Iodine is the heaviest member in a family of chemical elements called halogens. The halogen group includes fluorine, chlorine, bromine, astatine, and iodine. Halogens readily combine with other elements to form salts. At room temperature, iodine is a shiny, dark black, nonmetallic, crystalline solid. Good food sources of iodine are fish and shellfish from the sea, as well as other seaweeds. Milk and eggs, vegetables and fruit contain small amounts of iodine.
Pure iodine is never found in nature—it is always combined with other elements. When iodine is heated it sublimes (passes directly from a solid to a vapor, skipping the liquid state). The iodine vapor is violet colored and has an irritating odor like that of chlorine. The vapor rapidly condenses on a cold surface. Pure iodine is toxic (poisonous).
Iodine is only slightly soluble (capable of being dissolved) in water, but it dissolves easily in a potassium iodide solution. It is also soluble in alcohol, chloroform, and other organic substances. Tincture of iodine (iodine dissolved in alcohol) is commonly used to kill germs on cuts and scrapes.
Iodine compounds are found in seawater, soil, and rocks. Iodine is obtained as sodium iodate, an impurity in the sodium nitrate beds in the South American country of Chile (the world's largest source of commercial iodine). Other important sources of iodine are underground pockets of brine (saltwater) found in Michigan, California, and Louisiana.
Most plants and animals require small amounts of iodine for normal growth. In man and other mammals, iodine is concentrated in the thyroid gland (in the neck), where it helps the body make thyroxine and other bio-chemicals that are important in metabolism (the process by which cells provide energy for bodily functions). Without enough iodine, a person's growth may be stunted (halted or slowed), and he may develop a condition called goiter (in which the thyroid gland swells into a large lump).
Today these conditions have been mostly wiped out in the industrialized world by the introduction of table salt containing potassium iodide or sodium iodide.
Courtois Discovers Iodine
Iodine was discovered by French chemist Bernard Courtois (1777-1838) in 1811. Courtois was barely making a living in his family's business of manufacturing saltpeter, which was used to make gunpowder. Saltpeter was made from the ashes of seaweed, which were treated with acid to remove sulfur compounds. One day, Courtois accidentally added too much acid, producing clouds of vapor having an attractive violet color. When the vapor condensed on cold objects, it formed dark, shiny crystals.
Although Courtois investigated the properties of the new substance by combining it with several other elements, he did not have enough time or money to follow through on his discovery. He asked for help from two French chemist friends, Charles Bernard Desormes and Nicholas Clement, who completed the work and made the research public in 1813.
Later that year, Humphry Davy and Joseph Gay-Lussac, working independently, showed that iodine was a new element. This research was made public in 1813. Although Davy's and Gay-Lussac's research over-lapped, it was Gay-Lussac who gave the new element its name after the Greek word "iodes," meaning "purple." Gay-Lussac went on to study the substance and its compounds, such as hydrogen iodide, in great detail.
In 1831 Courtois received a prize from a French scientific institute for his work. Despite this small measure of fame, Courtois's saltpeter business declined. With the end of the Napoleonic Wars (1804-1815), the demand for gunpowder dropped, and Courtois's factory failed. Although he continued to produce and sell iodine, Courtois had little success, and he died in poverty.
In 1814 J. J. Colin made an important discovery in microanalytical chemistry when he showed that iodine reacted with starch to produce a blue color so bright that iodine could be detected in amounts as low as one part in 400,000. In 1819 Jean-Baptiste Dumas proved that sponges used for many years to treat goiter contained iodine. By 1820 iodine had been discovered in a seaweed called kelp, which had also been used as a goiter treatment. Kelp was one of mankind's earliest sources of iodine. In Japan, kelp is still harvested from the sea and dried under the sun to provide a raw material for iodine production. It is also considered a delicious food by the Japanese, so their diets do not need to be supplemented with iodized salt.
In the mid-1800s, French agricultural chemist Jean Boussingault first suggested that iodine compounds might be able to cure goiter. A young doctor had asked Boussingault to analyze samples of certain salts used by South American Indians to treat goiter. Boussingault found iodine in the salts and suggested the cure, but it was not until 1896 that this treatment was confirmed.
German chemist Eugen Baumann (1846-1896) discovered that the thyroid gland was rich in iodine, which had never been found before in animal tissue. Baumann also determined that the thyroid was the only tissue containing iodine. Just two years later, an Austrian psychiatrist named Julius Wagner von Jaurreg (1857-1940) established that goiter could be prevented by taking iodine tablets regularly. He also proposed that iodized salt be sold in areas where goiter was widespread; Austria and Switzerland later adopted this idea.
The modern use of iodine in the prevention of goiter was a result of studies by D. Marine in the United States. Marine used iodine to prevent goiter in schoolchildren in Akron, Ohio, where the disorder was common. The success of his experiments led to the adoption of this use of iodine in many regions of the world where goiter was a health problem.
Other Iodine Uses
Iodine's most important use is in the health sciences. One of iodine's radioactive isotopes, 1-131, is widely used in medical diagnosis as a radioactive tracer. It can also be used to treat thyroid cancer.
In addition to its use as a goiter treatment, iodine serves as an important antiseptic thanks to its germ-killing properties. Tincture of iodine was frequently used to disinfect open wounds. Because of the tincture's irritating sting, however, more complex iodine compounds have been developed for first-aid purposes. Iodine combined with cleaning agents is used in common sanitizers. Iodine is also used to sterilize drinking water.
Iodine has several major industrial uses. Silver iodide is the main light-sensitive substance in photographic film emulsions and photographic papers. Silver iodide is also used by weather scientists for "cloud seeding" in rain-making experiments. Other iodine-containing compounds are used as dyes, in engraving, as an indicator in analytic chemistry, and in special soaps and lubricants. Commercial bakeries add sodium iodate to certain kinds of flour to improve the quality of the bread. Some inorganic iodides are used in producing high-purity titanium and silicon metals.