SODIUM ACETATE: STRUCTURE, PROPERTIES, SYNTHESIS, USES - CHEMISTRY - 2024

The sodium acetate is a sodium salt of acetic acid with the molecular formula C ₂ H ₃ O ₂ Na. It consists of a deliquescent white powder, essentially present in two forms: anhydrous and trihydrated. Both are very soluble in water, the universal solvent; but not so soluble in alcohols or acetone.

The anhydrous form has a density, a melting point and a boiling point with higher values ​​than those presented by the trihydrate form of sodium acetate. This is because water molecules interfere between the interactions of the Na ⁺ and CH ₃ COO ^- ions.

Sodium acetate appearance

Sodium acetate is stable, especially when stored between 2 and 8 ° C; but it is susceptible to the action of strong oxidizing agents and halogens.

It can be prepared by reacting sodium bicarbonate with acetic acid. Also, with the reaction of acetic acid with sodium hydroxide. Both reactions are simple to produce and inexpensive; the first can even be done at home.

This salt is a low toxic compound. Produces skin irritation, only after frequent and continuous contact. It is slightly irritating to the eyes but can irritate the respiratory tract. There is no information about a harmful effect of its ingestion.

It has numerous uses and applications, highlighting its pH buffering function, along with acetic acid. The acetate buffer has a pKa = 4.7; which confers high efficiency in pH regulation in an acidic environment with pH values ​​between 3 and 6.

Due to its low toxicity and properties, it has been widely used to enhance the flavor of food, as well as an agent that provides protection against spoilage due to its action against microbes.

Structure of sodium acetate

Acetate and sodium ions. Source: Shu0309

The image above shows the ions that make up anhydrous sodium acetate crystals (without water). The purple sphere corresponds to the Na ⁺ cation, and the molecular ion to its left is acetate, CH ₃ COO ^-, with its oxygen atoms represented by red spheres.

These pairs, according to the formula CH ₃ COONa, are in a 1: 1 ratio; For each CH ₃ COO ^- anion, there must be a Na ⁺ cation attracted to its negative charge and vice versa. Thus, the attractions between them, and the repulsions between equal charges, end up establishing structural patterns that define a crystal, whose minimum expression is the unit cell.

Said unit cell, like the crystal as a whole, varies depending on the arrangement of the ions in space; it is not always the same, even for the same crystalline system. For example, anhydrous sodium acetate can form two orthorhombic polymorphs, one of which is depicted below:

Unit cell of orthorhombic crystals of sodium acetate. Source: Benjah-bmm27

Note the arrangement of the ions: four CH ₃ COO ^- ions enclose a Na ⁺ in such a way that they “draw” a distorted square-based pyramid. Each of these CH ₃ COO ^- in turn interacts with another adjacent Na ⁺.

Hydrated crystals

Sodium acetate has a high affinity for water; in fact it is deliquescent, that is, it retains moisture until it dissolves in it. The higher the humidity, the sooner it "melts". This is because both CH ₃ COO ^- and Na ⁺ can hydrate, surround themselves with water molecules that direct their dipoles towards their charges (Na ⁺ OH ₂, CH ₃ COO ^- HOH).

When this salt is prepared in the laboratory or at home, its affinity for water is such that even at normal temperatures it is already obtained as a hydrate; CH ₃ COONa · 3H ₂ O. Their crystals stop being orthorhombic and become monoclinic, since they now incorporate three water molecules for each CH ₃ COO ^- and Na ⁺ pair.

Properties

Names

-Sodium acetate.

-Sodium ethanoate (IUPAC).

Molar mass

-Anhydrous: 82.03 g / mol.

-Trihydrate: 136.03 g / mol.

Appearance

Deliquescent white powder.

Odor

To vinegar when heated to decomposition.

Density

-Anhydrous: 1.528 g / cm ³ at 20 ° C.

-Trihydrate: 1.45 g / cm ³ at 20 ° C.

Melting point

-Anhydrous: 324 ° C (615 ° F, 597 K).

-Trihydrate: 58 ° C (136 ° F, 331 K).

Boiling point

-Anhydrous: 881.4 ° C (1,618.5 ° F, 1,154.5 K).

-Trihydrate: 122 ° C (252 ° F, 395 K). It decomposes.

Solubility

In water

-Anhydrous: 123.3 g / 100 mL at 20ºC.

-Trihydrate: 46.4 g / 100 mL at 20 ºC.

In methanol

16 g / 100 g at 15 ° C.

In ethanol

5.3 g / 100 mL (trihydrate).

In acetone

0.5 g / kg at 15 ° C.

Acidity

pKa: 24 (20 ° C).

Basicity

pKb: 9.25. Here it is evident that sodium acetate is a basic salt because it has a pKb less than its pKa.

Refractive index (ηD)

1,464

Caloric capacity

-100.83 J / mol · K (anhydrous).

-229.9 J / mol · K (trihydrate).

ignition point

Greater than 250ºC.

Auto ignition temperature

600 ° C.

pH

8.9 (0.1M solution at 25 ° C).

Stability

Stable. Incompatible with strong oxidizing agents and halogens. Sensitive to humidity.

Reactions

Sodium acetate can form an ester by reacting with an alkyl halide; for example, bromoethane:

CH ₃ COONa + BrCH ₂ CH ₃ => CH ₃ COOCH ₂ CH ₃ + NaBr

Sodium acetate decarboxylates to methane (pyrolysis) in the presence of NaOH:

CH ₃ COONa + NaOH => CH ₄ + Na ₂ CO ₃

The reaction is catalyzed by cesium salts.

