Iodine was discovered by accident in 1811 by the French chemist Bernard Courtois. It is one of the better known elements among the general public, due to the very valuable property that it kills germs, mainly bacteria. Iodine (also known as Lugol after the physician who discovered how to make it soluble) is sold in chemists (pharmacies) as a yellow antiseptic lotion or a cream and is daubed on grazes, cuts and small wounds. It is also commonly added to table salt, which is then called iodized table salt. This is done because a dietary deficiency of iodine causes the thyroid gland in the throat to grow very large as it attempts to manufacture essential thyroid hormones. This condition is known as “goitre.”
Iodine and its derivatives are therefore principally used in the pharmaceutical and medical industry, for sanitation or as disinfectants, but also in animal feed, and in catalysts, inks, colorants, photographic equipment and stabilizers. Iodine is used as a test for the presence of starch because starch turns deep blue in its presence.
Iodine is in fact an element – one of the basic building blocks of all matter. The chemical symbol for Iodine is I, and it occurs naturally as one stable isotope,127I. It is element number 53 in the periodic table (the atomic number = 53) and occurs in group VIIA along with fluorine, chlorine and bromine. This group is called the halogens. Elemental iodine is a solid non-metallic bluish black crystal. It does not dissolve easily in water unless it bonds with potassium (Lugol’s discovery), but it does dissolve in other liquids (e.g., alcohol). When heated, it changes directly from the solid phase to the gas phase in a process called sublimation. Iodine gas has a characteristic and unmistakable violet colour and a strong odor. The name iodine is derived from the Greek word for violet “iodes.”
Melting point = 113.5°C (236.3°F).
Boiling point = 184°C (363°F).
Density @ 293 K = 4.93 g/cm3.
Atomic Mass = 126.9045
Elemental iodine is not found in nature, but it occurs rather as an iodide compound, usually with the alkali metals sodium and potassium. Iodide is thus merely the name for the iodine molecule in a compound that contains iodine and something else (e.g., potassium iodide). Iodide occurs in seawater, in brines from old sea deposits, in brackish water from oil wells and salt wells, and in saltpetre and nitrate-bearing soils called caliche (found in Chile). Iodides are absorbed from seawater by seaweeds such as brown kelp, which then enables its uptake by fish, especially reef fish, which in turn provide a source for human consumption. Shellfish have very high levels of iodine.
Allergic reactions to seafood are not due to their iodine content. Nor is the reaction to radiocontrast materials. Some people are hypersensitive to iodine, but that can be established by a simple skin test.
Iodine is, however, an irritant and can cause skin and membrane irritations. The gas should especially be handled with care.