AMMONIUM ACETATE: STRUCTURE, PROPERTIES, USES AND RISKS - CHEMISTRY - 2024

The ammonium acetate is an inorganic salt having the chemical formula NH ₄ CH ₃ COOH. It is derived from the mixture of acetic acid, present in commercial vinegar at a concentration of 5%, and ammonia. Both starting substances have characteristic odors, so it is possible to get an idea why this salt smells of vinegar-ammonia.

However, the most outstanding aspect of this salt is not its smell, but its low melting point. It is so low that in any laboratory it could be obtained in its liquid version, where ions flow freely to transport electrical charges.

Ammonium acetate crystals. Source: Vidak.

On the other hand, ammonium acetate is deliquescent; that is, it absorbs water or humidity from the environment until it dissolves completely. That is why, although in anhydrous state its crystals are white, they quickly turn bright (like the ones in the beaker in the image above).

Being a solid source of ammonia, it should be handled in such a way that inhalation of its vapors is minimized. But, despite this negative characteristic, NH ₄ CH ₃ COOH is useful for the preparation of buffer solutions that preserve food, as well as being a component of certain protein extracting solvents.

Structure of ammonium acetate

Ions of ammonium acetate. Source: CCoil

The image above shows the ions that make up ammonium acetate in a spheres and bars model. On the left is the cation of tetrahedral geometry, NH ₄ ⁺, while on the right is the molecular anion with two delocalized electrons between two oxygen atoms, CH ₃ COO ^- (the dotted line between the red spheres).

Thus both ions, NH ₄ ⁺ and CH ₃ COO ^-, are held together by their electrostatic attraction that, together with the repulsions between equal charges, end up defining a crystal. This ammonium acetate crystal has an orthorhombic structure, which could be observed in microscopic samples or even of visible sizes.

Not only is ionic bonding important for this salt, but so are hydrogen bonds. NH ₄ ⁺ can donate up to four of these bridges; that is, in each of the vertices of its tetrahedron an oxygen atom from a neighboring CH ₃ COO ^- (H ₃ N ⁺ -H- OCOCH ₃) is located.

In theory, the forces within your crystals should then be very strong; but experimentally the opposite occurs, since it only melts at 114 ° C. Therefore, hydrogen bonds do not compensate for the weakness of its ionic bond, nor the low lattice energy of the orthorhombic crystal of NH ₄ CH ₃ COO.

Other compositions and water

Ammonium acetate was originally said to be prepared by mixing acetic acid and ammonia. Therefore, the salt can also be expressed as: NH ₃ · CH ₃ COOH. Thus, depending on the composition, other structures can be obtained: NH ₃ · 2CH ₃ COOH, or NH ₃ · 5CH ₃ COOH, for example.

Also, it was mentioned that it absorbs a lot of moisture. In doing so, it incorporates water molecules to its crystals, which hydrolyze to give NH ₃ or CH ₃ COOH; and that is why salt gives off the smell of ammonia or vinegar.

Properties

Physical appearance

Deliquescent white crystals with a vinegar and ammonia odor.

Molar mass

77.083 g / mol.

Density

1.073 g / mL at 25 ° C.

Melting point

114 ° C. This value is considerably lower compared to other salts or ionic compounds. In addition, it lacks a boiling point due to the formation of ammonia, implying the decomposition of the salt.

Solubility

143 g / 100 mL at 20 ° C. Note its extraordinary solubility in water, which demonstrates the affinity felt by water molecules for NH ₄ ⁺ and CH ₃ COO ^- ions, hydrating them in aqueous spheres.

Its solubility is not such in less polar solvents. For example, 7.89 g of NH ₄ CH ₃ COO are dissolved in 100 mL of methanol at 15 ° C.

Stability

It is deliquescent, so you should avoid storing in humid spaces. Also, while absorbing water it releases ammonia, and therefore, it decomposes.

pKa

9.9.

This constant corresponds to the acidity of the ammonium ion:

NH ₄ ⁺ + B <=> NH ₃ + HB

Where HB is a weak acid. If the base B is about water, it will have its hydrolysis reaction:

NH ₄ ⁺ + H ₂ O <=> NH ₃ + H ₃ O ⁺

In which the species H ₃ O ⁺ defines the pH of the solution.

On the other hand, acetate also contributes to pH:

CH ₃ COO ^- + H ₂ O <=> CH ₃ COOH + OH ^-

Thus, both species H ₃ O ⁺ and OH ^- are neutralized giving a neutral pH 7. However, according to Pubchem, highly concentrated ammonium acetate solutions have an acidic pH; which means that the hydrolysis of NH ₄ ⁺ predominates over that of CH ₃ COO ^-.

Standard formation enthalpy

Δ _f H ₂₉₈ = -615 kJ / mol.

Applications

Analytical

Aqueous solutions of sodium acetate allow lead, iron and zinc sulfates to be solubilized, and their concentration can subsequently be determined by atomic absorption spectroscopy.

Medium conductor

As it melts at a low temperature compared to other salts, its liquid can be used to conduct the electric current that lights the circuit of a light bulb.

Buffer

It can regulate pH changes in acidic or basic ranges, which is used to maintain a constant pH in, for example, meats, chocolates, cheeses, vegetables or other food products.

On ice and soils

It is a relatively cheap and biodegradable salt, used to de-ice frozen roads. Likewise, as it is a source of nitrogen soluble in water, it is used for agricultural purposes to determine potassium levels.

Precipitating agent

This salt is used to precipitate proteins for chromatographic analysis.

Medicine

Ammonium acetate works as a diuretic agent in veterinary medicine, and it is also a necessary salt for the synthesis of insulin and penicillin.

Risks

Next, and finally, some risks or negative consequences caused by ammonium acetate are listed:

- It can cause slight irritation to the skin, but without being absorbed into it.

- When ingested it causes stomach upset, diarrhea, diuresis, anxiety, increased need to urinate, tremors and other symptoms related to ammonia poisoning, as well as damage to the liver.

- Its inhalation irritates the nose, throat and lungs.

For the rest, it is unknown if it can cause cancer, and any possible fire risk from this salt is ruled out (at least under normal storage conditions).

References

1. Inger Nahringbauer. (1967). Hydrogen Bond Studies. XIV. The Crystal Structure of Ammonium Acetate. Institute of Chemistry, University of Uppsala, Uppsala, Sweden. Acta Cryst. 23, 956.
2. National Center for Biotechnology Information. (2019). Ammonium acetate. PubChem Database. CID = 517165. Recovered from: pubchem.ncbi.nlm.nih.gov
3. Sullivan Randy. (2012). Conductivity of Ammonium Acetate. Recovered from: chemdemos.uoregon.edu
4. Viachem, Ltd. (sf). Ammonium acetate. Recovered from: viacheminc.com
5. Wikipedia. (2019). Ammonium acetate. Recovered from: en.wikipedia.org
6. New Jersey Department of Health. (2012). Ammonium Acetate - Hazardous Substance Fact Sheet.. Recovered from: nj.gov
7. Xueyanghu. (sf). Uses And Risks Of Ammonium Acetate. Recovered from: xueyanghu.wordpress.com