Finding iodine

There are thirty recognized isotopes of iodine, but only one of these, iodine–127, is counted among the stable isotopes, and is found in nature. Radioactive iodine-125 is routinely used in tracing problems with the thyroid gland, and another isotope, iodine-131 has been commonly used to treat overactive thyroid conditions. The “iodine” which is commonly used on small wounds is tincture of iodine, a solution of potassium iodide and iodine in ethanol.

Iodine is element number 53 in the periodic table, atomic weight 126.90. This element was first isolated in 1811 by Bernard Courtois (1777 - 1838). 

Like the Germans in the First World war, the French found themselves restricted by a British naval blockade which stopped them accessing American sources of potash during the Napoleonic wars.  The potassium carbonate was used to make potassium nitrate for French gunpowder, but the seaweed also contained a variety of other chemicals, one of which was an iodide.

In treating seaweed ash with acid to get rid of sulfur compounds, Courtois noticed a purple vapour, which condensed to make crystals of iodine. He later passed this information on to Sir Humphry Davy, who proposed the name “iodine”, from the Greek word for the colour violet, iodes. The credit for suggesting the name is sometimes given to Joseph Gay-Lussac, but this is incorrect.

As mentioned above, iodine is needed in the production of thyroxin, and a deficiency in dietary iodine leads to goitre, so that foods (especially table salt and bread) in many parts of the world now have traces of iodine added, although this is unnecessary in areas where seafood is available.

Part of the hormone ‘picture’ was already there in 1905, because a number of diseases were linked to disorders in particular glands: goiter and cretinism were associated with an enlarged thyroid gland, but this was rightly regarded as a deficiency disease caused by a lack of iodine. Many folk remedies used iodised salts or sea foods rich in iodine, even before we knew iodine existed (the element was detected in 1813). Its role in preventing goitre became more obvious after Eugen Baumann (1849–1896) showed in 1896 that iodine was only concentrated in the thyroid gland.

Curiously, Courtois also discovered that major fascination for undergraduates of a certain kind, nitrogen triiodide, which forms tremendously unstable crystals that will even explode when hot water falls on them.

I have no intention of revealing how I discovered this fact, as I conclude now that I had a lucky escape: Pierre Dulong  lost three fingers and an eye investigating this substance — which may explain why, when he was formulating what is now called “Dulong and Petit’s Law”, he chickened out, and did not investigate tellurium, fraudulently manufacturing the data for that and several other elements.

The reason is probably that when you handle tellurium, it is absorbed, and you get “tellurium breath “. Not to mince words, you stink of stale garlic for months after working with tellurium compounds. Dulong  either feared that, or perhaps he was attached to his remaining fingers and wished to stay that way.

Everything (other than Dulong's fingers, perhaps) is connected.