Synthesis

Sodium acetate can be produced cheaply in the laboratory by reacting sodium bicarbonate with acetic acid:

NaHCO ₃ + CH ₃ COOH => CH ₃ COONa + H ₂ CO ₃

This reaction is accompanied by the presence of intense bubbling in the solution, due to the decomposition of carbonic acid into water and carbon dioxide.

H ₂ CO ₃ => H ₂ O + CO ₂

Industrially, sodium acetate is produced by reacting acetic acid with sodium hydroxide.

CH ₃ COOH + NaOH => CH ₃ COONa + H ₂ O

Applications

Thermal bags

Sodium acetate is used to manufacture thermal bags.

Initially, the salt crystals are dissolved in a volume of water to prepare a solution that becomes supersaturated.

Then the solution is heated to a temperature higher than 58 ºC, which is its melting point. The supersaturated solution is allowed to cool to room temperature and no crystal formation is observed; solution is supercooled.

The absence of crystals is explained by the fact that the molecules of the liquid are too disorganized and do not have the proper orientation to reach the crystallization temperature. The liquid is in a metastable state, a state of imbalance.

Given the instability of the supercooled liquid, any disturbance is sufficient to initiate crystallization. In the case of thermal bags, a mechanical attachment is pressed to stir the liquid and initiate the formation of crystals and the solidification of the sodium acetate trihydrate solution.

As crystallization occurs, the temperature increases. Sodium acetate trihydrate crystals take on the appearance of ice, but are hot, which is why they are called "hot ice."

Industrial

-Sodium acetate is used in the textile industry as a mordant in the fabric dyeing process

-Neutralizes sulfuric acid waste

-It has use in the processing of cotton for the production of cotton pads, usable in personal cleaning and for medical purposes

-It is used in the pickling of metals, prior to their chrome plating

-Collaborate so that chloroprene vulcanization does not occur in the synthetic rubber production process

-It takes part in the purification of glucose

-Used in leather tanning.

Medical use

Sodium acetate is one of the compounds used to deliver electrolytes given to patients intravenously.

It is used to correct sodium levels in hyponatremic patients, also in the correction of metabolic acidosis and alkalinization of the urine.

PH buffer solution

It is used as a pH regulator in many, many enzymatic reactions that occur between pH 3 and pH 6.

The different pH of the acetate buffer solution are reached by the variation of the concentration of acetic acid and sodium acetate.

Thus, for example, to obtain a pH 4.5, the buffer solution has an acetic acid concentration of 3.8 g / L and an anhydrous sodium acetate concentration of 3.0 g / L.

The concentration of the acetate buffer can be increased, increasing the components of the solution necessary to obtain the desired pH in the same proportion.

Acetate / acetonitrile buffer is used in capillary electrophoresis in the separation of photoberberines.

Research laboratories

-Sodium acetate is a weak nucleophilic agent that is used in the anionic polymerization of β-lactone.

-It is used as a reagent in the nuclear magnetic resonance of biological macromolecules.

-Used in the extraction of DNA from cells. Sodium is a cation that interacts with the negative charges of the phosphates present in DNA, which helps in their condensation. In the presence of ethanol, DNA forms a precipitate that can then separate into the aqueous layer.

Preservation of food

-Helps prevent bacterial growth, in addition to preventing the generation of excess acidity that causes food deterioration, thus maintaining a specific pH.

-The sodium present in acetate enhances the flavor of food.

-Sodium acetate is used in the preparation of pickled vegetables: cucumber, carrot, onion, etc. In addition, it is used in the preservation of meat.

Conservation of concrete

Concrete is damaged by the action of water, which shortens its duration. Sodium acetate acts as a concrete sealing agent that makes it waterproof, prolonging its initial characteristics.

Youth Experiments

Experiment 1

A simple experiment is the synthesis of sodium acetate by reacting sodium bicarbonate with vinegar (5% acetic acid).

30 mL of vinegar is placed in a beaker and approximately 3.5 g of bicarbonate are added.

Sodium acetate and carbonic acid are formed in the reaction. Acid breaks down into carbon dioxide and water. The generation of carbon dioxide will produce an intense bubbling within the solution.

To collect the sodium acetate, the water is caused to evaporate, heating the solution.

Experiment 2

Another simple experiment is the formation of crystals of sodium acetate trihydrate.

To do this, about 20 g of sodium acetate trihydrate are weighed and placed in a beaker, then 10 mL of water is added. The solution is heated to a temperature above 58 ° C.

The sodium acetate is completely dissolved to ensure that the solution is supersaturated. Previously, a Pietri dish is placed on a cold surface.

The contents of the beaker with the acetate trihydrate are slowly poured into the Pietri dish. The temperature of the liquid in the capsule begins to drop, even below the melting point, without observing the crystallization or solidification of the sodium acetate trihydrate.

Usually to produce crystallization of sodium acetate trihydrate a small amount of the salt is added to serve as the crystallization core. Other times a small disturbance of the solution occurs, to initiate the crystallization of sodium acetate.

Sodium acetate trihydrate crystals have the appearance of ice, but touching them will show that they are relatively hot. For this reason, salt is called "hot ice."

Toxicity

Sodium acetate is a very low toxic compound. It is also not classified as a sensitizer for the skin and respiratory tract.

Furthermore, sodium acetate is not classified as a germ cell mutagenic, carcinogenic, or reproductive toxic agent.

In short, it is slightly irritating to the eyes. Respiratory tracts may be irritated after inhalation. Frequent and constant contact with the skin can cause irritation.

References

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