Acetic acid is an organic acid and a byproduct of sugar fermentation. It’s also known by other names, including ethanoic acid, ethylic acid, and methane carboxylic acid.
Acetic acid is primarily produced by a group of Gram-negative bacteria called acetic acid bacteria (AAB). They produce acetic acid as metabolic waste from fermenting sugar or ethanol. Our bodies also make small amounts of acetic acid, which play an important role in metabolising carbs and fats.
Many people think acetic acid is vinegar itself. However, vinegar is actually just 4% to 6% acetic acid dissolved in water with other organic and inorganic impurities. Aside from its famous culinary uses, acetic acid in its pure form has many industrial applications. For example, it’s used to produce metal acetates, which are a pivotal part of the printing processes.
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Acetic acid explained
Acetic acid is chemically composed of two carbon, four hydrogen, and two oxygen atoms. Its chemical formula is commonly written as CH3COOH to indicate the functional groups, which are the methyl group (—CH3), hydroxyl group (—OH), and carbonyl group (C=O).
The carbon in the carbonyl group serves as the central carbon to which the functional groups are attached. See the illustration below, which shows the molecular structure of acetic acid. 
Just like many other organic acids, acetic acid is classified as a weak acid. This is because it does not completely dissociate into ions when dissolved in water. When it’s in an aqueous solution, its ions separate into the negatively charged acetate ion (CH3COO–) and positively charged hydrogen ion (H+).
However, only four per 100 molecules of acetic acid dissociate into ions when dissolved in water. Hence, the dynamic equilibrium of the system is represented by this chemical equation:
CH3COOH(aq) + H2O(l) ⇌ CH3COO–(aq)+ H3O+(aq)
Acetic acid is one of the simplest types of carboxylic acid, second only to formic acid. Industrially, it’s produced both through synthetic processes and bacterial fermentation. The bacterial fermentation process follows a metabolic pathway that produces acetic acid either from glucose or ethanol. 
Methanol carbonylation is the main method for the industrial production of acetic acid. Around 75% of the world’s acetic acid is made using this synthetic pathway, with just 10% produced through the organic biological method of fermentation.
So how does it work? Methanol reacts with carbon monoxide using a catalyst, while iodomethane is an intermediate in the process. 
Methanol carbonylation involves three steps, which include metal carbonyl as a catalyst of carbonylation.
- Step 1: CH3OH + HI → CH3I + H2O
- Step 2: CH3I + CO → CH3COI
- Step 3: CH3COI + H2O → CH3COOH + HI
What is the use of acetic acid?
Most of us are familiar with acetic acid as vinegar, which is mainly used as a preservative or flavouring in food preparation. Humans have been using vinegar for thousands of years, with the very first recorded use dating back to ancient Babylonians at around 5000 BCE.
Other uses of acetic acid include:
- Paints and adhesives production – acetic acid is used to make vinyl acid monomer, which can then be polymerised into polyvinyl acetate and used to manufacture paints and adhesives.
- Ester production – esters can be produced from the catalytic reactions between acetic acid and a corresponding alcohol. They have a wide range of industrial applications and are commonly used to manufacture inks, coatings, and paints.
- Acetic anhydride – up to 30% of the world’s acetic acid supply is used to produce acetic anhydride, making it one of the main applications of acetic acid. Acetic anhydride is used to make other materials, such as cellulose acetate, a synthetic textile. It also acts as a precursor when manufacturing medicines such as aspirin and the illicit drug heroin.
- Solvent – a great polar protic solvent, acetic acid is used as a solvent for the recrystallisation and purification of organic compounds.
- Medicine – acetic acid has been used as an antiseptic for thousands of years. It was also used to treat tumours in the 1800s and is a good test agent for cervical cancer.
What products contain acetic acid?
As an additive, preservative, and flavouring, acetic acid from vinegar can be found in several food products. These include marinades, mustard, ketchup, salad dressings, mayonnaise and canned fruits.
It’s also found in many pickled products, including pickled sausages. Other products, as previously mentioned, are manufactured using acetic acid but they do not contain acetic acid in the true sense.
Is acetic acid harmful?
Although acetic acid is not harmful at low concentrations, if the concentration exceeds 25% it can have damaging effects.
Highly concentrated acetic acid can irritate the mucous membranes of the nasal cavity and throat, and can even damage the eyes. In addition, inhaling the vapour may cause coughing, breathing difficulties and headaches. It can also result in fever and confusion.
How acetic acid affects the body
Acetic acid only has adverse effects on the body at high concentrations of over 25%. It can burn the lining of the mouth and throat when ingested, making it difficult to breathe and swallow.
It may also trigger vomiting and stomach pain. In some serious cases, highly concentrated acetic acid can damage the airways and cause fluid to accumulate in the lungs.
What is acetic acid also known as?
Acetic acid is more commonly known as vinegar, although vinegar per se only contains around 4% to 6% acetic acid. The acid is systematically named ethanoic acid, but it’s also known as ethylic acid and methane carboxylic acid.
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