History and Production
From Greek iodes, meaning violet. First discovered by B. Courtois in 1811 by treating the ash of seaweed with concentrated sulfuric acid. The name was subsequently given by J.L. Gay Lussac in 1813 to reflect its
characteristic coloration. There are many commercial sources for iodine extraction. For example, from saltpetre, salt wells and old sea deposits. Very pure iodine can be prepared from the reaction of potassium iodide with copper sulfate.
Iodine and its compounds are used in photography, medicine and also in organic synthesis. The artificial isotope 131I is used to treat the thyroid gland.
It is a bluish-black, lustrous solid that readily vaporized to give irritating violet fumes. It is only slightly soluble in water but readily soluble in organic solvents such as chloroform and carbon tetrachloride.
It consists of 0.46 ppm of crustal rocks, mostly exist in salt well, brines associated with oil wells and in seaweeds. The average concentration of iodine in sea water is only 0.05 ppm, which is too low for commercial
Interatomic distance: 266.2 pm
Melting point: 113.7°C
Boiling point: 184.4°C
Thermal conductivity/Wm-1K-1: 0.449 (27°C)
Density/kgm-3: 4930 (20°C)
Standard Thermodynamic Data (atomic gas)
Enthalpy of formation: 106.8 kJ/mol
Gibbs free energy of formation: 70.2 kJ/mol
Entropy: 180.8 J/mol K
Heat capacity: 20.8 J/mol K
Electronic configuration: [Kr] 4d10 5s2 5p5
Term symbol: 3P3/2
Electron affinity: 295.1524 kJ/mol Electronegativity (Pauline): 2.66
Ionization energy (first, second, third): 1008.39, 1845.89, 3184.02 kJ/mol
Test for iodide ion:
(1) Silver nitrate with nitric acid gives a yellow precipitate of silver iodide that is insoluble in ammonia.
(2) Addition of an oxidizing agent such as concentrated sulfuric acid, hydrogen peroxide and potassium dichromate yields iodine. The iodine can be extracted with carbon tetrachloride which gives a purple
coloration at the organic layer.
(3) Addition of mercury(II) chloride solution to a solution of an iodide gives a scarlet precipitate of mercury(II) iodide, soluble in excess of iodide, forming a complex [HgI4]2-.