Hydrochloric acid (HCl) is a corrosive and strongly acidic solution that is made by dissolving gaseous hydrogen chloride in water. It is made industrially as well as in the body.
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Properties of HCl
Hydrochloric acid is the simplest chlorine-based acid that contains water. In spite of its acidity, it is also the least hazardous strong acid to handle because of its non-toxic and non-reactive chloride ion.
Different concentrations of hydrochloric acid have different properties, especially when it comes to things like boiling and melting points. In general, it has a pungent odour and colourless appearance.
The reason hydrochloric acid is a strong acid is because it becomes completely dissociated in water. It is also an excellent acidifying agent because, at intermediate concentrations, HCl is very stable and can maintain its concentration.
Discovered by Jabir ibn Hayyan in c. 800 AD, hydrochloric acid has always been a frequently used chemical through history. Its presence has been noted down throughout time, from the middle ages to the industrial revolution:
- In the 13th century Jabir ibn Hayyan described obtaining aqua regia, a mixture of nitric acid and hydrochloric acid, by dissolving sal ammoniac in nitric acid
- In the 15th century Basil Valentine, a Benedictine monk, was said to have obtained pure hydrochloric acid when he heated together green vitriol and salt
- In the 16th century Libavius did a similar experiment where he heated salt in clay crucibles. This led to his discovery of free hydrochloric acid
- In the 17th century a German-Dutch alchemist, Johann Glauber, prepared sodium sulphate by using sodium chloride salt and sulphuric acid. This experiment, known as the Mannheim process, released hydrogen chloride gas
- In 1772 Joseph Priestley, a chemist from Leeds, was able to use this information to prepare a sample of pure hydrogen chloride
- In 1808 the famous Cornish chemist, Humphry Davy, analysed hydrogen chloride and was able to prove that its chemical composition included the elements hydrogen and chlorine
How is Hydrochloric Acid Produced?
Hydrochloric acid is prepared by dissolving hydrogen chloride gas in a solution of water. Since there are many ways that hydrogen chloride can be formed, there are many different precursors to HCl.
Interestingly, any large-scale production of hydrochloric acid is almost always a by-product of the industrial-scale production of other chemicals, like in the Industrial Revolution where hydrochloric acid was first produced on an industrial scale during the manufacture of sodium carbonate.
Prior to the early nineteenth-century, the production of soap was a small-scale, household industry. It was produced by boiling fat (obtained from tallow) with alkali (obtained from the ashes of burnt vegetable matter).
This method of obtaining alkali from vegetables was not sustainable; and as soap became a commercialised product, the demand quickly began to take over the supply. This is why the Industrial Revolution was marked by a sky-rocketing demand for alkali substances.
It was the French chemist Nicolas Leblanc who introduced a process that would lead to the cheap yet large-scale production of sodium carbonate, an alkaline substance also known as soda ash. Known as the Leblanc process, this method used sulphuric acid, limestone and coal to convert common salt into soda ash. The Leblanc process also released one major by-product: hydrogen chloride.
This excess gas would be vented into the air until the 1863 Alkali Act. Under this new law, soda ash producers were prohibited from releasing the excess gas into the air. Instead, they had to absorb the waste hydrogen chloride in water. Little did they know that this would be the first time hydrochloric acid was produced on an industrial scale.
In the Body
HCl may be a strong acid, but it is actually produced in our bodies and is the main constituent of stomach acid.
Also known as gastric acid, stomach acid is a crucial part of our digestion process. It is formed from hydrochloric acid, potassium chloride and sodium chloride. Hydrochloric acid is secreted by parietal cells which are located in the stomach’s gastric glands. It helps to activate digestive enzymes and breaks down ingested proteins into smaller molecules that these enzymes can easily digest.
The pH of stomach acid ranges between 1.5 and 3.5. It is especially acidic after a big meal because the enzymes that aid digestion, known as proteases, work best under acidic conditions. When you eat, the stomach releases hydrochloric acid as well as the protease in order to achieve this ideal environment.
Hydrochloric acid is extremely corrosive, so how can the body handle such an intensely acidic substance in the stomach? The answer lies in mucus, which forms a protective barrier against the acid.
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