Acetic Acid
Properties
| State | Liquid (pungent vinegar odor) |
| Color | Colorless |
| Solubility | Miscible with water in all proportions |
| Melting Point | 16.6°C |
| Boiling Point | 118.1°C |
About Acetic Acid
Acetic acid is the carboxylic acid that almost every general-chemistry weak-acid problem ends up using as its example, and there's a reason: its Ka of 1.8 × 10⁻⁵ sits in the textbook-friendly middle of the dissociation range, its conjugate base (acetate) is well-behaved, and the acetate buffer pair anchors the pH region around 4–5 that comes up constantly in biology and food chemistry. The two-carbon structure — methyl group attached to a carboxyl — is the simplest place where the full chemistry of carboxylic acids actually shows up; formic acid behaves a little oddly because its α-carbon is just a hydrogen. Pure (glacial) acetic acid freezes at 16.6 °C, which is why a winter cold-room can produce ice-like crystals in a bottle that was perfectly liquid in summer. Industrially, almost all acetic acid is now made by the Cativa or Monsanto process — methanol carbonylation with a rhodium or iridium catalyst — at a scale of around 18 million tonnes a year. About a third of that goes into vinyl acetate monomer for paints and adhesives, another big chunk into terephthalic acid for PET, and the remainder splits across acetic anhydride, ester solvents, and direct use as vinegar.
Where you'll encounter it
Outside a chemistry lab, you meet acetic acid as vinegar — a 4–8% aqueous solution made by feeding ethanol to Acetobacter bacteria, the same biology that turns wine into vinegar if a bottle gets left open too long. In a chemistry teaching lab, it's the substance students use to demonstrate buffer behavior: a 0.1 M acetic acid / 0.1 M sodium acetate mixture holds pH near 4.74 (the pKa) even when challenged with small additions of strong acid or base, which is the standard introduction to the Henderson–Hasselbalch equation. In an analytical setting, glacial acetic acid is the solvent for non-aqueous titrations of weak bases, since its low autoprotolysis constant lets a strong acid like perchloric acid stay protonated and give a sharp endpoint.
Common Uses
- Vinyl acetate monomer feedstock for paints and adhesives
- Solvent for terephthalic acid synthesis (PET production)
- Acetate-buffer component for biology and analytical work
- Aspirin and pharmaceutical synthesis precursor
- Glacial-form solvent for non-aqueous titrations
Safety Information
Concentrated (>50%) acetic acid is corrosive — burns skin and eyes on contact and the vapor irritates the respiratory tract. Glacial acetic acid is also flammable, with a flash point near 39 °C. Dilute solutions (vinegar concentrations) are food-safe. GHS pictograms are GHS05 (corrosive) and GHS02 (flammable) for the concentrated form.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.