Sodium acetate is a sodium salt of acetic acid with the chemical formula CH₃COONa. It is a colorless, crystalline solid that is highly soluble in water and has a faint odor similar to vinegar due to its acetic acid component. Sodium acetate is commonly encountered in its anhydrous form or as a trihydrate (CH₃COONa·3H₂O).
It is widely used in various industries — in food preservation, buffer solutions, textile dyeing, and chemical synthesis. In everyday life, it is known for its role in “hot ice” packs, where a supersaturated sodium acetate solution releases heat upon crystallization.
Structure
The structure of sodium acetate consists of two main parts:
- Cation: Sodium ion (Na⁺)
- Anion: Acetate ion (CH₃COO⁻)
The acetate ion is derived from acetic acid (CH₃COOH) when the hydrogen atom from the carboxylic acid group (-COOH) is replaced by a sodium ion.
Structural Representation:
- The acetate ion (CH₃COO⁻) contains a methyl group (CH₃) attached to a carboxylate group (COO⁻).
- The negative charge is delocalized over the two oxygen atoms due to resonance, giving the ion extra stability.
- The sodium ion (Na⁺) is ionically bonded to the acetate anion, balancing the negative charge.
Physical Properties
- Appearance:
Sodium acetate is a colorless, crystalline solid that may also occur as a white powder. - Molecular Weight:
- Anhydrous: 82.03 g/mol
- Trihydrate (CH₃COONa·3H₂O): 136.08 g/mol
- Melting Point:
- Anhydrous: 324 °C (decomposes on heating)
- Trihydrate: 58 °C
- Solubility:
- Highly soluble in water, forming a mildly basic solution.
- Slightly soluble in alcohol and practically insoluble in ether.
- Density:
- Anhydrous: 1.528 g/cm³
- Trihydrate: 1.45 g/cm³
- Taste and Odor:
It has a salty, slightly vinegary taste and a faint acetic odor. - Hygroscopic Nature:
The trihydrate form readily absorbs moisture from the air. - pH:
A 1 M aqueous solution has a pH around 8.9, indicating mild basicity.
Chemical Properties
Reaction with Acids:
Sodium acetate reacts with strong acids (like hydrochloric acid) to liberate acetic acid:
CH3COONa+HCl→CH3COOH+NaCl.
This reaction is used to produce acetic acid in laboratories.
Reaction with Bases:
When reacted with strong bases like sodium hydroxide, there is no significant reaction, as both are salts of a strong base (NaOH) and weak acid (CH₃COOH).
Hydrolysis in Water:
Sodium acetate undergoes hydrolysis in water, forming a slightly basic solution:
CH3COONa+H2O⇌CH3COOH+NaOH.
This occurs because the acetate ion reacts weakly with water, generating hydroxide ions.
Thermal Decomposition:
On heating strongly, sodium acetate decomposes to form methane (CH₄) and sodium carbonate
(Na₂CO₃): 2CH3COONa+NaOH→CH4+Na2CO3.
(This reaction is also used in the laboratory preparation of methane.)
Buffer Formation:
Sodium acetate, when mixed with acetic acid, forms an acetate buffer solution, which maintains a nearly constant pH:
CH3COOH⇌CH3COO−+H+.
The presence of CH₃COONa shifts equilibrium, resisting pH changes upon addition of acids or bases.
Preparation
Sodium acetate can be prepared by several methods:
From Acetic Acid and Sodium Carbonate or Sodium Bicarbonate:
2CH3COOH+Na2CO3→2CH3COONa+H2O+CO2↑
or
CH3COOH+NaHCO3→CH3COONa+H2O+CO2↑
This is the most common and convenient laboratory method.
From Acetic Acid and Sodium Hydroxide:
CH3COOH+NaOH→CH3COONa+H2O
This is a neutralization reaction between a weak acid and a strong base, producing sodium acetate and water.
By Heating Acetaldehyde and Sodium Hydroxide (Industrial Process):
In the presence of air, acetaldehyde can oxidize to acetic acid, which then reacts with NaOH to form sodium acetate.
Uses
In the Food Industry
- Food Preservative (E262):
- Sodium acetate is used as a food additive and preservative, labeled as E262, to control acidity and prevent microbial growth in processed foods such as sauces, bread, and meat products.
- Flavoring Agent:
It adds a mild salty and tangy flavor, often used in salt and vinegar flavored chips. - pH Control Agent:
Helps stabilize food pH, maintaining freshness and preventing spoilage.
In the Textile Industry
- Neutralizing Agent:
Used in dyeing processes to neutralize sulfuric acid waste after fabric dyeing. - Buffer in Dye Baths:
Maintains consistent pH during textile dyeing and printing, ensuring even color distribution on fabrics.
In Laboratories and Chemical Analysis
- Buffer Preparation:
Combined with acetic acid, sodium acetate forms an acetate buffer, commonly used to maintain constant pH in biochemical and analytical experiments. - Reagent:
Used in titrations and chemical synthesis where a mild basic environment is required. - DNA Precipitation:
In molecular biology, sodium acetate is used to precipitate DNA and RNA from aqueous solutions during extraction and purification procedures.
In the Pharmaceutical and Medical Field
- Electrolyte Component:
Used in intravenous (IV) solutions to replenish electrolytes and maintain acid-base balance in patients. - Buffering Agent:
Helps stabilize drug formulations and maintain their optimal pH. - Antacid Mixtures:
Occasionally used in some mild antacid preparations.
In Heating Pads and Hand Warmers
- “Hot Ice” Applications:
Sodium acetate trihydrate is used in reusable heating pads.
When a metal disc inside the pad is clicked, it triggers crystallization of supersaturated sodium acetate solution, releasing latent heat (an exothermic process).
The pad can be reheated to dissolve the crystals and reused.
In the Leather and Rubber Industry
- Tanning Agent:
Used in leather tanning to control pH and improve texture. - Latex Coagulation:
Helps in the coagulation of latex during rubber production.
In Waste Treatment and De-icing
- Biological Waste Treatment:
Acts as a carbon source for bacteria in wastewater treatment plants, enhancing denitrification. - De-icing Agent:
A less corrosive alternative to sodium chloride for de-icing airport runways and roads, as it reduces ice formation while being environmentally safer.
In Education and Demonstration
- Chemical Demonstrations:
Commonly used in classroom experiments to show exothermic crystallization (“hot ice”) and buffer action in acid–base reactions.