Sodium acetate

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Sodium acetate
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Template:Longitem Template:Chembox Elements/molecular formula
Molar mass Template:Chem molar mass
Appearance White deliquescent powder or crystals
Odor Vinegar (acetic acid) odor when heated to decomposition[1]
Density 1.528 g/cm3 (20 °C, anhydrous)
1.45 g/cm3 (20 °C, trihydrate)[2]
Melting point Template:Chembox CalcTemperatures
Boiling point Template:Chembox CalcTemperatures
Solubility Soluble in alcohol, hydrazine, SO2[4]
Solubility in methanol 16 g/100 g (15 °C)
16.55 g/100 g (67.7 °C)[4]
Solubility in ethanol Trihydrate:
5.3 g/100 mL
Solubility in acetone 0.5 g/kg (15 °C)[4]
Acidity (pKa) 24 (20 °C)[4]
4.75 (when mixed with CH3COOH as a buffer)[5]
Basicity (pKb) 9.25
Template:Longitem −37.6·10−6 cm3/mol
Template:Longitem 1.464
Template:Longitem Monoclinic
Template:Longitem 100.83 J/(mol·K) (anhydrous)[6]
229 J/(mol·K) (trihydrate)[7]
Template:Longitem 138.1 J/(mol·K) (anhydrous)[6]
262 J/(mol·K) (trihydrate)[2]
Template:Longitem −709.32 kJ/mol (anhydrous)[4]
−1604 kJ/mol (trihydrate)[2]
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Template:Longitem Sodium formate
Sodium propionate
Template:Longitem Potassium acetate
Calcium acetate
Template:Longitem Sodium diacetate

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Sodium acetate, CH3COONa, also abbreviated NaOAc,[8] is the sodium salt of acetic acid. This salt is colorless, deliquescent, and hygroscopic.

Applications

Biotechnological

Sodium acetate is used as the carbon source for culturing bacteria. Sodium acetate can also be useful for increasing yields of DNA isolation by ethanol precipitation.

Industrial

Sodium acetate is used in the textile industry to neutralize sulfuric acid waste streams and also as a photoresist while using aniline dyes. It is also a pickling agent in chrome tanning and helps to impede vulcanization of chloroprene in synthetic rubber production. It is also used to reduce static electricity during production of disposable cotton pads.

Concrete longevity

Sodium acetate is used as a sealant to mitigate water damage to concrete. It is environmentally benign and cheaper than the commonly used epoxy alternative for sealing concrete against water permeation.[9]

Food

Sodium acetate (anhydrous) is widely used as a shelf-life extending agent and pH control agent.[10] It is safe to eat at low concentration.[11]

Buffer solution

A solution of sodium acetate (a basic salt of acetic acid) and acetic acid can act as a buffer to keep a relatively constant pH level. This is useful especially in biochemical applications where reactions are pH-dependent in a mildly acidic range (pH 4–6).

Heating pad

File:Hand warmer.jpg
A hand warmer contains a supersaturated solution of sodium acetate which releases heat upon crystallization

Sodium acetate is also used in heating pads, hand warmers, and "hot ice". A supersaturated solution of sodium acetate in water is supplied with a device to initiate crystallization, a process that releases substantial heat.

File:Solubilite NaC2H3O2.png
Solubility from CRC Handbook

Sodium acetate trihydrate crystals melt at Script error: No such module "convert".,[12][13] and the liquid sodium acetate dissolves in the released water of crystallization. When heated past the melting point and subsequently allowed to cool, the aqueous solution becomes supersaturated. This solution is capable of cooling to room temperature without forming crystals. By pressing on a metal disc within the heating pad, a nucleation center is formed, causing the solution to crystallize back into solid sodium acetate trihydrate. The process of crystallization is exothermic.[14] The latent heat of fusion is about 264–289 kJ/kg.[12] Unlike some types of heat packs, such as those dependent upon irreversible chemical reactions, a sodium acetate heat pack can be easily reused by immersing the pack in boiling water for a few minutes, until the crystals are completely dissolved, and allowing the pack to slowly cool to room temperature.[15]

Heat stores and batteries

Sodium acetate trihydrate can also be used as a phase-change material to store heat, especially to provide domestic hot water for heat pump applications. The heat store consists of a well-insulated container filled with the salt through which pass a pair of copper coils. One coil is used to melt the material by passing hot water from either solar thermal panels or a heat pump. Cold mains water passes through the other coil where its temperature is raised to 40 or 50 ˚C to provide water for washing or cleaning. This process can be cycled almost indefinitely.

