Iodide

An iodide ion is the ion I⁻.[2] Compounds with iodine in formal oxidation state −1 are called iodides. This page is for the iodide ion and its salts, not organoiodine compounds. 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 intellectual disability.[3]

"i-" redirects here. For the Internet-related prefix i-, see Wiktionary's entry i-.

Iodide

Names
Systematic IUPAC name Iodide[1]
Identifiers
CAS Number	20461-54-5^Y
3D model (JSmol)	Interactive image
Beilstein Reference	3587184
ChEBI	CHEBI:16382
ChEMBL	ChEMBL185537^Y
ChemSpider	28015^Y
ECHA InfoCard	100.125.457
Gmelin Reference	14912
KEGG	C00708^Y
PubChem CID	30165
UNII	09G4I6V86Q^Y
CompTox Dashboard (EPA)	DTXSID80912339
InChI InChI=1S/HI/h1H/p-1^Y Key: XMBWDFGMSWQBCA-UHFFFAOYSA-M^Y
SMILES [I-]
Properties
Chemical formula	I^−
Molar mass	126.90447 g·mol^−1
Conjugate acid	Hydrogen iodide
Thermochemistry
Std molar entropy (S^o298)	169.26 J K^−1 mol^−1
Related compounds
Other anions	Fluoride Chloride Bromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Infobox references

Structure and characteristics of inorganic iodides

Iodide is one of the largest monatomic anions. It is assigned a radius of around 206 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, iodide 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 compared to 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.[2]

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 ⇌ I⁻
₃

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⁻      E° = 0.54 volts (versus 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:[4]

2 I⁻ + peroxidase + H₂O₂ + tyrosine, histidine, lipid, etc. → iodo-compounds + H₂O + 2 e⁻ (antioxidants).

Representative iodides

Compound	Formula	Appearance	Use or occurrence
Potassium iodide	KI	white crystals	iodine component of iodized salt
Hydrogen iodide	HI	colourless gas	strong mineral acid
Silver iodide	AgI	yellow powder that darkens in light	photoactive component of silver-based photographic film
Thyroxine (3,5,3′,5′-tetraiodothyronine)	C15H11I4NO4	pale yellow solid	hormone essential for human health

Natural occurrence

Iodargyrite—natural, crystalline silver iodide—is the most common iodide mineral currently known. Iodide anions may sometimes also be found combined with mercury, copper and lead, but minerals with such compositions are even more scarce.[5]

Other oxyanions

Iodine can assume oxidation states of −1, +1, +3, +5, or +7. A number of neutral iodine oxides are also known.

Iodine oxidation state	−1	+1	+3	+5	+7
Name	iodide	hypoiodite	iodite	iodate	periodate
Formula	I^−	IO^−	IO^− 2	IO^− 3	IO^− 4 or IO^5− 6

References

1. "Iodide - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
3. McNeil, Donald G. Jr (2006-12-16). "In Raising the World's I.Q., the Secret's in the Salt". New York Times. Retrieved 2008-12-04.
4. Küpper FC; Carpenter LJ; McFiggans GB; et al. (2008). "Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry" (Free full text). Proceedings of the National Academy of Sciences of the United States of America. 105 (19): 6954–8. Bibcode:2008PNAS..105.6954K. doi:10.1073/pnas.0709959105. PMC 2383960. PMID 18458346.
5. http://www.mindat.org

External links

• "Seaweed use iodine as an antioxidant". Chemistry World blog. Archived from the original on 2010-07-03. Retrieved 2008-05-15.
• "Stressed seaweed contributes to cloudy coastal skies, study suggests". Retrieved 2008-05-15.