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Iodine sublimating in a fume hood.

Iodine is the element with the symbol I and atomic number 53. It is a reactive halogen which is volatile and has many uses in inorganic and organic chemistry.



Metallic crystals of iodine encrusting the inside of a beaker.

Iodine can be seen in the form of royal dark purple crystals, or in a gas in a lighter color. It is most commonly found as a silvery metallic looking chemical. Iodine, like other halogens, has its own unique smell, somewhat similar to chlorine, or described as being "clean".


Iodine is not very soluble in water, but is very soluble in a solution of sodium or potassium iodide in water. When dissolved like this, the solution has a color from light yellow to dark orange, to almost black brown. This change in color and solubility is due to the formation of the triiodide(I3-) ion. If reacted while in water with a sufficient reducing agent, such as phosphorus or hydrogen sulfide, iodine is converted to hydrogen iodide, which dissolves in solution as hydroiodic acid. Iodine possesses some metallic properties, despite being a Halogen.


Metallic iodine prepared by melting in H2SO4

Iodine can be isolated by acidifying a soluble iodide (I-) solution to form hydroiodic acid, adding an oxidizing agent like hydrogen peroxide to precipitate insoluble iodine, and purifying by sublimation. Adding concentrated sulfuric acid or nitric acid will produce iodine directly. Wet iodine can be dried by melting under concentrated sulfuric acid. Iodine is not easily purchased in the United States and other countries that tightly control drug precursors, as iodine is often used in the clandestine manufacture of methamphetamine.


Iodine is a controlled substance in many countries due to its use in producing illicit drugs, and while possession of iodine itself may not be a crime, its presence in a lab may be used to build a case against an innocent amateur chemist.




Iodine, like other halogens, is toxic in large quantities and its vapors should not be inhaled. Objects stained with iodine solution or with solid iodine on them should not be brought inside or left out, as any iodine will sublimate given time. Iodine stains can be cleaned with a solution of sodium thiosulfate.


Elemental iodine is very difficult to store properly, as it tends to sublime at ambient temperatures and its vapors will escape the storage bottle, and stain the environment where it's stored. Sealing the bottle with parafilm or teflon tape will reduce the escaping iodine vapors. Do NOT use aluminium or any other metal foil, as it will corrode.

A more permanent way is to store the iodine in a sealed glass ampoule.


Iodine can be neutralized by reacting it with thiosulfate.


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