Acetic acid (CAS 64-19-7) is a fundamental organic acid with the formula CH3COOH, used across the chemical, pharmaceutical, food, textile, and laboratory industries. One of its most important physical properties is its excellent solubility in water: acetic acid is completely miscible with water in all proportions. This behavior directly influences how the acid is stored, transported, diluted, and formulated, and it is the reason acetic acid appears in applications as diverse as food preservation and polymer synthesis.
Does Acetic Acid Dissolve in Water?
Yes. Acetic acid and water mix in any ratio to form a homogeneous solution. The molecular basis is the carboxyl group (-COOH), which is strongly polar and forms hydrogen bonds with water molecules. When acetic acid dissolves, part of the molecules ionize into acetate ions (CH3COO-) and hydrogen ions (H+), making the solution weakly acidic; a 1 mol/L solution has a pH of about 2.4, similar to the acidity of vinegar at comparable concentration. Because the two liquids are fully miscible, there is no separate layer formation, no solubility limit, and no need for emulsifiers or co-solvents.
Key Specification of Glacial Acetic Acid
| Property | Typical Value |
|---|---|
| Chemical Formula | CH3COOH |
| CAS Number | 64-19-7 |
| Appearance | Colorless transparent liquid |
| Purity (Glacial Grade) | 99% or higher |
| Boiling Point | 117.9°C |
| Melting Point | 16.6°C |
| Density (20°C) | About 1.049 g/cm3 |
| pH of 1 mol/L Solution | About 2.4 |
Glacial acetic acid is the name given to acetic acid at concentrations of about 99% or higher. It is called glacial because it freezes at 16.6°C, forming ice-like crystals in cold conditions, and it must be stored and handled with attention to its freezing behavior.
Industrial Applications Enabled by Water Miscibility
Food industry: used as an acidulant, preservative, and flavoring agent in vinegar-based products, pickles, sauces, and dressings.
Chemical synthesis: key feedstock for vinyl acetate monomer (used in paints and adhesives), terephthalic acid (for PET plastics), cellulose acetate, and acetate esters.
Textile processing: pH adjustment in dyeing with disperse, acid, and cationic dyes, and neutralization after alkaline pretreatment.
Laboratories: preparation of buffer solutions, acid-base titrations, and general reagent use.
Pharmaceuticals: intermediate and pH-control agent in drug synthesis and formulation.
Dilution, Storage, and Transportation
Mixing glacial acetic acid with water is exothermic: significant heat is released, so acid should be added to water gradually with stirring rather than the reverse, to avoid local boiling and splashing. Store acetic acid in containers made of compatible materials such as stainless steel (316L) or suitable plastics such as HDPE for dilute solutions, and keep glacial grade in tightly sealed containers because the vapor is corrosive and the product is hygroscopic.
Because glacial acetic acid freezes at 16.6°C, cold-season storage requires heated tanks or insulated facilities in regions with low winter temperatures; frozen product can rupture containers and piping. Acetic acid is corrosive to skin, eyes, and mucous membranes, and concentrated vapor is irritating, so handling requires protective equipment and adequate ventilation. Transportation must comply with regulations for corrosive liquids, with proper packaging, labeling, and documentation.
Frequently Asked Questions
Why is acetic acid completely soluble in water?
The polar carboxyl group forms strong hydrogen bonds with water, and the small carbon chain does not create a significant hydrophobic barrier. As a result, acetic acid and water mix in all proportions without phase separation.
What is the difference between acetic acid and glacial acetic acid?
Glacial acetic acid is concentrated acetic acid, typically 99% or higher, which freezes at 16.6°C into ice-like crystals. Dilute acetic acid, such as vinegar at 4-8%, is the same compound in aqueous solution.
Is it safe to add water to acetic acid?
Always add acid to water, not water to acid, and stir while mixing because dilution releases heat. Adding water rapidly to glacial acetic acid can cause splashing and localized heating.
What pH does acetic acid solution have?
A 1 mol/L solution has a pH of about 2.4. Higher dilutions approach neutral pH, while concentrated acetic acid is a weak acid that ionizes only partially in water.
Why is about 60% of glacial acetic acid used in chemical production?
The largest chemical uses are vinyl acetate monomer for paints and adhesives and terephthalic acid for PET, which together account for the majority of acetic acid consumption, followed by acetate esters, cellulose acetate, and other derivatives.
What happens if glacial acetic acid freezes?
At temperatures below 16.6°C it solidifies into ice-like crystals. This can block piping and rupture containers, so heated storage or temperature-controlled facilities are required in cold climates.
