Iodine

MELTING POINT: 113.7°C
BOILING POINT: 184.3°C
DENSITY: 4.94 g/cm ³
MOST COMMON IONS: IO ⁻ , IO ₃ ⁻ , IO ₆ ⁵⁺

Iodine is the heaviest of the halogen family of elements, excluding the radioactive element astatine. It was discovered in 1811 by French chemist Bernard Courtois, who isolated the element from seaweed. The element is named for its color in the gas phase (the Greek word iodes means "violet").

At normal temperatures and pressures, iodine is a shiny, purplish-black or gray solid. Near room temperature, iodine sublimes (i.e., it does not melt to form a liquid but goes directly from the solid to the gas phase). It is found at a level of about 60 parts per billion (ppb) by weight in seawater but its concentration is enhanced in marine organisms.

As with the other halogens, iodine is a diatomic molecule. It is always found in nature in a combined state, often as iodide salts where it has a −1 oxidation number. Compounds in which iodine is found to have oxidation numbers of 7, 3, 5, and 1 are also well known. Iodine is prepared commercially by treatment of natural salt solutions (seawater or brines) with chlorine (a more reactive halogen), according to the reaction:

2 I ⁻ + Cl ₂ → I ₂ 2Cl ⁻

Iodine is necessary for the proper function of the thyroid gland in humans. Dietary deficiencies can be avoided by the occasional consumption of seafood or by using iodized salt, which combines common table salt (NaCl) with potassium iodide (KI). Iodine is a useful antiseptic, either as tincture of iodine (an alcohol solution of I ₂ ), or as an aqueous solution of providone iodine (Betadine).

SEE ALSO H ALOGENS ; I NORGANIC C HEMISTRY .

John Michael Nicovich

Bibliography

Lide, David R., ed. (2003). The CRC Handbook of Chemistry and Physics, 84th edition. Boca Raton, FL: CRC Press.

Internet Resources

Winter, Mark. "Iodine." The University of Sheffield and WebElements Ltd., U.K. Available from http://www.webelements.com .