256 | The History and Use of Our Earth’s Chemical Elements Common฀Uses

256 | The History and Use of Our Earth’s Chemical Elements Common฀Uses

One of the most important uses of iodine is in the treatment of hypothyroidism, a condi- tion in which the thyroid gland is deficient in iodine. Iodine deficiency may lead to the forma- tion of a goiter, wherein the gland that surrounds the windpipe in the neck becomes enlarged. There are other causes of goiter, including cancer of the thyroid gland. A deficiency of iodine can also cause cretinism (infant hypothyroidism) in newborn babies, which can result in mental retardation unless the subject takes thyroid hormones for a lifetime. Green leafy foods, among other foods, contain iodine that when taken into the human body ends up in the thyroid gland. Some food grown in iodine-deficient soils do not contain adequate iodine for our diets. This is why iodine was added to table salt (about 0.01% potassium iodide) decades ago, specifically for people who live in regions with iodine-poor soils. The area around the Great Lakes in the United States is one region with soil that is deficient in iodine. A healthy diet requires 90 to 150 micrograms of iodine each day that, in addition to being available in iodized salt, can be obtained from eating a balanced diet, including seafood.

The isotope iodine-131 is an artificial radioisotope of iodine used as a tracer in biomedical research and as a treatment for thyroid disease. I-131 has a half-life of about eight days, which means it will be eliminated from the body in several weeks.

In industry, iodine is used for dyes, antiseptics, germicides, X-ray contrast medium, food and feed additives, pharmaceuticals, medical soaps, and photographic film emulsions and as a laboratory catalyst to either speed up or slow down chemical reactions.

Iodine is also used as a test for starch. When placed on starch (a potato for example), iodine turns the starch a dark blue color. Silver iodide is used in the manufacture of photographic film and paper. It is also used to “seed” clouds because of its ability to form a large number of crystals that act as nuclei upon which moisture in the clouds condenses, forming raindrops that may result in rain.

I-125, another artificially radioactive isotope of iodine, is used in pellets to treat prostate cancer. Pellets, the size of rice grains, are inserted into the cancerous prostate gland. Since the half-life of I-125 is about 17 days, the grains of I-125 deliver a strong dose of radiation for a week or so. They are either removed or become less radioactive in a few weeks. This treatment is localized and thus spares other parts of the body exposure to potentially harmful doses of radiation.

Trace amounts of iodine are required for a healthy body. Iodine is part of the hormone thyroxin produced by the thyroid gland. Thyroid secretions control the physical and mental development of the human body. A goiter, a swelling of the thyroid gland, is caused by the lack of iodine. Adding thyroid medication and iodized salt to the diet helps prevent this disease. Radioactive iodine (I-131), with a half-life of eight days, is used to treat some diseases of the thyroid gland.

Potassium iodide (KI) is used to manufacture photographic film, and when mixed with alcohol, it is used as an antiseptic to kill bacteria.

Examples฀of฀Compounds Like all the lighter halogens, iodine, even though it is the heaviest halogen, easily combines

with group 1 and group 2 metals to form iodide salts. Following are examples of the –1 oxida- tion state of iodine forming compounds:

Guide to the Elements |

Sodium iodide (NaI): Na 1- +I → NaI.

Potassium iodide (KI): K 1- +I → KI. Iodine also has an oxidation state of +5, when forming the iodate –1 ion:

3 . And it exhibits a +7 oxidation state in the following reaction:

I 1- + 3O → IO

Potassium metaperiodate (KIO 2-

Hazards Iodine is a poison, and as such, care must be taken when handling and using it. Even in

less than pure form, it can damage the skin, eyes, and mucous membranes. Both the elemental form and its compounds (gases, liquids, or solids) are toxic if inhaled or ingested. Even in diluted form (e.g., a tincture of iodine to treat minor skin wounds), it should be used with care.

Although a poison in high concentrations, iodine is required as a trace element in our diets to prevent thyroid problems and mental retardation in the very young.

ASTATINE SYMBOL:฀At฀ PERIOD:฀6฀ GROUP:฀17฀(VIIA)฀ ATOMIC฀NO:฀85