The element iodine is more prevalent in the sea than on land. In 1811, French chemist Berenard Courtois was adding sulfuric acid to the ash he got from burning seaweed. Too much sulfuric acid resulted in a violet vapor that condensed into dark crystals. Courtois distributed samples of these crystals for testing, and he is credited with discovering iodine.
Although iodine is present in seawater and sea life, other sources are more economical. Iodine is extracted from underground brine reservoirs found above natural gas fields in the United States and Japan. The largest producing source of iodine, however, is the mineral deposits of the Atacama Desert in Chile (right). Chile produced 50% of the world's iodine in 2017, which total was estimated to be about 40,000 tonnes.
Properties and Uses
Iodine forms dark blue crystals that evaporate easily into a violet gas. Iodine behaves similarly to the elements above it in the periodic table, bromine, chlorine, and fluorine, although it is not as reactive. Many iodine compounds dissolve in water, which is why it is most abundant in the oceans.
Iodine is most significant for its uses in medicine and biology. Tincture of iodine, a solution of it in alcohol, is familiar as a disinfectant.
In the human body, iodine is needed by the thyroid, and lack of iodine causes the thyroid to swell. Table salt normally contains a bit of potassium iodide for this reason. One radioactive isotope of iodine (I131) is a common component of nuclear fallout that can concentrate in the thyroid. This can be forestalled by taking potassium iodide pills to crowd out the unwanted element.
Other isotopes of iodine are useful in medicine. As a rather heavy element that is safe in the body, iodine is good for enhancing contrast in x-ray images. A short-lived isotope introduced to the bloodstream will enhance the veins and arteries just long enough for imaging.