# Iodide

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An **iodide** [ion](/source/Ion) is I−.[2] Compounds with [iodine](/source/Iodine) in formal [oxidation state](/source/Oxidation_state) −1 are called **iodides**. In everyday life, iodide is most commonly encountered as a component of [iodized salt](/source/Iodized_salt), which many governments mandate. Worldwide, [iodine deficiency](/source/Iodine_deficiency) affects two billion people and is the leading preventable cause of [intellectual disability](/source/Intellectual_disability).[3]

## Structure and characteristics of inorganic iodides

Iodide is one of the largest monatomic [anions](/source/Anion). It is assigned a radius of around 206 [picometers](/source/Picometer). For comparison, the lighter halides are considerably smaller: [bromide](/source/Bromide) (196 pm), [chloride](/source/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](/source/Sodium_iodide) is highly soluble in acetone, whereas [sodium chloride](/source/Sodium_chloride) is not. The low solubility of [silver iodide](/source/Silver_iodide) and [lead iodide](/source/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](/source/Silver_nitrate) or [lead(II) nitrate](/source/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](/source/Triiodide) ion, which is brown:

- I− + I2 ⇌ I3−

### Redox, including antioxidant properties

Iodide salts are mild [reducing agents](/source/Reducing_agent) 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](/source/Redox_potential):

- 2I− ⇌ I2 + 2e− *E*° = 0.54 volts (versus [SHE](/source/Standard_hydrogen_electrode))

Because iodide is easily oxidized, some enzymes readily convert it into [electrophilic](/source/Electrophilic) iodinating agents, as required for the [biosynthesis](/source/Biosynthesis) of myriad iodide-containing [natural products](/source/Natural_product). Iodide can function as an antioxidant [reducing](/source/Redox) species that can destroy ozone[4] and [reactive oxygen species](/source/Reactive_oxygen_species) such as [hydrogen peroxide](/source/Hydrogen_peroxide):[5]

- 2 I− + [peroxidase](/source/Peroxidase) + H2O2 + [tyrosine](/source/Tyrosine), [histidine](/source/Histidine), lipid, etc. → iodo-compounds + H2O + 2 e−

## 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](/source/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.[6]

## Other oxoanions

[Iodine](/source/Iodine) can assume [oxidation states](/source/Oxidation_state) of −1, +1, +3, +5, or +7. A number of neutral [iodine oxides](/source/Iodine_oxide) are also known.

Iodine oxidation state −1 +1 +3 +5 +7 Name iodide hypoiodite iodite iodate periodate Formula I− IO− IO2− IO3− IO4− or IO65−

## References

1. ["Iodide - PubChem Public Chemical Database"](https://pubchem.ncbi.nlm.nih.gov/compound/30165). *The PubChem Project*. USA: National Center for Biotechnology Information.

1. McNeil, Donald G. Jr (2006-12-16). ["In Raising the World's I.Q., the Secret's in the Salt"](https://www.nytimes.com/2006/12/16/health/16iodine.html?fta=y). *New York Times*. Retrieved 2008-12-04.

1. Pillar, Elizabeth A.; Guzman, Marcelo I.; Rodriguez, Jose M. (2013-10-01). "Conversion of Iodide to Hypoiodous Acid and Iodine in Aqueous Microdroplets Exposed to Ozone". *Environmental Science & Technology*. **47** (19): 10971–10979. [Bibcode:2013EnST...4710971P](https://ui.adsabs.harvard.edu/abs/2013EnST...4710971P). [doi:10.1021/es401700h](https://doi.org/10.1021/es401700h). [ISSN 0013-936X](https://www.worldcat.org/issn/0013-936X). [PMID 23987087](https://pubmed.ncbi.nlm.nih.gov/23987087)

1. Küpper FC; Carpenter LJ; McFiggans GB et al. (2008). ["Iodide accumulation provides kelp with an inorganic antioxidant impacting atmospheric chemistry"](http://bib-pubdb1.desy.de/record/85506/files/Kuepper%20et%20al%20Fig%203.pdf) (Free full text). *Proceedings of the National Academy of Sciences of the United States of America*. **105** (19): 6954–8. [Bibcode:2008PNAS..105.6954K](https://ui.adsabs.harvard.edu/abs/2008PNAS..105.6954K). [doi:10.1073/pnas.0709959105](https://doi.org/10.1073/pnas.0709959105). [PMC 2383960](https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2383960). [PMID 18458346](https://pubmed.ncbi.nlm.nih.gov/18458346)

1. ["Mineral/rock/commodity names containing 'iodide'"](https://www.mindat.org/search.php?search=iodide). *mindat.org*. Retrieved 2022-08-09.

## External links

- ["Seaweed use iodine as an antioxidant"](https://web.archive.org/web/20100703233405/http://prospect.rsc.org/blogs/cw/?p=956). *Chemistry World blog*. Archived from [the original](http://prospect.rsc.org/blogs/cw/?p=956) on 2010-07-03. Retrieved 2008-05-15.
- ["Stressed seaweed contributes to cloudy coastal skies, study suggests"](https://web.archive.org/web/20080511185319/http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php). Archived from [the original](http://www.eurekalert.org/pub_releases/2008-05/uom-ssc050608.php) on 2008-05-11. Retrieved 2008-05-15.

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Adapted from the Wikipedia article [Iodide](https://en.wikipedia.org/wiki/Iodide) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Iodide?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.