(Gr. iodes: violet) Discovered by Courtois in 1811, Iodine, a halogen, occurs sparingly in the form of iodides in sea water from which it is assimilated by seaweeds, Chilean saltpeter, nitrate-bearing earth (known as caliche), brines from old sea deposits, and in brackish waters from oil and salt wells.
Ultrapure iodine can be obtained from the reaction of potassium iodide with copper sulfate. Several other methods of isolating the element are known.
Iodine is a bluish-black, lustrous solid, volatizing at ordinary temperatures into a blue-violet gas with an irritating odor; it forms compounds with many elements, but is less active than the other halogens, which displace it from iodides. Iodine exhibits some metallic-like properties. It dissolves readily in chloroform, carbon tetrachloride, or carbon disulfide to form beautiful purple solutions. It is only slightly soluble in water.
Thirty isotopes are recognized. Only one stable isotope, 127I is found in nature. The artificial radioisotope 131I, with a half-life of 8 days, has been used in treating the thyroid gland. The most common compounds are the iodides of sodium and potassium (KI) and the iodates (KIO3). Lack of iodine is the cause of goiter.
Iodine compounds are important in organic chemistry and very useful in medicine. Iodides, and thyroxine which contains iodine, are used internally in medicine, and as a solution of KI and iodine in alcohol is used for external wounds. Potassium iodide finds use in photography. The deep blue color with starch solution is characteristic of the free element.
Care should be taken in handling and using iodine, as contact with the skin can cause lesions; iodine vapor is intensely irritating to the eyes and mucus membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m3 (8-hour time-weighted average - 40-hour).