Iodine

Iodine (standardly iodine, but this is technically obsolete) is a chemical element with the element symbol I (before the international element symbol introduction it was J) and the atomic number 53. In the periodic table it is in the 7th main group or the 17th IUPAC group and thus belongs to the halogens. The name is derived from the ancient Greek word "ioeides" (ιο-ειδής) meaning "violet-colored, violet". Vapours produced during heating are characteristically violet.

Older names are iodium, iodina and iodine.

Iodine is a solid at room temperature, poorly soluble in water but highly soluble in aqueous potassium iodide solution and very soluble in ethanol and other organic solvents.

In everyday language, iodine is the common spelling, also in the older chemical and predominantly in the current medical literature. The spelling iodine is listed in the Duden with the reference to iodine and refers to the technical language. The element symbol I has been used in the chemical literature since 1970 in the IUPAC publication, in the older German-language literature the element designation J is found throughout.

Iodine is an indispensable component of the animal and human organism and is absorbed with food. The highest concentration in humans is in the thyroid gland, where it is used in the hormones thyroxine and triiodothyronine and as diiodotyrosine. Iodine deficiency in drinking water and food is usually responsible for the occurrence of goiter. As a preventive measure, the consumption of sea fish once or several times a week and the use of so-called iodized salt (table salt with sodium or potassium iodate added) is recommended. This individual iodine prophylaxis and the iodization of feedstuffs partially compensate for the lack of iodine in the soil in Germany. It was temporarily achieved that in the total iodine supply of the population the specifications of the WHO were just met. As of 2020, according to the Federal Institute for Risk Assessment, the iodine supply is showing a downward trend and is not optimal.

History

The pharmacological significance of iodine-containing preparations was already known in ancient times. Thus, already 1500 years before our era, goitre patients were prescribed the iodine-containing thyroid glands of sheep or ashes of sea sponges.

Iodine was first extracted from the ashes of seaweed in 1811 by the Parisian saltpetre boilermaker and soda manufacturer Bernard Courtois in the manufacture of gunpowder. However, its elemental character (which Humphry Davy, for example, publicly announced early on and which gave iodine its name) was not explored until 1813 by the French natural scientists Nicolas Clément and Joseph Louis Gay-Lussac, who gave it its present name a year later.

Occurrence

Iodine, apart from astatine, is much rarer than the other halogens. It is widespread in nature, but only in the form of its compounds, for example enriched (0.02-1 %) in Chile saltpetre, mainly in the form of sodium iodate (NaIO3), but also sodium periodate (NaIO4) and lautarite (Ca(IO3)₂). It is detectable in small traces in soils and rocks. On average, 1000 grams of anhydrous fine soil from the German region contains about 2.5 milligrams of iodine. The iodine content of the soil is essential for the supply of the population with natural iodine. As hydrogen iodide, it occurs in minute quantities in volcanic gases.

Soluble iodine compounds such as alkali and alkaline earth iodides are released by rainwater during the weathering of rocks or decay at higher temperatures. In this way, they enter the groundwater and eventually the oceans. Some mineral waters contain iodine. The mineral spring at Woodhall Spa in Lincolnshire (England) produces water that is coloured brown by iodine. In sea water, the amount of iodine is 0.05 milligrams per liter. It occurs there in the form of iodide (I-) and iodate (IO3-) at a concentration of about 500 nmol/L. The distribution in surface water generally varies from 0-200 nmol I-/L.

In the Earth's atmosphere, iodine is found in the form of organic compounds or inorganically in the form of iodine oxide (IO), iodine nitrate or higher oxides. For the stratosphere there is little information and an upper limit of 0.1 ppt for inorganic iodine. High concentrations of more than 10 ppt IO have been detected over coastal algal fields and the iodine oxide radical has also been detected on the tropical Atlantic.

Organic iodine compounds can be isolated from marine algae (19 grams of iodine per kilogram of dry matter), seaweed and sponges (up to 14 grams of iodine per kilogram of dry matter). An important store for organically bound iodine is the thyroid gland.