CID 517045
CH3COONa
Sodium acetate
- C 2
- H 3
- O 2
- Na 1
- IUPAC name
- sodium acetate
- Molar mass
- 82.0340(76) g/mol
- Atoms
- 8
- SMILES
- CC(=O)[O-].[Na+]
- InChIKey
- VMHLLURERBWHNL-UHFFFAOYSA-M
- PubChem CID
- 517045
Some reusable heat packs hold a sodium acetate solution that stays liquid until a small strip inside is flexed, then crystallises and gives off heat. The compound is the sodium salt of acetic acid, an organic salt in which one sodium ion, Na+, partners each acetate ion, CH3COO−. Its molar mass is 82.03 g/mol. Its name under the rules of the International Union of Pure and Applied Chemistry (IUPAC) is the everyday one, sodium acetate.
Industry makes it by letting acetic acid act on an alkali, for example caustic soda or soda ash. It also comes out as a by-product of manufacturing acetanilide, acetylsalicylic acid (aspirin) and similar acetylated compounds. Reports to the US Environmental Protection Agency put national volume for 2023 between 10 million and 50 million pounds. The dry salt is a white powder, and the International Chemical Safety Card describes its solution in water as a weak base.
Structure and bonding
Eight atoms make up the drawing, in two ions that no bond connects. Sodium, Na+, stands alone as one. The acetate ion is the other: a carbon bearing three hydrogen atoms, singly bonded to a second carbon that holds two oxygen atoms. One oxygen is double-bonded; the other is single-bonded and carries the −1 charge. Together they are one formula unit of an ionic lattice, not a free-standing particle.
Which oxygen gets the double bond and which the charge is arbitrary, since the two could trade roles. Chem1 treats a structure offering two equivalent arrangements of single and double bonds as a resonance hybrid, a blend of both.
The drawing shows the anhydrous salt, whose monoclinic crystals are hygroscopic, taking up moisture from the air. The trihydrate, the form used in warmers, holds water of crystallisation in colourless monoclinic prisms. These melt at 58 °C, far below the anhydrous salt’s 324 °C in the Hazardous Substances Data Bank (HSDB) or 328 °C on the safety card. In warm, dry air the trihydrate effloresces, losing that water.
Where it turns up
Foot warmers and milk-bottle warmers use the trihydrate, HSDB records. Hospitals and clinics use a single-use infant heel warmer filled with a food-grade sodium acetate solution, cleared by the US Food and Drug Administration (FDA) in 2015. It warms a baby’s heel to raise blood flow there before blood is drawn for tests. Dialysis solutions rely on the salt as a source of sodium ions.
HSDB lists meat preservation among its uses, and it can migrate into food from packaging. In 1983, by HSDB’s breakdown, textiles, leather, dyestuffs and chemical intermediates took 75 per cent of the supply. Photography and cosmetics use it as a buffer. With acetic acid it forms a conjugate acid-base pair, two species that differ by a single proton. That pair, the core of any buffer, can absorb small additions of hydrogen or hydroxide ions.
Why it matters
The heat pack of US Patent 4,077,390, published in 1978, carries its sodium acetate solution supercooled: liquid below the point where it would normally crystallise. Flexing an activator strip inside produces the first crystals and releases heat, and those crystals spread crystallisation through the whole solution. The inventors believed the bending extends tiny fractures in the strip, and that these fresh cracks start the process. In the FDA-cleared heel warmer, a flexible metal disc does the starting, and contact with tiny crystals sets off the heat-releasing change.
Once absorbed, the acetate ion is metabolised rapidly and completely, and tissues outside the liver handle it readily. In effect, sodium acetate ends up acting like sodium bicarbonate, raising the pH of blood and urine. Medicine uses it as an alkaliser and as an electrolyte replenisher: in parenteral solutions, given other than by mouth, it replaces sudden losses of fluid. HSDB credits it with no real advantage over bicarbonate solution except a lower cost and longer storage in hot climates. Vets use it against ketosis in cattle.
Properties
| Property | Value | Unit | Source |
|---|---|---|---|
| Formula (Hill order) | C2H3NaO2 | ||
| Molar mass | 82.0340(76) g/mol | g/mol | IUPAC/CIAAW 2024 |
| Carbon atoms | 2 | ||
| Hydrogen atoms | 3 | ||
| Oxygen atoms | 2 | ||
| Sodium atoms | 1 | ||
| PubChem molecular weight | 82.03 g/mol | g/mol | PubChem |
Safety
As a food additive or the solution sealed in a heat pack, sodium acetate is essentially harmless. In 95.4% of reports to the European Chemicals Agency it meets no Globally Harmonized System (GHS) hazard criteria. The safety card calls the solid combustible and mildly irritating to eyes and skin; it decomposes on heating or on contact with strong acids, giving off acetic acid fumes. The PubChem Laboratory Chemical Safety Summary is the sheet to consult.
Questions
What is the chemical formula of sodium acetate?
It is written CH3COONa, which sets the acetate group apart, or C2H3NaO2 in PubChem's record. Either way the formula gives a ratio, not a molecule: one sodium ion, Na+, for each acetate ion, CH3COO−, in an ionic lattice. The trihydrate, the form used in foot warmers, also holds three water molecules for every formula unit, as water of crystallisation.
Is sodium acetate acidic or basic?
Weakly basic, according to the International Chemical Safety Card. Acetate is the conjugate base of acetic acid, the ion left when that weak acid gives up a proton. Some acetate ions therefore undergo hydrolysis, each taking a proton from a water molecule. Chem1 states the general pattern: the weaker the parent acid, the more alkaline a solution of its salt. Adding a strong acid shrinks the proportion of acetate, turning it back into acetic acid.
Why does the liquid in a heat pack not crystallise on its own?
In the reusable pack of US Patent 4,077,390 the solution is supercooled, and a crystal needs somewhere to begin. In Chem1's account, a tiny cluster must grow to a critical size and stay stable enough to act as a nucleus, the surface on which the crystal builds. Most nucleation is thought to happen on some other particle, perhaps a speck of dust. Without one, a supersaturated solution, holding more dissolved salt than equilibrium allows, ordinarily forms no solid.
Is sodium acetate safe to eat?
US Food and Drug Administration (FDA) rules affirm sodium acetate added directly to food as generally recognised as safe, provided it meets the stated requirements. JECFA, the food-additives committee run jointly by the UN Food and Agriculture Organization and the World Health Organization, lists it as an acidity regulator. The US Environmental Protection Agency's Safer Choice programme gives it a green circle, meaning verified as of low concern.
Made of the same elements
- NaHCO3 Sodium bicarbonate 6 atoms in one formula unit of an ionic lattice.
Sources
- PubChem CID 517045, sodium acetate
- IUPAC/CIAAW, Abridged Standard Atomic Weights 2024
- PubChem Laboratory Chemical Safety Summary for sodium acetate (CID 517045)
- US Patent 4,077,390, Reusable heat pack containing supercooled solution and means for activating same (1978)
- US Food and Drug Administration, 510(k) K150627, Rapid Aid Instant Disposable Infant Heel Warmer (2015)
- Stephen Lower, Chem1 General Chemistry, §12 Solubility Equilibria (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §13.4 Conjugate Pairs and Buffers (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §10.3 Acid-Base Reactions à la Brønsted (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §10.4 Acid-Base Reactions (LibreTexts, CC BY 3.0)
- Stephen Lower, Chem1 General Chemistry, §9.7 The Hybrid Orbital Model II (LibreTexts, CC BY 3.0)