Iodine belongs to halogen group in the periodic table. Iodine is an essential mineral commonly found in sea food. Iodine plays an important role in proper functioning of thyroid hormone in the human body.
Discovery and History
During Napoleonic Wars, Bernard Courtois, a French chemist discovered Iodine in 1811, while he was extracting sodium and potassium compounds from seaweed ash. He noticed a cloud of violet gas when sulfuric acid was added to the ash and proposed that a distinct element is present in the ashes. Two years later Joseph Louis Gay-Lussac gave the name “iode” to this element, that is the Greek word for ‘’violet colored’’. Iodized table salt was sold first time in Michigan in 1924 [1].
Occurrence
Iodine is the 61st most abundant element on Earth. It is a rare element and does not exist in free -state. About 50mg per metric tons of iodine is present in the sea water [2]. Iodine is the least abundant halogens and is present in only 0.46 ppm (parts per million) of earth crust rocks. The mineral Caliche (sedimentary rock), is the main source of iodine. Brine (high concentration of iodine in water) is also used for the extraction of iodine and Japan and USA have major deposits of brine. Iodine is also formed in oysters, seaweeds and cod liver. Human body contains iodine in the form of thyroxin produced by the thyroid glands. Iodine is chiefly obtained from sodium iodate (NaIO3) and sodium periodate (NaIO4). Chile and Japan are prime producer of iodine in the world.
Physical characteristics
Iodine is bluish black, shiny solid and has a pungent odor. It is non -metal but show some metallic properties. Iodine is hardly dissolved in water but gives a yellow color solution. Iodine belongs to group halogens, that are the salt formers. Iodine when dissolved in chloroform gives purple color solution. Melting point and boiling point of iodine are highest as compared to other halogens. Its melting point is 113.70C and boiling point is about 184.30C. Atomic number of iodine is 63 and easily attached with organic compounds. Iodine’s atomic weight is 126.90. The density of iodine is very low, about 4.94g/ml.
Chemical characteristics
Iodine is reactive element among the halogens. It has lowest ionization energy and is easily oxidized. Iodine has various oxidation states, including +1 (iodides), +3, +5 (iodates) and +7 (periodate). and is more stable than bromine and chlorine. Iodine molecules act as Lewis acid with combined with many Lewis bases. Electron affinity of iodine is also similar to other halogen atoms. The simplest compound of iodine is hydrogen iodide. It is a colorless gas that reacts with oxygen to give water and iodine. Iodine does not react with oxygen or nitrogen. Iodine reacts with nonmetals and forms iodides, for example, silver and aluminum are converted into iodides. The iodide ion is a strong reducing agent and gives up electron easily. Iodide solutions are colorless but gives brownish tint due to oxidation. Iodine reacts with zinc and forms zinc iodide. This reaction is very exothermic and produce violet color vapors of iodine.
Significance and Uses
- Iodine in form of potassium iodide and alcohol is widely used as a disinfectant.
- Radioactive isotopes of iodine 131I is used to treat thyroid cancer.
- Iodine is used as catalyst in the preparation of acetic acid.
- Iodine is widely used in laboratories to test the presence of starch in solution.
- Iodine is used in iodoform test to detect the presence of methyl ketones.
- Iodine has antiviral and antimicrobial action.
- Erythrosine, an organo-iodine compound is important food coloring agent.
- Potassium iodide is used to make photographic films.
- Iodine is added in table salt for nutrition to prevent goiter in the thyroid gland.
- Tungsten iodide is used to stabilize filaments in light bulbs.
- Iodine is used in making of dyes and various pharmaceuticals products.
- Iodine- 129 is used in rainwater studies.
- Potassium iodide can be used for the treatment of people in a nuclear disaster area.
Health effects
Iodine is an essential part of human diet because the body does not make iodine and it is needed to produce thyroid hormones that regulate the growth and metabolism of the body. Deficiency of iodine may cause enlargement of thyroid gland (goiter) hypothyroidism and intellectual disabilities in infants and children. Goiter disease is also present in animals like dog, cattle, goat and birds. People who consume iodine based food on daily bases can also face various health problems, such as disturbed heart beats and loss of weight. Pure iodine is dangerous and poisonous if ingested. In U.S, the recommended daily dose of iodine is 110 to 113µg for infants to 12 months, 90 µg for 1 to 18 years old, 220µg for pregnant women and 290µg for lactation. U.S Food and Drug recommends an intake of 150µg per day of iodine for both men and women.
Isotopes of Iodine
Iodine has 34 isotopes with mass numbers ranging from 108 to 141. 127I is the only stable isotope of iodine [3]. The longest-lived isotope is 129I has half-life of 15.7 million years [4]. 125I has half-life of 59 days.
References
[1]. https://www.livescience.com/37441-iodine.html
[2]. https://www.britannica.com/science/iodine
[3]. https://www.chemicool.com/elements/iodine.html
[4]. Audi, G.; Bersillon, O.; Blachot, J.; Wapstra, A. H. (2003). “The NUBASE evaluation of nuclear and decay properties” (PDF). Nuclear Physics A. 729: 3–128.