Preparation

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File:Sodium acetate trihydrate crystal - 2009-01-28.jpg
A crystal of sodium acetate trihydrate (length 1.7 centimetres)

For laboratory use, sodium acetate is inexpensive and usually purchased instead of being synthesized. It is sometimes produced in a laboratory experiment by the reaction of acetic acid, commonly in the 5–18% solution known as vinegar, with sodium carbonate ("washing soda"), sodium bicarbonate ("baking soda"), or sodium hydroxide ("lye", or "caustic soda"). When sodium bicarbonate is used the reaction between the bicarbonate ion and acetic acid forms carbonic acid which readily decomposes under normal conditions into gaseous carbon dioxide and water. This is the reaction taking place in the well-known "volcano" that occurs when the household products, baking soda and vinegar, are combined.

CH3COOH + NaHCO3 → CH3COONa + Template:Chem/link
Template:Chem/link → Template:Chem/link + Template:Chem/link

Industrially, hydrated sodium acetate is prepared by reacting acetic acid with sodium hydroxide using water as the solvent.[17]

CH3COOH + NaOH → CH3COONa + H2O.

To manufacture anhydrous sodium acetate industrially, the Niacet Process is used. Sodium metal ingots are extruded through a die to form a ribbon of sodium metal, usually under an inert gas atmosphere such as N2, and then immersed in anhydrous acetic acid.

2 CH3COOH + 2 Na →2 CH3COONa + H2.

The hydrogen gas is normally a valuable byproduct.

Structure

The crystal structure of anhydrous sodium acetate has been described as alternating sodium-carboxylate and methyl group layers.[18] Sodium acetate trihydrate's structure consists of distorted octahedral coordination at sodium. Adjacent octahedra share edges to form one-dimensional chains. Hydrogen bonding in two dimensions between acetate ions and water of hydration links the chains into a three-dimensional network.[19][20]

Comparison of anhydrous and trihydrate crystal structures
Degree of hydration Anhydrous[18] Trihydrate[19][20]
Na coordination File:Sodium-acetate-form-I-xtal-coordination-at-Na2-3D-bs-17.png File:Sodium-acetate-trihydrate-xtal-Na-coordination-3D-bs-17.png
Strongly bonded aggregation File:Sodium-acetate-form-I-xtal-sheet-3D-sf.png
2D sheet 		File:Sodium-acetate-trihydrate-chain-from-xtal-3D-bs-17.png
1D chain
Weakly bonded aggregation File:Sodium-acetate-form-I-xtal-packing-c-3D-bs-17.png
sheets stacked with
hydrophobic surfaces in contact 		File:Sodium-acetate-trihydrate-chain-packing-and-hydrogen-bonding-in-xtal-3D-bs-17.png
chains linked by hydrogen bonds
(one chain highlighted in light blue)

Reactions

Sodium acetate can be used to form an ester with an alkyl halide such as bromoethane:

CH3COONa + BrCH2CH3 → CH3COOCH2CH3 + NaBr

Sodium acetate undergoes decarboxylation to form methane (CH4) under forcing conditions (pyrolysis in the presence of sodium hydroxide):

CH3COONa + NaOH → CH4 + Na2CO3

Calcium oxide is the typical catalyst used for this reaction. Caesium salts also catalyze this reaction.Script error: No such module "Unsubst".

References

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  5. ↑ Sigma-Aldrich Co., Sodium acetate. Retrieved on 2014-06-07.
  6. ↑ a b Acetic acid, sodium salt in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD) (retrieved 2014-05-25).
  7. ↑ Acetic acid, sodium salt, hydrate (1:1:3) in Linstrom, Peter J.; Mallard, William G. (eds.); NIST Chemistry WebBook, NIST Standard Reference Database Number 69, National Institute of Standards and Technology, Gaithersburg (MD) (retrieved 2014-05-25).
  8. ↑ Script error: No such module "citation/CS1".
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  12. ↑ a b Ibrahim Dincer and Marc A. Rosen. Thermal Energy Storage: Systems and Applications, page 155.
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External links

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