Iodine is a nonmetallic, bluish-black, lustrous solid element, that becomes a blue-violet gas with an irritating odor at room temperature. Iodine occurs sparingly in brines; brackish waters from oil and salt wells; saltpeter and nitrate-bearing earth, which is known as caliche; and seawater. "Brine" is the term for water that is saltier than seawater. Iodine production in the United States comes from underground brines in Oklahoma. Chile, Japan, and the United States are the world's major producers. The United States is the world's largest consumer and imports about three-fourths of its needs every year. Iodine is used mainly in chemicals and organic compounds, disinfectants, and X-ray contrast media. Radioactive iodine is used to treat thyroid cancer in humans.
When iodine is heated, it sublimates, that is, it goes from a solid to a vapor without going through the liquid phase.
Iodine is essential to many life forms, including humans, and is found in thyroid hormones. A lack of iodine in the body will result in a condition known as a goiter where the thyroid gland in the neck becomes enlarged. In order to assure an adequate amount of iodine in the diet, table salt is iodized. This approach has greatly reduced the incidence of goiter since so many people regularly use table salt.
Iodine is primarily retrieved from underground brines (water with many dissolved salts and ions) that are associated with natural gas and oil deposits. It is also retrieved as a by-product with nitrate deposits in caliche deposits. Chile's production of iodine is from this source. Seawater contains 0.05 ppm (parts per million) iodine which means that there are approximately 76 billion pounds of iodine in the world's oceans. Iodine was first discovered in seaweed. Dried seaweeds, particularly those of the Liminaria family, contain as much as 0.45% iodine. Seaweed was a major source of iodine before 1959. Seaweed is a significant source for iodine in the diets of many people around the world. Production from caliche is presently the most economical of the options listed here.
Chile is the world's leading iodine producing nation. Japan is second. Russia also produces significant amounts of elemental iodine.
Iodine is used in a number of chemical and biological applications. Silver iodide is used in photography. Iodide is used as a disinfectant. Iodine compounds are used as a catalyst. It is used as a supplement in animal feeds. Potassium iodine is included in table salt as a simple way to assure adequate iodine in the human diet. It is also used to make inks and colorants.
For many of iodine's uses, there is no adequate substitute. For example, other substances cannot replace its applications in pharmaceuticals, and human and animal nutrition. There are some chemical applications for iodine that can be accomplished using other chemicals. For example, bromine and chlorine can be used in place of iodine for ink and colorant purposes, and for disinfectant purposes.