Everyday Chemicals 8 – Acetic Acid
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It’s been a little while since the last post in the ‘Everyday Chemicals’ series, but it’s back today, and with a revamped look (which will also be applied out to the previous posts in the series over the next week or so). The latest post looks at acetic acid; this compound is well known for its presence in vinegar, but has a role in the manufacture of other chemicals we regularly encounter. It’s also an important part of a classic home science experiment!

Acetic acid also has the name ethanoic acid, though this is less commonly used. Most people will know of its presence in vinegar, from which its name is derived – it originates from the latin word for vinegar. In vinegar acetic acid is generated by fermentation, which produces ethanol, and then subsequent oxidation of this ethanol. The method of acetic acid’s production by the oxidation of ethanol also helps explain why wines can begin to taste vinegary if the bottle is left open.

Vinegars will contain between 4-18% acetic acid; table condiment varieties tend to have a maximum concentration of 8%, with the commercial vinegars used for pickling slightly stronger. The remainder of the vinegar is simply water, with trace compounds providing colourings and nuances in flavour, in cases such as malt vinegar and balsamic vinegar. The significant differences in different vinegars stem from their origin. Malt vinegar, popular in the UK, is created from malted barley (much like beer), whilst balsamic vinegar is made from white grape juice, then aged in barrels.

Though it might be the best known of acetic acid’s uses, vinegar accounts for a minimal amount of the acetic acid that’s produced worldwide every year. Estimates put the production of acetic acid at 6.5 million tonnes per year, with the majority of this produced industrially from petrochemicals, rather than by fermentation. The major process used is the carbonylation of methanol, in which methanol and carbon monoxide are combined along with a catalyst to produce acetic acid. The rhodium-catalysed Monsanto process was also previously used, but has been largely supplanted by the iridium-catalysed Cativa process, which requires less water, and reduces the quantity of unwanted side-products from the reaction.

Approximately one third of all of the acetic acid produced worldwide is reacted with ethene and oxygen (with a palladium catalyst) to produce vinyl acetate. This is a compound that can be used as a monomer to produce the polymer polyvinyl acetate (PVA), found in wood glues, and it can also be polymerised with other monomers to produce various other useful polymers. These include polyvinylpyrrolidone, used in some hair gels.

Vinyl acetate isn’t the only chemical that acetic acid is useful for producing. It’s also useful for producing acetate esters, which include cellulose acetate, which can be used to make some photographic films (a use that has presumably declined now that digital photography has become prevalent). It can also be used in the preparation of some drugs – for example, aspirin can be produced by the reaction between acetic acid and salicylic acid. However, acetic anhydride, another compound that can be produced from acetic acid, is more commonly used to produce aspirin.

Pure acetic lab finds use in chemical laboratories as a water-free solvent for other chemicals. In pure form, it’s known as glacial acetic acid, as its melting point being only just below room temperature at 16.7˚C means that it can often freeze in its container on cold days. Although we don’t think of vinegar as being a particularly dangerous substance, pure acetic acid is a very corrosive chemical.

Back to the household, and acetic acid often finds uses as a household cleaner. Particularly, it can be found in some descales for removing limescale. Limescale is a mixture of minerals, but primarily calcium carbonate, a basic compound. As everyone learns in school chemistry lessons, the combination of an acid with a basic compound will initiate a neutralisation reaction; in the case of limescale, this helps turn the insoluble calcium carbonate into soluble calcium acetate, removing it from surfaces.

Acetic acid has also been shown to have antibacterial and anti-fungal properties by research studies, active even when significantly diluted to a low concentration. This reinforces its use over past centuries for the disinfection of wounds, and as such is often suggested as a kitchen surface cleaner – as long as you don’t mind the faint smell of vinegar afterwards!

Finally, there’s also a chemical reaction you can carry out with vinegar, and doubtless many will have previously, in the comfort of your own home. Mixing vinegar with baking powder (which is chemically called sodium bicarbonate) produces a volcano-like fizzing eruption effect – particularly effective if a few drops of red food colouring are also added into the mix! This reaction is a result of the neutralisation occurring between the acetic acid in the vinegar and the basic sodium bicarbonate. It produces carbon dioxide gas as one of the products, which leads to the bubbling effect observed.

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