IODINE

Iodine has atomic number 53 with a symbol I, is a solid nonmetallic element of the Halogen family. Group VIIA in the periodic table, a group that includes Flourine, Chlorine, and Bromine. At room temperature iodine is a lustrous, blue black, crystalline solid of atomic weight 126.9045. Iodine is the least water soluble halogen, but it dissolves readily in alcohol, chloroform, carbon tetrachloride, and benzene.

Iodine is poisonous, but as a trace element it is essential to plant and animal growth. In higher mammals it is concentrated in the Thyroid gland and is involved in the synthesis of thyroxine and other biochemicals that grown metabolic activities. Besides affecting growth, iodine deficiency can also cause goiter, so iodine salts are added to table salts in regions where iodine levels are low.

History
Iodine was first observed in 1811 by a French saltpeter manufacture, Bernard Desormes and Nicholas Clement confirmed its nature and announced the discovery of the new element, in 1813. Sir Humphry Davy's electrical experiments of Joseph Gay Lussac, who named the element for the color of its vapor.

In 1819, Jean Baptise Dumas provided proof that iodine existed in the sponges that had long been used to treat goiter. By 1820, data had been published linking kelp as a goiter treatment to the presence of iodine in this sea plant. Earlier, in 1814 J.J. Colin advanced microanalytical chemistry by the discovery that the starch reaction with iodine produce a blue coloration so intense that iodine could be detected in amounts as low as one part in 400,000.

Occurrence
Iodine is the 44th most abundant element in the cosmos, the 62d most abundant in the Earth's crust, and the17th most abundant of those dissolved in seawater (gases excluded). Even though iodine is widely distributed in nature, no large deposits exist. Chilean nitrates in which it is an impurity serve as one important source. Several organisms in which it becomes concentrated are also important sources, such as the seaweeds known as Kelp.

Chemical Properties of Iodine
Like all members of the halogen family, iodine is very reactive, although it is somewhat less reactive than the other halogens. Both bromine and chlorine liberate free iodine from aqueous solutions of iodine by reactions similar to:

 2 NaI + Br  -----> 2 NaBr  + I

Iodine vapor react with water to establish the equilibrium I2 + H2O <===> H+ + I- + HOI.

Iodine has several oxidation states: -1, +1, +3, +5, and +7. They help explain the compounds of halogens with halogens, such as ICl, ICl3, IF5, IF7, K(IF6), IOF3 and IO2F, all of which are stable at room temperature. About half of all commercial iodine is produce from Chilean nitrate deposits, which contain about 0.2% calcium iodate (Ca(IO3)2. Iodine are recovered from solutions of the nitrate using sodium bisulfite (NaHSO3) as a reducing agent. Other significant sources of iodine are underground brines, such as those found in Michigant. Sea weeds that are able to concentrate the element are now less important in the United States but are still major sources of iodine in some countries.

Iodine Uses:
The principle use of iodine is in the health sciences. Almost from the year of its discovery it has been used to prevent goiter. An alcoholic solution of the element known as tincture of iodine has been used as a disinfectant , although iodine complexes now predominate in this application. Iodine complexed with surfactants are used in common sanitarizers. Radioactive iodine, 132I, has found important use in tracer steochemistry studies, in dyes, as a catalyst, as an indicator in analytic chemistry, in engraving, in special soaps and lubricants, in rainmaking experiments, and as a measure of the degree of unsaturation of organic coumpound.

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