Iodine was first discovered by Barnard Courtois, a French chemist in 1811 while extracting potassium and sodium from seaweed ash. When he accidentally added sulfuric acid, a violet colored cloud evolved from the mass. The resulting gas was condensed into dark crystals-the first occurrence of observed solid iodine.
Solid at 298 K
CAS Registry ID
Group in periodic table
Period in periodic table
Block in periodic table
Violet-dark grey, lustrous
386.85 K (113.7°C or 236.7°F)
457.55 K (184.4°C or 364°F)
Phase at room temperature
Although the concentration of Iodine in the Earth's crust is only around 0.3 to 0.5 ppm, it can be obtained in large quantities from sea water, sea kelp, brine or brackish water. Small amount of iodine can also be obtained from silver ores present in Mexico, and sodium iodate deposits and sodium periodate deposits in Chile and Bolivia.
Iodine has 34 isotopes with known half-lives and mass numbers ranging from 108 to 141.
127I is the only stable isotope. The longest-lived isotope is 129I with a half-life of 15.7 million years.
Iodine can be produced in a number of ways, which are listed below.
Seaweed is first washed with water to remove potassium sulfate, potassium chloride and sodium chloride. The residue is then heated with concentrated sulfuric acid and manganese dioxide to liberate iodine.
2 I(-) + MnO
2 + H(+) → Mn(2-) + 2 H 2O + I 2
Iodine can also be prepared by heating sodium iodide or potassium iodide with manganese dioxide and dilute sulfuric acid.
2 KI + MnO
2 + 3 H 2SO 4 → I 2 + 2 KHSO 4 + MnSO 4 +2 H 2O
Furthermore, Iodine can be obtained from the left-over liquor after extracting potassium nitrate from saltpeter.
Elemental Iodine is not particularly dangerous, although prolonged exposure can cause irritation in the lungs and eyes. Iodine 131, on the other hand, is a long-lived radionucleotide used in nuclear weapons which will increase the risk of cancer and other thyroid-related diseases.
The following are the key properties of iodine:
It is a bluish-black, lustrous solid
It undergoes sublimation when heated
It is the most electropositive halogen
It is slightly soluble in water and soluble in many other solvents like carbon tetrachloride
It is less reactive than other halogens
Some of the applications of iodine include the following:
It is used as an antiseptic for external wounds
It can be used for treating diseases in the thyroid gland
It is used for manufacturing dyestuffs and drugs
Silver iodine is used in making photographic films
It is used as a reagent in laboratories for performing analytical techniques
It is used as water purification tablets and also animal feeds
In industries, it is used as a catalyst.