A wonderful purple cloud rose from the ashes to which Bernard Courtois from Dijon, France, had just added sulfuric acid. Courtois was a producer of saltpeter, the main ingredient of gunpowder, which France was in great need of during all the wars of the early 19th century. Saltpeter was made with sodium carbonate extracted from the ashes of burnt seaweed from Normandy. On that day in May 1811, Courtois was mistaken about the amount of sulfuric acid, and saw the violet cloud condense on colder surfaces into shiny dark crystals.
Courtois had no money to research the substance. He gave samples to his friends, including the chemist Joseph Louis Gay-Lussac, and through and through the British chemist Sir Humphry Davy also got his hands on it. Both published a description of the new element in 1813, which Davy named iodine, after the Greek word for violet-colored.
Iodine is a mineral in the outer layer of the earth’s crust, especially in coastal areas and in sea water and all plants and animals in it. Like the other elements from group 17 of the periodic table, it is counted among the salt formers (halogens), but it is the least reactive in that group. It readily forms positive ions in chemical reactions. In nature, it occurs only in compounds.
Underactive thyroid gland
Soon after the discovery, a Swiss doctor noticed that taking iodine helped patients with an enlarged, underactive thyroid (goitre): the gland in their neck shrank as a result. In the years that followed, scientists saw that in areas where no salt with iodine occurs naturally, many more people have goitre.
Iodine is essential for the production of the thyroid hormones thyroxine (T4) and 3-iodothyronine (T3). They are important for growth, the development of the nervous system and metabolism. In children, an iodine deficiency leads to stunted growth and reduced learning ability. We need 150 milligrams of iodine daily, pregnant women even more. To combat an iodine deficiency in the population, many countries enrich table salt with iodine. From 1942, bakers in the Netherlands were obliged to use iodised salt in bread.
Iodine tablets at home
The purple cloud brought much more good to the world. Iodine is not just a thyroid medication and dietary supplement. It is also a wound disinfectant, dissolved in 70 percent alcohol or as povidone iodine: it kills bacteria, fungi and viruses. It is contained in contrast agents for X-rays, as lithium iodide in batteries, as tungsten iodide in halogen lamps, and is widely used as a catalyst in the chemical industry. People who live near a nuclear power plant have iodine tablets at home. In the event of a nuclear disaster, they have to take it to prevent their thyroid from absorbing radioactive iodine.
Since 2008, the Dutch have been able to choose whether they want to use iodised kitchen salt or baker’s salt – most bakers do the latter. Iodized table salt contains 21 milligrams of iodine per gram of salt. Baker’s salt contains 65 milligrams of iodine per gram, so that a slice of bread contains 24 grams of iodine. People who eat little or no bread are at risk of an iodine deficiency. Even those who only eat organic or home-baked bread should pay attention: they often do not contain baker’s salt. They can prevent an iodine deficiency by regularly eating sea fish, seafood, dairy or eggs. Or the plant in which iodine was discovered: seaweed.