Iodine, I, is a dark violet (Greek, ioeides, violet) non-metallic halogen element belonging to Group VIIb (i.e. the Halogen Group of elements) of the periodic table. There is one stable isotope, iodine-127 and fourteen radioactive isotopes.


Iodine was discovered by B Courtois in 1811AD. He treated the liquor obtained from the extraction of kelp (i.e. seaweed), which is now known to contain iodide salts, with sulphuric acid to produce a vapour with a violet colour.

2 NaI + H2SO4 ==> l2 + Na2SO4

In 1812AD, Guy Lussac demonstrated that iodine was an element and its chemical relationship to chlorine. About the same time, Davy confirmed many of these properties.


Because of its reactivity, iodine does not exist in the free elemental state in nature, but small quantities are combined in many silver ores found in Mexico.

Compounds of iodine occurs naturally in seaweed.

Iodide salts are found in liquor from the extraction of saltpeter from deposits in Chile.


Iodine is extracted from kelp, which is obtained by burning seaweed. Salts such as sodium chloride, potassium chloride, and potassium sulphate are removed from the kelp by washing with water. The residue is then heated with manganese dioxide and concentrated sulphuric acid, and the iodine is liberated.

              2 I(-)   +   MnO2   +   H(+)   ==>   Mn(2-)   +   2 H2O   +   I2


Iodine can be prepared in the laboratory by heating potassium Iodide or sodium iodide with dilute sulphuric acid and manganese dioxide.

2 KI + MnO2 + 3 H2SO4 ==> I2 + 2 KHSO4 + MnSO4 +2 H2O


Iodine is manufactured industrially


Iodine is a violet dense solid, which is insoluble in water, but it is soluble in non-polar solvents. When heated it gives a violet vapour that sublimes. Iodine is soluble in a solution of potassium iodide due to the formation of the soluble potassium tri-iodide.

		KI   +   I2   ==>   KI3	


Iodine resembles chlorine and bromine in many of its properties, but it is much less reactive.

Iodine combines directly with many elements, although it is usually necessary to provide heat and/or a catalyst.

Iodine reacts quantitatively with sodium thiosulphate

	I2   +   2 Na2S2O3   ==>   2 NaI   +   NaS4O6	

This reaction is of importance in volumetric analysis for the estimation of oxidising agents.


Iodine is used

Detection and Analysis

Iodine also produces a deep blue colour when it forms a complex with starch solution. Thus, a starch solution is used as an indicator for free elemental iodine in solution during titration's.

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