Iodid

Iodid, or iodide, is the anion I−. It forms when iodine gains one electron, and is the reduced counterpart to elemental iodine. In salts it pairs with metal cations such as sodium, potassium, or ammonium (for example NaI and KI). In aqueous solution iodide is typically colorless, but it is readily oxidized to iodine (I2). In presence of oxidants, I− converts to I2, and iodine can form the triiodide ion I3−, imparting a brown color to solution.

Iodide ions are large and highly polarizable and participate in redox chemistry. They serve as good leaving groups in organic synthesis and form polyiodide species when mixed with iodine, affecting color and reactivity in solution and in solid films.

Iodide is abundant in seawater and is an essential nutrient for humans. It is taken up by the thyroid gland and used to synthesize the hormones thyroxine (T4) and triiodothyronine (T3). Iodide deficiency can cause goiter and developmental impairments; iodized salt helps prevent these conditions.

Industrial iodide salts are produced from iodine by reduction or by neutralizing hydroiodic acid with bases. Sodium iodide and potassium iodide are used in organic synthesis, photography, medicine, and as sources of iodide in various processes.

Safety and environment: iodide is of relatively low hazard at typical exposures, but excessive intake can disturb thyroid function. Environmental releases are monitored due to potential effects on aquatic life and its role in iodine cycles.