Potassium iodide

Potassium iodide
General
Systematic name	Potassium iodide
Other names	Kalium iodide, knollide, potide
Molecular formula	KI
Molar mass	166.00 g/mol
Appearance	white crystalline solid
CAS number	^*
Properties
Density and phase	3.13 g/cm³, solid
Solubility in water	128 g/100 ml (6 °C)
Melting point	680 °C (953 K)
Boiling point	1330 °C (1600 K)
Acidity (pK_a)	?
Basicity (pK_b)	?
Structure
Molecular shape	?
Coordination geometry	?
Crystal structure	?
Dipole moment	? D
Hazards
MSDS	External MSDS
Main hazards	Slightly hazardous
NFPA 704
R/S statement	R: 36, 38, 42-43, 61 S: 26, 36-37, 39, 45
RTECS number	TT2975000
Supplementary data page
Structure & properties	n, ε_r, etc.
Thermodynamic data	Phase behaviour Solid, liquid, gas
Spectral data	UV, IR, NMR, MS
Related compounds
Other anions	potassium bromide potassium chloride
Other cations	sodium iodide rubidium iodide calcium iodide
Related compounds	?
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) Chemical infobox

Potassium iodide is a white crystalline salt with chemical formula , used in photography and radiation treatment. It finds widespread application as an iodide source because it is less hygroscopic than sodium iodide, making it easier to work with. KI can turn yellow upon heating in air or upon standing in moist air for long periods, because of oxidation of the iodide to iodine.

Chemical properties

Potassium iodide behaves as a simple ionic salt, K⁺I^-. Since the iodide ion is a mild reducing agent, I^- is easily oxidised to I₂ by powerful oxidising agents such as chlorine:

2 KI(aq) + Cl₂(aq) → 2 KCl + I₂(aq)

Even air will oxidize iodide as evidenced by the observation of a purple extract when KI is rinsed with dichloromethane. Under acidic conditions, KI is oxidised even more easily, due to the formation of hydroiodic acid (HI), which is a powerful reducing agent.

KI forms I₃^- when combined with elemental iodine.

KI(aq) + I₂(s) → KI₃(aq)

Unlike I₂, I₃^- salts can be highly water-soluble. I₂ and I₃^- have virtually identical redox potentials (0.535 and 0.536 V vs NHE, respectively), i.e. they are both mild oxidants relative to H₂. Therefore, this reaction allows the iodine to be used in aqueous solutions for redox titrations.

Potassium iodide also serves in some organic reactions as a source of iodide ion (see "uses" below).

Preparation

Potassium iodide may be prepared by the reaction of a potassium base with hydroiodic acid, for example:

HI + KHCO₃ → KI + H₂O(l) + CO₂(g)

Alternatively iron(II) iodide, prepared using scrap iron and iodine (made from iodide rich brines or from Chile saltpeter, can be treated with potassium carbonate:

FeI₂ + K₂CO₃ → 2 KI + FeCO₃

Uses

Potassium iodide is used in photography, in the preparation of silver(I) iodide for high speed photographic film:

KI(aq) + AgNO₃(aq) → AgI(s) + KNO₃(aq)

Potassium iodide is also added to table salt in small quantities to make it "iodized". In a saturated solution, it is also used as an expectorant to treat lung congestion.

KI is often used as a source of iodide ion in organic synthesis. A useful application is in the preparation of aryl iodides from arenediazonium salts^{^*}, for example:

Saturated solution of potassium iodide is also used as treatment for sporotrichosis, a fungal infection.

Role of potassium iodide in radiological emergency preparedness

Potassium iodide may also be used to protect the thyroid from radioactive iodine in the event of an accident or terrorist attack at a nuclear power plant, or other nuclear attack, especially where a nuclear reactor is breached and the volatile radionuclides, which contain significant amount of ¹³¹I, are released into the environment. Radioiodine is a particularly dangerous radionuclide because the body concentrates it in the thyroid gland. Potassium iodide cannot protect against other causes of radiation poisoning, however, nor can it provide any degree of protection against a dirty bomb unless the bomb happens to contain a significant amount of radioactive iodine.

See fission products for more details.

Precautions

Mild irritant, wear gloves. Chronic overexposure can have adverse effects on the thyroid.

References

N. N. Greenwood, A. Earnshaw, Chemistry of the Elements, Pergamon Press, Oxford, UK, 1984.
Handbook of Chemistry and Physics, 71st edition, CRC Press, Ann Arbor, Michigan, 1990.
The Merck Index, 7th edition, Merck & Co., Rahway, New Jersey, 1960.
H. Nechamkin, The Chemistry of the Elements, McGraw-Hill, New York, 1968.
(a) L. G. Wade, Organic Chemistry, 5th ed., pp. 871-2, Prentice Hall, Upper Saddle RIver, New Jersey, 2003. (b) J. March, Advanced Organic Chemistry, 4th ed., pp. 670-1, Wiley, New York, 1992.

External links