Iodine

Iodine (from the Gr. Iodes, meaning "violet"), is a chemical element in the periodic table that has the symbol I and atomic number 53. This is an insoluble element that is required as a trace element for living organisms. Chemically, iodine is the least reactive of the halogens, and the most electropositive metallic halogen. Iodine is primarily used in medicine, photography and in dyes.

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Notable characteristics

Iodine is a bluish-black, lustrous solid that sublimes at standard temperatures into a blue-violet gas that has an irritating odor. This halogen also forms compounds with many elements, but is less active than the other member of its series and has some metallic-like properties. Iodine dissolves easily in chloroform, carbon tetrachloride, or carbon disulfide to form purple solutions (It is only slightly soluble in water). The deep blue color with starch solution is characteristic of the free element.

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Applications

In areas where there is little iodine in the diet - typically remote inland areas where no marine foods are eaten - iodine deficiency gives rise to goitre, so called endemic goitre. In many (but not all) such areas, this is now prevented by the addition of small amounts of sodium iodide to table salt - this product is known as iodised salt Other uses:

One of the halogens, it is an essential trace element; the thyroid hormones, thyroxine and triiodotyronine contain iodine.
Tincture of iodine (3% elemental iodine in water/ethanol base) is an essential component of any emergency survival kit, used both to disinfect wounds and to sanitize surface water for drinking (3 drops/L, let stand for 30 minutes)
Iodine compounds are important in the field of organic chemistry and are very useful in medicine.
Iodides and thyroxine which contains iodine, are both used in internal medicine and, in combination with alcohol (as tincture of iodine) are used externally to disinfect wounds
Potassium iodide is used in photography
Tungsten iodide is used to stabilise the filaments in light bulbs
Nitrogen triiodide is an explosive, too unstable to be used commercially, but is commonly used in college pranks
Iodine-131 is used as a tracer in medicine
Potassium Iodide (KI) tablets can be given to people in a nuclear disaster area. KI prevents the body from absorbing the radioactive iodine produced at the disaster area.

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History

Iodine (Gr. iodes meaning violet) was discovered by Barnard Courtois in 1811. He was the son of a manufacturer of Saltpeter (potassium nitrate, a vital part of gunpowder). At the time France was at war and gunpowder was in great demand. Saltpeter was isolated from seaweed washed up on the coasts of Normandy and Brittany. To isolate the potassium nitrate, seaweed was burned and the ash then washed with water. The remaining waste was destroyed by adding hydrochloric acid. One day Curtois added too much sulfuric acid and cloud of purple vapor rose. Curtois noted that the vapor crystallised on cold surfaces making dark crystals. Curtois suspected that this was a new element but lacked the money to pursue his observations.

However he gave samples to his friends, Charles Bernard Desormes (1777-1862) and Nicolas Clément (1779-1841) to continue research. He also gave some of the substance to Joseph Louis Gay-Lussac (1778–1850), a well-known chemist at that time, and to André-Marie Ampère (1775-1836). On November 29 1813 Dersormes and Clément made public Curtois’ discovery. They described the substance to a meeting of the Imperial Institute of France. On December 6 Gay-Lussac announced that the new substance was either an element or a compound of oxygen. Ampère had given some of his sample to Humphrey Davy (1778-1829). Davy did some experiments on the substance and noted its similarity to chlorine. Davey sent a leter dated December 10 to the Royal Society of London stating that he had identified a new element. A large argument erupted between Davy and Gay-Lussac over who identified iodine first but both scientists acknowledged Bernard Curtois as the first to isolate the element.

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Occurrence

Iodine can be prepared in an ultrapure form through the reaction of potassium iodide with copper (II) sulfate. There are also several other methods of isolating this element.

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Isotopes

There are thirty isotopes of iodine and only one, I-127, is stable. The artificial radioisotope I-131 (a beta emitter) which has a half-life of 8 days, has been used in treating cancer and other pathologies of the thyroid glands. The most common compounds of iodine are the iodides of sodium and potassium (KI) and the iodates (KIO₃).

I-129 (half-life 15.7 million years) is a product of Xe-129 spallation in the atmosphere, but is also the result of uranium and plutonium fission. I-129 was used in rainwater studies following the Chernobyl accident. It also has been used as a ground-water tracer and as an indicator of waste dispersion into the natural environment. Other applications may be hampered by the production of I-129 in the lithosphere through a number of decay mechanisms.

In many ways, I-129 is similar to Cl-36. It is a soluble halogen, fairly non-reactive, exists mainly as a non-sorbing anion, and is produced by cosmogenic, thermonuclear, and in-situ reactions. In hydrologic studies, I-129 concentrations are usually reported as the ratio of I-129 to total I (which is virtually all I-127). As is the case with Cl-36/Cl, I-129/I ratios in nature are quite small, 10^-14 to 10^-10 (peak thermonuclear I-129/I during the 1960s and 1970s reached about 10^-7). I-129 differs from Cl-36 in that its half-life is longer (1.6 vs 0.3 million years), it is highly biophilic, and occurs in multiple ionic forms (commonly, I- and iodate) which have different chemical behaviors.

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Precautions

Direct contact with skin can cause lesions so care needs to be taken in handling iodine. Iodine vapor is very irritating to eyes and mucous membranes. The maximum allowable concentration of iodine in air should not exceed 1 mg/m³ (8-hour time-weighted average - 40-hour).

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References

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