Iodide

An iodide ion is the ion I&minus;. Compounds with iodine in formal oxidation state &minus;1 are called iodides. This page is for the iodide ion and its salts. For information on organoiodides, see organohalides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of mental retardation.

Structure and characteristics of inorganic iodides
Iodide is one of the largest (monoatomic) anions. It is assigned a radius of around 220 picometers. For comparison, the lighter halides are considerably smaller: bromide (196 pm), chloride (181 pm), and fluoride (133 pm). In part because of its size, iodine forms relatively weak bonds with most elements.

Most iodide salts are soluble in water, but often less so than the related chlorides and bromides. Iodide, being large, is less hydrophilic than are the smaller anions. One consequence of this is that sodium iodide is highly soluble in acetone, whereas sodium chloride is not. The low solubility of silver iodide and lead iodide reflects the covalent character of these metal iodides. A test for the presence of iodide ions is the formation of yellow precipitates of these compounds upon treatment of a solution of silver nitrate or lead(II) nitrate.

Aqueous solutions of iodide salts dissolve iodine better than pure water. This effect is due to the formation of the triiodide ion, which is brown:
 * I− +  I2   I3−

Redox, including antioxidant properties
Iodide salts are mild reducing agents and many react with oxygen to give iodine. A reducing agent is a chemical term for an antioxidant. Its antioxidant properties can be expressed quantitatively as a redox potential:
 * I−   1/2 I2  +  e− (electrons) =  - 0.54 Volt vs SHE

Because iodide is easily oxidized, some enzymes readily convert it into electrophilic iodinating agents, as required for the biosynthesis of myriad iodide-containing natural products. Iodide can function as an antioxidant reducing species that can destroy reactive oxygen species such as hydrogen peroxide:
 * 2 I− + Peroxidase +  H2O2 + tyrosine,  histidine,  lipid,  etc.  →   iodo-Compounds + H2O  + 2 e− (antioxidants).