Iodine evaporating into a purple gas
17 November 2023
The origin of the word iodine is a story of cooperation and rivalry between scientists of warring nations in the Napoleonic era.
Iodine is a semi-lustrous, non-metallic solid at room temperature and pressure with atomic number 53 and the symbol I. Iodine is essential for human health, and a deficiency can lead to goiters in adults and intellectual disabilities in children. (Nowadays, iodide is typically added to table salt to prevent such deficiency.) It has a variety of applications, including medicine, where it has long been used for its anti-microbial properties. More recently it has been used as a radiocontrast material in medical imaging and in treating thyroid cancer.
The name comes from the Greek ἰώδης (iodes, violet colored), after the purple color of its gaseous state.
Iodine was discovered in 1811 by Bernard Courtois. At the time, in the midst of the Napoleonic Wars, France was blockaded by Prussia and Austria on land and by the Royal Navy at sea and was thus unable to obtain nitrate for gunpowder from abroad. Courtois was engaged in finding ways to extract nitrate from kelp. One day, probably in November of that year—the exact date is not known—when using acid and heat to clean out the vats used to extract nitrate from the seaweed, Courtois noticed a purple vapor arising from the vessels and metallic-like crystals forming at their bottoms. Courtois sent samples of the substance to various colleagues for analysis, including Joseph Louis Gay-Lussac and André-Marie Ampère.
Two years passed with no discernable progress made on identifying the substance. Then in October 1813 English chemist Humphry Davy and his young lab assistant, twenty-two-year-old Michael Faraday, were traveling through France on their way to Italy. (It was a different time then, and despite being at war with England, Bonaparte was happy to grant passports to English scientists, especially an eminent one like Davy.) While in France, Davy met with Ampère, who shared his sample of the unknown substance. Davy, using the traveling laboratory he always took with him, got to work. Upon hearing this, Gay-Lussac was furious; he had already determined it was an element but had been slow in publishing his findings and did not want to be scooped by his English rival.
Davy and Gay-Lussac engaged in a bitter quarrel over who had priority in the discovery. Gay-Lussac was the first to publish, but he did so under Courtois’s name in October 1813, dubbing the new element iode:
La substance nouvelle, que depuis on a nommée iode à cause de la belle couleur violette de sa vapeur, a bien tout l’aspect d’un metal.
(The new substance, which has since been named iode because of the beautiful purple color of its vapor, has all the appearance of a metal.)
Davy wrote up his findings in December 1813, when he was still in Paris, changing Gay-Lussac’s coinage to iodine, and sent them off to the Royal Society, where they were read and published the following year:
The name ione has been proposed in France for this new substance from its colour in the gaseous state, from ἴον, viola; and its combination with hydrogen has been named hydroionic acid. The name ione, in English, would lead to confusion, for its compounds would be called ionic and ionian. By terming it iodine, from ἰώδης, violaceous, this confusion will be avoided, and the name will be more analogous to chlorine and fluorine.
Thus iodine was born out of a scientific rivalry between warring nations (not unlike the transuranic elements discovered during the Cold War of the twentieth century). Ironically, the young lab assistant Faraday would end up eclipsing both Davy and Lussac with his scientific achievements.
Sources:
Courtois, B. (Gay-Lussac, Joseph Louis) “Découverte d’une substance nouvelle dans le Vareck.” Annales de Chimie, 88, 31 October 1813, 304–10 at 305. Bibliothèque Nationale de France: Gallica.
Davy, Humphry. “Some Experiments and Observations on a New Substance Which Becomes a Violet Coloured Gas by Heat” (10 December 1813, read 20 January 1814). Philosophical Transactions of the Royal Society of London, 104, 74 at 93 at 91. DOI: 10.1098/rstl.1814.0007. https://doi.org/10.1098/rstl.1814.0007
Kelly, Francis C. “Iodine in Medicine and Pharmacy Since Its Discovery—1811–1961.” Proceedings of the Royal Society of Medicine, 54.10, October 1961, 831–926. DOI: 10.1177/003591576105401001.
Miśkowiec, Pawel. “Name Game: The Naming History of the Chemical Elements: Part 2—Turbulent Nineteenth Century.” Foundations of Chemistry, 8 December 2022. DOI: 10.1007/s10698-022-09451-w.
Oxford English Dictionary, second edition, 1989, s.v. iodine, n.
Image credit: Chemical Elements: A Virtual Museum, 2009. Wikimedia Commons. Licensed under a Creative Commons Attribution 3.0 Unported license.