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Sodium acetate
Chemical compound

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

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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 to mitigate water damage to concrete by acting as a concrete sealant, while also being environmentally benign and cheaper than the commonly used epoxy alternative for sealing concrete against water permeation.2

Food

Sodium acetate (anhydrous) is widely used as a shelf-life extending agent and pH-control agent.3 It is safe to eat at low concentration.4

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

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.

Sodium acetate trihydrate crystals melt at 58–58.4 °C (136.4–137.1 °F),56 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.7 The latent heat of fusion is about 264–289 kJ/kg.8 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.9

Preparation

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"). Any of these reactions produce sodium acetate and water or sodium acetate and carbonic acid. When a sodium and carbonate ion-containing compound is used as the reactant, the carbonate anion from sodium bicarbonate or carbonate, reacts with the hydrogen from the carboxyl group (-COOH) in acetic acid, forming carbonic acid. Carbonic acid 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 + H2CO3 H2CO3 → CO2 + H2O

Industrially, sodium acetate trihydrate is prepared by reacting acetic acid with sodium hydroxide using water as the solvent.

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 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.10 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.1112

Comparison of anhydrous and trihydrate crystal structures
Degree of hydrationAnhydrous13Trihydrate1415
Na coordination
Strongly bonded aggregation2D sheet1D chain
Weakly bonded aggregationsheets stacked withhydrophobic surfaces in contactchains 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. Cesium salts also catalyze this reaction.

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References

  1. Clayden, Jonathan; Greeves, Nick; Warren, Stuart; Wothers, Peter (2001). Organic Chemistry (1st ed.). Oxford University Press. ISBN 978-0-19-850346-0. 978-0-19-850346-0 ↩

  2. "Potato Chip Flavoring Boosts Longevity Of Concrete". Science Daily. 8 August 2007. https://www.sciencedaily.com/releases/2007/08/070806101941.htm ↩

  3. "Food Additive "Sodium Acetate (Anhydrous)"". Mitsubishi Chemical Corporation. Retrieved 16 September 2020. https://www.m-chemical.co.jp/en/products/departments/mcc/emulsifier/product/1206177_8006.html ↩

  4. Mohammadzadeh-Aghdash, Hossein; Sohrabi, Yousef; Mohammadi, Ali; Shanehbandi, Dariush; Dehghan, Parvin; Ezzati Nazhad Dolatabadi, Jafar (15 August 2018). "Safety assessment of sodium acetate, sodium diacetate and potassium sorbate food additives". Food Chemistry. 257: 211–215. doi:10.1016/j.foodchem.2018.03.020. ISSN 0308-8146. PMID 29622200. S2CID 4596295. Retrieved 16 September 2020. https://www.sciencedirect.com/science/article/pii/S0308814618304370 ↩

  5. Ibrahim Dincer and Marc A. Rosen. Thermal Energy Storage: Systems and Applications, page 155. https://books.google.com/books?id=EsfcWE5lX40C&q=latent+heat+of+fusion+sodium+acetate#search ↩

  6. Courty J.-M., Kierlik É. (2008-12-01). "Les chaufferettes chimiques". Pour la Science (in French). pp. 108–110. https://www.pourlascience.fr/sd/physique/les-chaufferettes-chimiques-892.php ↩

  7. "Crystallization of Supersaturated Sodium Acetate". Journal of Chemical Education. 2015-07-19. http://jchemed.chem.wisc.edu/JCESoft/CCA/CCA3/MAIN/ACETATE/PAGE1.HTM ↩

  8. Ibrahim Dincer and Marc A. Rosen. Thermal Energy Storage: Systems and Applications, page 155. https://books.google.com/books?id=EsfcWE5lX40C&q=latent+heat+of+fusion+sodium+acetate#search ↩

  9. "How do sodium acetate heat pads work?". HowStuffWorks. April 2000. Retrieved 2007-09-03. http://www.howstuffworks.com/question290.htm ↩

  10. Hsu, Leh-Yeh; Nordman, C. E. (1983). "Structures of two forms of sodium acetate, Na+.C2H3O2−". Acta Crystallogr. C. 39 (6): 690–694. Bibcode:1983AcCrC..39..690H. doi:10.1107/S0108270183005946. /wiki/Acta_Crystallographica ↩

  11. Cameron, T. S.; Mannan, K. M.; Rahman, M. O. (1976). "The crystal structure of sodium acetate trihydrate". Acta Crystallogr. B. 32 (1): 87–90. Bibcode:1976AcCrB..32...87C. doi:10.1107/S0567740876002367. /wiki/Acta_Crystallographica ↩

  12. Wei, K.-T.; Ward, D. L. (1977). "Sodium acetate trihydrate: a redetermination". Acta Crystallogr. B. 33 (2): 522–526. Bibcode:1977AcCrB..33..522W. doi:10.1107/S0567740877003975. /wiki/Acta_Crystallographica ↩

  13. Hsu, Leh-Yeh; Nordman, C. E. (1983). "Structures of two forms of sodium acetate, Na+.C2H3O2−". Acta Crystallogr. C. 39 (6): 690–694. Bibcode:1983AcCrC..39..690H. doi:10.1107/S0108270183005946. /wiki/Acta_Crystallographica ↩

  14. Cameron, T. S.; Mannan, K. M.; Rahman, M. O. (1976). "The crystal structure of sodium acetate trihydrate". Acta Crystallogr. B. 32 (1): 87–90. Bibcode:1976AcCrB..32...87C. doi:10.1107/S0567740876002367. /wiki/Acta_Crystallographica ↩

  15. Wei, K.-T.; Ward, D. L. (1977). "Sodium acetate trihydrate: a redetermination". Acta Crystallogr. B. 33 (2): 522–526. Bibcode:1977AcCrB..33..522W. doi:10.1107/S0567740877003975. /wiki/Acta_Crystallographica ↩