets.

This allows them to act as carbon, nitrogen and hydrogen reserves, which, receiving solar radiation, can be transformed into organic material such as amino acids.

In other words, these frozen ammonia blocks could be carriers of “the wheel that starts the machinery of life” in the cosmos. For these reasons, his interest in the field of astrobiology and biochemistry is growing.

Uses / applications

It is used as a leavening agent, since when heated it produces carbon dioxide and ammonium gases. Ammonium carbonate is, if you will, a precursor to modern baking powders and can be used to bake cookies and flatbreads.

However, it is not recommended for baking cakes. Due to the thickness of the cakes, the ammonium gases are trapped inside and produce an unpleasant taste.

It is used as an expectorant, that is, it relieves cough by decongesting the bronchial tubes. It has fungicidal action, being used for this reason in agriculture. It is also a regulator of acidity present in foods and is used in the organic synthesis of urea under high pressure conditions, and of hydantoins.

Risks

Ammonium carbonate is highly toxic. Produces acute irritation of the oral cavity in humans on contact.

In addition, if it is ingested it causes gastric irritation. A similar action is observed in eyes exposed to ammonium carbonate.

Inhaling the gases from the decomposition of salt can irritate the nose, throat and lungs, causing coughing and respiratory distress.

Acute exposure to fasted dogs ammonium carbonate at a dose of 40 mg / kg of weight causes them vomiting and diarrhea. Higher doses of ammonium carbonate (200 mg / kg body weight) are usually lethal. A heart damage is indicated as the cause of death.

If heated to very high temperatures and in oxygen-enriched air, it releases toxic NO 2 gases .

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