99.5 percent acetic acid, also known as glacial acetic acid, is the highly concentrated form of acetic acid with CAS number 64-19-7 and the formula C2H4O2. At concentrations of 99.5 percent or higher the acid is a clear, colorless liquid with a strong, pungent vinegar-like odor, and it is used across food, pharmaceutical and chemical manufacturing industries.
What 99.5% Acetic Acid Is
Glacial acetic acid is essentially anhydrous: it contains less than half a percent of water and has a molar mass of 60.05 g/mol. The name comes from the fact that the nearly water-free liquid solidifies at about 16.6 °C and forms ice-like crystals in a cold room. It is fully miscible with water, alcohols, ethers and many other organic liquids and is one of the most important bulk organic intermediates in the chemical industry.
Because it contains almost no water, glacial acid is the grade of choice for reactions and processes that would be disturbed by moisture, and it is also the economical grade when the acid has to be transported long distances, since water is not being paid for and shipped.
Technical and Food-Grade Specifications
Technical grade glacial acetic acid commonly meets the following limits.
| Parameter | Limit |
|---|---|
| Acetic acid content | 99.5% minimum |
| Formic acid | 0.06% maximum |
| Acetaldehyde | 0.05% maximum |
| Residue on evaporation | 0.01% maximum |
| Moisture | 0.15% maximum |
| Iron, as Fe | 0.00004% maximum |
| Color value | 10 Pt-Co maximum |
| Potassium permanganate reduction time | 30 minutes minimum |
Food grade glacial acetic acid is specified more tightly on trace metals: acetic acid content of 99.8 percent minimum, evaporation residue of 0.005 percent maximum, freezing point of 16.2 °C minimum, heavy metals expressed as lead of 2 mg/kg maximum and arsenic of 1 mg/kg maximum.
Chemical Manufacturing: VAM, Anhydride and Esters
The largest use of acetic acid is in the production of vinyl acetate monomer, abbreviated as VAM, which is a building block for adhesives, paints and polymer emulsions. Two molecules of acetic acid can also undergo a condensation reaction to form acetic anhydride, an important acetylating agent used for cellulose acetate and a range of other derivatives.
Acetate esters such as ethyl acetate and butyl acetate, used as solvents in coatings, inks and printing.
Cellulose acetate for textile fibers, filter tow and film products.
Purified terephthalic acid manufacture, where acetic acid serves as the reaction medium for oxidation of p-xylene.
Acetic acid as a process solvent in the production of other organic intermediates.
Iron, moisture and reducing impurities are the main concerns in these duties, because they can disturb catalyst activity and colour the final polymer, which is why the iron and permanganate time limits in the specification are written so tightly.
Food, Pharmaceutical and Textile Applications
As the major constituent of vinegar, acetic acid is used in the pickling of vegetables and as an acidity regulator in food processing, where it carries the food additive identity E260. Food grade material must meet strict purity limits for heavy metals and arsenic before it can be released to a food plant.
Diluted acetic acid has antibacterial properties and is used as an antiseptic in some medical and hygiene applications; it also acts as a pH adjuster and excipient in pharmaceutical production, where all medical uses follow approved clinical protocols and regulatory guidance. In textiles, the manufacture of rayon fiber consumes acetic acid, and the acid is also used in rubber processing and in the manufacture of various perfumes and fragrance esters.
Safety and Handling
Glacial acetic acid is corrosive and can cause severe burns on contact with skin or eyes. Vapors are irritating to the respiratory tract, and the liquid is flammable. Chemical-resistant gloves, goggles and protective clothing are required when handling the acid, and the material should be used only in well-ventilated areas, away from oxidizers and ignition sources.
In case of skin contact, rinse immediately with plenty of water and remove contaminated clothing. For eye contact, flush for at least 15 minutes and seek medical advice without delay. Spill kits, eye-wash stations and safety showers should be available wherever the acid is decanted or dosed.
Storage Guidelines
Store glacial acetic acid in corrosion-resistant containers, typically stainless steel or approved plastic, in a cool, dry, ventilated area. The freezing point is about 16.6 °C, so storage areas and transfer lines may need insulation or mild heating to prevent solidification in cold climates, and the food grade specification sets a minimum freezing point of 16.2 °C.
Keep containers tightly closed, because the acid absorbs moisture from humid air and would lose part of its glacial character, and keep them separated from incompatible materials such as strong oxidizers, strong bases and active metals. Tanks, pumps and gaskets should be selected for the concentrated acid, and regular inspection of seals and vents prevents both leakage and pressure build-up.
Frequently Asked Questions
Q: What is 99.5% acetic acid?
It is glacial acetic acid, the concentrated form of acetic acid with a minimum content of 99.5 percent, the formula C2H4O2 and CAS number 64-19-7.
Q: What is glacial acetic acid used for?
It is used in chemical manufacturing, food processing, pharmaceutical production and textile applications, including the production of vinyl acetate monomer, acetic anhydride, cellulose acetate and rayon fiber.
Q: Is glacial acetic acid the same as vinegar?
No. Vinegar is a dilute solution of acetic acid, typically around 4 to 8 percent, while glacial acetic acid is nearly pure acid above 99 percent.
Q: Is glacial acetic acid dangerous?
Yes. It is corrosive and can cause severe burns to skin and eyes, its vapors irritate the respiratory tract, and the liquid is flammable, so protective equipment is required.
Q: What is the freezing point of glacial acetic acid?
The freezing point is about 16.6 °C, and the food grade specification requires a minimum of 16.2 °C, so the acid can solidify in cold storage conditions.
Q: How does 99.5% acetic acid differ from the 80% grade?
The 80 percent grade is a water-based solution that stays liquid in cold plants and is used mainly for descaling and pH control, while the 99.5 percent grade is nearly anhydrous and is used where water must be excluded.