Iodine (I) - Discovery, Occurrence, Production, Properties and Applications of Iodine

Chemical Formula



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.

Basic Information

Name Iodine
Symbol I
Atomic number 53
Atomic weight 126.9 amu
Standard state Solid at 298 K
CAS Registry ID 7553-56-2
Group in periodic table 17
Group name Halogen
Period in periodic table 5
Block in periodic table p-block
Color Violet-dark grey, lustrous
Classification Non-metallic
Melting point 386.85 K (113.7°C or 236.7°F)
Boiling point 457.55 K (184.4°C or 364°F)
Density 4.9 g/cm3
Phase at room temperature Solid


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 - Periodic Table of Videos


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(-) + MnO2 + H(+) → Mn(2-) + 2 H2O + I2

Iodine can also be prepared by heating sodium iodide or potassium iodide with manganese dioxide and dilute sulfuric acid.

         2 KI + MnO2 + 3 H2SO4 → I2 + 2 KHSO4 + MnSO4 +2 H2O

Furthermore, Iodine can be obtained from the left-over liquor after extracting potassium nitrate from saltpeter.

Health Aspects

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.

Key Properties

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.


G.P. Thomas

Written by

G.P. Thomas

Gary graduated from the University of Manchester with a first-class honours degree in Geochemistry and a Masters in Earth Sciences. After working in the Australian mining industry, Gary decided to hang up his geology boots and turn his hand to writing. When he isn't developing topical and informative content, Gary can usually be found playing his beloved guitar, or watching Aston Villa FC snatch defeat from the jaws of victory.


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