Potassium iodide

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

2 KI(aq) + Cl₂(aq) → 2 KCl + I₂(aq)

If dichloromethane is present, the purple colour of elemental iodine may be seen in the organic phase. Under acidic conditions, KI is oxidised even more easily, due to the formation of hydroiodic acid (HI), which is a powerful reducing agent.

KI also forms a complex ion I₃^- when combined with elemental iodine, and in fact this reaction is frequently used to render I₂ more soluble in water:

KI(aq) + I₂(s) → KI₃(aq)

The reaction is slightly reversible, allowing the iodine to be used in aqueous solutions for redox titrations.

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

Preparation

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

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

It may also be used to protect the thyroid from radioactive iodine in the event of an accident at a nuclear power plant, a dirty bomb or other minor nuclear attack (it cannot protect against other causes of radiation poisoning, however). In a saturated solution, it is also used as an expectorant to treat lung congestion.

Potassium iodide is also added to table salt in small quantities to make it "iodized".

Role of Potassium Iodide in Radiological Emergency Preparedness

The purpose of radiological emergency preparedness is to protect people from the effects of radiation exposure after an accident at a nuclear power plant. Evacuation is the most effective protective measure in the event of a radiological emergency because it protects the whole body (including the thyroid gland and other organs) from all radionuclides and all exposure pathways. However, in situations when evacuation is not feasible and in-place sheltering is substituted as an effective protective action, administering potassium iodide is a reasonable, prudent, and inexpensive supplement to sheltering. Potassium iodide is a special kind of protective measure in that it offers very specialized protection. Potassium iodide protects the thyroid gland against internal uptake of radioiodines that may be released in the unlikely event of a nuclear reactor accident. When potassium iodide is ingested, it is taken up by the thyroid gland. In the proper dosage, and taken at the appropriate time, it will effectively saturate the thyroid gland in such a way that inhaled or ingested radioactive iodines will not be accumulated in the thyroid gland. The risk of thyroid effects is reduced. Such thyroid effects resulting from radioiodine uptakes due to inhalation or ingestion, or both, could result in acute, chronic, and delayed effects. Acute effects from high doses include thyroiditis, while chronic and delayed effects include hypothyroidism, thyroid nodules, and thyroid cancer. The FDA has approved potassium iodide as an over-the-counter medication. As with any medication, individuals should check with their doctor or pharmacist before using it.

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], for example:

Precautions

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