Acetic acid

• 1. Introduction

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  Acetic acid has been inside a bottle of vinegar on your kitchen shelf for as long as vinegar has existed, which is at least 10,000 years. It is the chemical compound that makes vinegar sour, the molecule your body uses to burn carbohydrates and fats, and one of the most produced industrial chemicals on the planet. By 2023, global demand had reached 17.88 million metric tonnes per year. That single figure spans an enormous range of uses, from the food on your table to photographic film, from wood glue to the treatment of cervical cancer in clinics across the developing world. How did a molecule first noticed as the thing that makes wine go bad become so fundamental to modern industry and to life itself? And what is it about acetic acid that makes it appear, in one form or another, almost everywhere chemistry touches human existence?

• 2. Ten Thousand Years of Sour

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  Hippocrates prescribed vinegar as an antiseptic and as a remedy for fever, constipation, ulcers, and pleurisy, placing acetic acid among the earliest recorded medicines. The Greek philosopher Theophrastus, writing in the third century BCE, described how vinegar acted on metals to produce pigments used in art, among them white lead and verdigris, a green mixture of copper salts that included copper(II) acetate. Ancient Romans found a different application. They boiled soured wine to produce a sweet syrup called sapa. When sapa was made in lead pots, it became rich in lead acetate, a substance also known as sugar of lead, and the Roman aristocracy consumed it in quantity. Historians believe this contributed significantly to lead poisoning among Rome's ruling class.

  For centuries, chemists working with vinegar were puzzled by a property of acetic acid that seemed almost paradoxical. The presence of water in vinegar so dramatically changes the compound's physical behaviour that chemists long believed glacial acetic acid, the pure water-free form, and the acid in vinegar were actually two different substances entirely. The name "glacial" comes from the ice-like crystals that form when anhydrous acetic acid is agitated at slightly below room temperature, around 16.6 degrees Celsius. It was French chemist Pierre Adet who eventually proved the two were identical. In 1845, German chemist Hermann Kolbe went further by synthesizing acetic acid from inorganic compounds for the first time, a reaction sequence involving chlorination of carbon disulfide through to electrolytic reduction.

• 3. Inside the Body

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  At physiological pH levels, acetic acid is fully ionized to acetate in aqueous solution, which is the form in which living cells actually use it. The acetyl group derived from acetic acid is, according to biochemists, fundamental to all forms of life. Bound to coenzyme A by acetyl-CoA synthetase enzymes, it sits at the centre of how cells metabolize both carbohydrates and fats. Without this molecular handshake between acetate and coenzyme A, the basic energy machinery of living organisms would not function.

  Acetic acid bacteria, notably the genus Acetobacter and Clostridium acetobutylicum, produce and excrete acetic acid. These bacteria are found universally in foodstuffs, water, and soil, which is why food left exposed to air spoils with a sour note. Acetic acid also appears as a component of vaginal lubrication in humans and other primates, where it appears to act as a mild antibacterial agent. The artificial triglyceride triacetin, or glycerine triacetate, is a common food additive found also in cosmetics and topical medicines; once consumed, the body metabolizes it back into glycerol and acetic acid.

• 4. Making Acid at Scale

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  By 1910, the standard route to glacial acetic acid was destructive distillation of wood, producing a liquid called pyroligneous liquor. The acid was isolated by treatment with milk of lime, and the resulting calcium acetate was then acidified with sulfuric acid to recover acetic acid. Germany alone was producing 10,000 tons of glacial acetic acid at that time, with roughly 30% of it going into the manufacture of indigo dye.

  Henri Dreyfus at British Celanese developed a methanol carbonylation pilot plant as early as 1925, but the high pressures required, 200 atmospheres or more, meant that no practical materials existed to contain the corrosive reaction mixture. German chemical company BASF cracked the problem in 1963 with the first commercial methanol carbonylation process, using a cobalt catalyst. Then in 1968 a rhodium-based catalyst was discovered that could operate efficiently at much lower pressure with almost no by-products. Monsanto Company built the first plant using this catalyst in 1970, and the so-called Monsanto process became the dominant production method. In the late 1990s, BP Chemicals introduced the Cativa catalyst, promoted by iridium, which is greener and more efficient. The Cativa process has since largely supplanted the Monsanto process, often in the very same production plants.

• 5. What Industry Does With It

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  In 2008, the production of vinyl acetate monomer was estimated to consume a third of the world's entire acetic acid output. Vinyl acetate can be polymerized into polyvinyl acetate, a key component in paints and adhesives including wood glue. Acetic anhydride, produced from two molecules of acetic acid, accounts for a further 25% to 30% of global production; its main application is cellulose acetate, the synthetic textile that also serves as the base for photographic film.

  About 20% of worldwide acetic acid, as of 2006, went into the production of terephthalic acid, the raw material for polyethylene terephthalate, better known as PET plastic. Another 15% to 20% is consumed in ester production: ethyl acetate, n-butyl acetate, isobutyl acetate, and propyl acetate are all widely used as solvents for inks, paints, and coatings. One of the ether acetates, ethylene glycol monoethyl ether acetate, has been shown to be harmful to human reproduction. The chemical company Showa Denko opened an ethylene oxidation plant in Oita, Japan, in 1997, commercializing a cheaper single-stage route from ethylene directly to acetic acid, competitive with methanol carbonylation for smaller plants in the range of 100 to 250 thousand tonnes per year.

• 6. Medicine and the Body's Risk

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  Acetic acid injection into tumors has been used as a cancer treatment since the 1800s. In many parts of the developing world today, it serves a diagnostic purpose: applied to the cervix, a white area appearing after about a minute indicates a positive result in cervical cancer screening. As a 1% solution, it is an effective antiseptic with a broad spectrum of activity against streptococci, staphylococci, pseudomonas, and enterococci, and is listed on the World Health Organization's List of Essential Medicines as a treatment for otitis externa, infection of the outer ear.

  The compound is not without hazard to workers who handle it in industrial quantities. Prolonged inhalation at 10 parts per million over eight hours can irritate the eyes, nose, and throat. At 100 ppm, marked lung irritation and possible tissue damage may result. A study of five workers exposed for seven to twelve years to concentrations of 80 to 200 ppm at peaks found blackening and hyperkeratosis of the skin of the hands, conjunctivitis, bronchitis, pharyngitis, and erosion of the incisors and canines. Concentrated solutions at or above 25% are corrosive to skin, and burns may not appear until hours after exposure.

• 7. Found in Deep Space

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  Acetic acid was first detected in the interstellar medium inside the Sagittarius B2 North molecular cloud, also known as the Sgr B2 Large Molecule Heimat source. Its detection carried a distinction beyond mere novelty: it was the first molecule ever identified in the interstellar medium using solely radio interferometers. In every previous discovery of molecules in the millimetre and centimetre wavelength regimes, single-dish radio telescopes had played at least a partial role. The fact that a molecule integral to vinegar, to cellular metabolism, and to the plastics in modern packaging also forms in the space between stars points toward how broadly distributed the basic chemistry of life's building blocks may be across the universe.