What Is Glacial Acetic Acid?

Acetic acid (CH₃COOH) is the common name for ethanoic acid. It is an organic chemical compound that has a distinctive pungent odor and sour flavor, recognizable as the scent and flavor of vinegar. Vinegar is about 3-9% acetic acid.

How Glacial Acetic Acid Is Different

Acetic acid that contains a very low amount of water (less than 1%) is called anhydrous (water-free) acetic acid or glacial acetic acid. The reason it's called glacial is because it solidifies into solid acetic acid crystals just cooler than room temperature at 16.7 °C. Removing the water from acetic acid lowers its melting point by 0.2 °C.

Glacial acetic acid may be prepared by dripping acetic acid solution over a "stalactite" of solid acetic acid (which could be considered to be frozen). Like a water glacier contains purified water, even if it's floating in the salty sea, pure acetic acid sticks to the glacial acetic acid, while impurities run off with the liquid.

Caution: Although acetic acid is considered a weak acid, safe enough to drink in vinegar, glacial acetic acid is corrosive and can injure skin on contact.

More Acetic Acid Facts

Acetic acid is one of the carboxylic acids. It is the second simplest carboxylic acid, after formic acid. The main uses of acetic acid are in vinegar and to make cellulose acetate and polyvinyl acetate. Acetic acid is used as a food additive (E260), where it is added for flavor and to regular acidity. It's an important reagent in chemistry, too. Worldwide, around 6.5 metric tons of acetic acid are used per year, of which approximately 1.5 metric tons per year are produced by recycling. Most acetic acid is prepared using petrochemical feedstock.

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What Is the Chemical Composition of Vinegar?

By Anne Marie Helmenstine, Ph.D.

Acetic Acid and Ethanoic Acid Naming

The IUPAC name for the chemical is ethanoic acid, a name formed using the convention of dropping the final "e" in the alkane name of the longest carbon chain in the acid (ethane) and adding the "-oic acid" ending.

Even though the formal name is ethanoic acid, most people refer to the chemical as acetic acid. In fact, the usual abbreviation for the reagent is AcOH, partly to avoid confusion with EtOH, a common abbreviation for ethanol. The common name "acetic acid" comes from the Latin word acetum, which means vinegar.

Acidity and Use as a Solvent

Acetic acid has an acidic character because the hydrogen center in the carboxyl group (-COOH) separates via ionization to release a proton:

CH₃CO₂H → CH₃CO₂⁻ + H⁺

This makes acetic acid a monoprotic acid with a pKa value of 4.76 in aqueous solution. The concentration of the solution greatly affects the dissociation to form the hydrogen ion and the conjugate base, acetate (CH₃COO⁻). At a concentration comparable to that in vinegar (1.0 M), the pH is around 2.4 and only around 0.4 percent of the acetic acid molecules are dissociated. However, in very dilute solutions, over 90 percent of the acid dissociates.

Acetic acid is a versatile acidic solvent. As a solvent, acetic acid is a hydrophilic protic solvent, much like water or ethanol. Acetic acid dissolves both polar and nonpolar compounds and is miscible in both polar (water) and nonpolar (hexane, chloroform) solvents. However, acetic acid is not fully miscible with higher alkanes, such as octane.

Importance in Biochemistry

Acetic acid ionizes to form acetate at physiological pH. The acetyl group is essential to all life. Acetic acid bacteria (e.g., Acetobacter and Clostridium acetobutlicum) produce acetic acid. Fruits produce acetic acid as they ripen. In humans and other primates, acetic acid is a component of vaginal lubrication, where it acts as an antibacterial agent. When the acetyl group binds to coenzyme A, the holoenzyme is used in the metabolism of fats and carbohydrates.

Acetic Acid in Medicine

Acetic acid, even at 1 percent concentration, is an effective antiseptic, used to kill Enterococci, Streptococci, Staphylococci, and Pseudomonas. Dilute acetic acid may be used to control skin infections of antibiotic bacteria, particularly Pseudomonas. The injection of acetic acid into tumors has been a cancer treatment since the early 19th century. The application of dilute acetic acid is a safe and effective treatment for otitis externa. Acetic acid is also used as a quick cervical cancer screening test. Acetic acid swabbed onto the cervix turns white in one minute if cancer is present.

Additional References

• Fokom-Domgue, J.; Combescure, C.; Fokom-Defo, V.; Tebeu, P. M.; Vassilakos, P.; Kengne, A. P.; Petignat, P. (3 July 2015). "Performance of alternative strategies for primary cervical cancer screening in sub-Saharan Africa: systematic review and meta-analysis of diagnostic test accuracy studies". BMJ (Clinical research ed.). 351: h3084.
• Madhusudhan, V. L. (8 April 2015). "Efficacy of 1% acetic acid in the treatment of chronic wounds infected with Pseudomonas aeruginosa: prospective randomised controlled clinical trial". International Wound Journal. 13: 1129–1136.