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]

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

HI																	He
LiI	BeI₂											BI₃	CI₄	NI₃	I₂O₄, I₂O₅, I₄O₉	IF, IF₃, IF₅, IF₇	Ne
NaI	MgI₂											AlI₃	SiI₄	PI₃, P₂I₄	S	ICl, ICl₃	Ar
KI	CaI₂	Sc	TiI₄	VI₃	CrI₃	MnI₂	FeI₂	CoI₂	NiI₂	CuI	ZnI₂	Ga₂I₆	GeI₂, GeI₄	AsI₃	Se	IBr	Kr
RbI	SrI₂	Y	ZrI₄	Nb	Mo	Tc	Ru	Rh	Pd	AgI	CdI₂	InI₃	SnI₄, SnI₂	SbI₃	TeI₄	I	Xe
CsI	BaI₂		Hf	Ta	W	Re	Os	Ir	Pt	AuI	Hg₂I₂, HgI₂	TlI	PbI₂	BiI₃	Po	AtI	Rn
Fr	Ra		Rf	Db	Sg	Bh	Hs	Mt	Ds	Rg	Cn	Nh	Fl	Mc	Lv	Ts	Og
		↓
		La	Ce	Pr	Nd	Pm	SmI₂	Eu	Gd	TbI₃	Dy	Ho	Er	Tm	Yb	Lu
		Ac	ThI₄	Pa	UI₃, UI₄	Np	Pu	Am	Cm	Bk	Cf	Es	Fm	Md	No	Lr

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 IO5− 6

References

↑ "Iodide - PubChem Public Chemical Database". The PubChem Project. USA: National Center for Biotechnology Information.
1 2 Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0-08-037941-9.
↑ 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.
↑ 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.

External links

"Seaweed use iodine as an antioxidant". Chemistry World blog. Retrieved 2008-05-15.
"Stressed seaweed contributes to cloudy coastal skies, study suggests". Retrieved 2008-05-15.

Iodine compounds

Iodine(-I)	HI AtI ICN I⁻

Iodine(I)	HIO ICl IF IBr

Iodine(III)	HIO₂ ICl₃ IF₃

Iodine(V)	HIO₃ IF₅ I₂O₅

Iodine(VII)	HIO₄ H₅IO₆ IF₇

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