Iodine | Research & Encyclopedia Articles

Iodine

Iodine is the heaviest member in a family of elements called halogens. It has an atomic number of 53, an atomic weight of 126.9045 and is denoted by the atomic symbol I. Although iodine is somewhat less reactive than other halogens, it does form compounds, both organic and inorganic, with nearly all elements. At normal temperatures, iodine exists as a lustrous, blue-black solid. When heated, it usually changes directly from a solid into a purple vapor, skipping the liquid phase. The element, which is diatomic (I₂), has one stable isotope, along with 22 artificial isotopes.

Most plants and animals require small amounts of iodine to promote normal growth. In many parts of the United States, drinking water and local foods do not contain enough iodine for humans, so manufacturers of common table salt add iodine to supplement people's diets. But iodine's nutritional properties remained unknown long after the element was discovered by French chemist Bernard Courtois (1777-1838) in 1811. Courtois was making a living in his family 's business of manufacturing saltpeter which was used to make gunpowder. Saltpeter in those days was derived 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. So, he enlisted help of two French chemist friends, who made his research public in 1813. Later that year, Sir Humphry Davy and Joseph Gay-Lussac, working independently, showed that iodine was a new element. Although Davy's and Gay-Lussac's research overlapped, it was Gay-Lussac who gave the new element its name, after the Greek word for "purple" (iodes). Gay-Lussac went on to study the substance and its compounds, such as hydrogen iodide, in great detail.

However, Courtois is recognized as the discoverer of iodine. In 1831, he received a prize from the French scientific institute for his work. Despite this small measure of fame, Courtois' saltpeter business declined. With the end of the Napoleonic Wars, the demand for gunpowder dropped, and Courtois's factory failed. Although he continued to produce and sell iodine, Courtois had little success, and died in poverty.

In the mid-1800s, French agricultural chemist Jean Boussingault first suggested that iodine compounds might be able to cure goiter, an enlargement of the thyroid gland in humans. 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. Baumann also determined that the thyroid was the only tissue containing iodine. Just two years later, Austrian psychiatrist, 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.

Iodine's influence on human growth has been clarified in much greater detail since. The thyroid gland, located in the neck, produces an iodine-containing hormone called thyroxine which controls the body's rate of physical and mental development. If people do not get enough iodine, their growth can be stunted. Iodine deficiency also causes the thyroid gland to enlarge into a goiter, or 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.

Iodine does not occur naturally in its elemental form, but iodine salts are widely distributed in mineral ores and soil. Although iodine 's concentration in seawater is very low, certain marine plants and animals, such as a type of seaweed called kelp, extract iodine from the ocean and accumulate it. 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. Since the mid-1800s Chilean saltpeter deposits have been an important source, but most iodine today is produced from saltwater pumped out of oil and gas wells in California, Louisiana, and Michigan.

In addition to its use as a goiter treatment, iodine serves as an important antiseptic because it has germ-killing properties. Iodine dissolved in alcohol (called tincture of iodine) is 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 is also used to sterilize drinking water. One of iodine 's radioactive isotopes, I-131, is widely used in medical diagnosis as a radioactive tracer. It can also be used to treat thyroid cancer.

Iodine has several major industrial uses. For example, silver iodide serves as the primary light-sensitive substance in fast photographic film emulsions. Silver iodide is also used by weather scientists to induce rainfall by "seeding " clouds. Other iodine-containing compounds are used as dyes in food processing and color photography. Commercial bakeries add sodium iodate to certain kinds of flour to improve the quality of the bread. And some inorganic iodides are used in producing high-purity titanium and silicon metals.