Things are looking up for pickled eggs

Acetic acid gives us vinegar and videos. And now it's easier to make than ever, says John Emsley

John Emsley
Sunday 21 July 1996 23:02

Iridium, one of the rarest metals, is set to transform a large part of the chemical industry, thanks to a group of British chemists based at Hull. They have discovered that an iridium catalyst can boost production of acetic acid by around 30 per cent. This is the acid used to make wallpaper, paint, curtains, carpets, clothes and nail varnish. It is also an essential ingredient in pickled onions, chutney, tomato ketchup and salt and vinegar crisps.

World production of acetic acid exceeds 5 million tons a year, and by the end of this decade two-thirds of it will be made using the new catalyst which was launched by BP Chemicals in June. Called Cativa, it is already in operation at the company's Texas City plant, where output has increased by 80,000 tonnes a year. There are also plans to introduce the new technology at BP's other plants around the world, including Humberside, where it was discovered.

Acetic acid, also known as ethanoic acid, is a small molecule that humans have been manufacturing for thousands of years, from the time we began making fermented drinks. When the bacterium azobacter gets into these drinks, it turns the alcohol into acetic acid, making it sour (hence the vinegar, from vin aigre, Old French for sour wine) and undrinkable, but not unusable.

Food pickled in vinegar can be preserved for months, and so alcoholic liquors were deliberately converted to wine, cider and malt vinegars. Acetic acid acts as a preservative partly due to its acidity, but mainly because it inhibits spoilage organisms, particularly yeasts and bacteria, in solutions as weak as 0.1 per cent. Acetic acid is used in bread-making to prevent the spores of Bacillus mesentericus from germinating, causing the bread to go ropy.

Pure acetic acid is a colourless liquid that boils at 118C, but is not as corrosive as other pure acids, so has been used to remove warts. Normally, we would only encounter acetic acid as a 5 per cent solution in water, the same strength as vinegar; like vinegar, it was once used as a household cleaner.

Although cooks have been adding vinegar to foods for thousands of years, acetic acid solution is regarded as a chemical additive and consequently has been assigned an E-number (E260). There is no difference between the two forms of the acid, which is why the BP variety finds its way into pickles, chutneys, cheeses, salad cream, brown sauces, mint jelly, and horseradish cream.

An early method of making acetic acid was to heat wood chippings. The liquid distilled was mainly a solution of acetic acid. Today, most companies make it from methanol, made from the mixture of carbon monoxide and hydrogen gas produced by reacting natural gas with steam. When methanol is mixed with more carbon monoxide it forms acetic acid - given the right catalyst.

Chemists at Monsanto, the US chemical company, discovered the first such catalyst, based on rhodium iodide, which BP has used until now. The new catalyst is the work of Mark Howard, John Cook and Derick Watson, who found that iridium acetate worked even better. Both rhodium and iridium belong to the so-called platinum group of metals, all of which are rare. Rhodium is five times rarer than gold, but iridium is 300 times rarer, yet much cheaper.

But iridium acetate alone is not enough. "We have devised a new system that depends on promoter molecules to activate the iridium catalyst to work efficiently, and with them we get a quicker reaction with fewer side products," claims John Aitken, BP's process technology manager. For the time being, the company is keeping these promoter molecules secret.

The largest single use of industrial acetic acid is to make vinyl acetate monomer, used in paints, adhesives, textile treatments and paper coatings. Another major use is making polyester which is turned into fibres, polyethylene bottles, audio and video tapes. Acetic acid is also needed for viscose and rayon yarns, for printing inks and for over-the-counter medicines such as aspirin and ibuprofen. Other products include vitamins A and C, washing powders, jeans, leather goods, photographic film and filter tips.

Dr John Emsley is science writer in residence at Imperial College, London.

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