Potassium iodide

Potassium iodide is a white crystalline salt with chemical formula KI, 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.

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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.^[1][2][3][4]

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).

Physical Properties

It occurs as odourless, colourless, transperant or somewhat opaque crystals or white granular powder. It is slightly hygroscopic, the taste is saline and slightly bitter. On long exposure to air, it becomes yellow due to the liberation of iodine and small quantities of iodate may be formed.

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.^[5][6] For example:

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

In medical use, it can also serve as an antiseptic for people suffering from sore throat. The dose is 0.5 g-1.0 g in 100 mL, with the accompany of iodine (0.5 g-1.0 g in 100 mL).

KI is also used as a fluorescence quenching agent in biomedical research because of collisional quenching by its iodide ion.

In aqueous solution with elemental iodine, it acts as a gold etchant and will attack and dissolve gold surfaces.

Radiation protection

Potassium iodide may also be used to protect the thyroid from radioactive iodine in the event of an accident or 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. In case of a nuclear emergency, iodine used for the cleaning of wounds should not be ingested.^[7] It is a poison.  

See fission products and the external links for more details.

Precautions

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

References

Reacts with various compounds since Iodine is a halogen and has greater -I effect