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Potassium iodide

Potassium iodide

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Potassium iodide is an inorganic compound with formula KI. This colorless salt is the most commercially significant iodide compound, approximately 37,000 tons/y were produced in 1985. It is less hygroscopic than sodium iodide, making it easier to work with. Aged and impure samples are yellow because of oxidation of the iodide to iodine.

Structure, production, properties

Potassium iodide is ionic, K+I. It crystallises in the sodium chloride motif. It is produced industrially by treating KOH with iodine.

Inorganic chemistry

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)
This reaction is employed in the isolation of iodine from natural sources. Even air will oxidize iodide as evidenced by the observation of a purple extract when aged samples of KI are rinsed with dichloromethane. As formed under acidic conditions, hydroiodic acid (HI) is a stronger reducing agent.

Like other iodide salts, KI forms I3 when combined with elemental iodine.

KI(aq) + I2(s) → KI3(aq)
Unlike I2, I3 salts can be highly water-soluble. Through this reaction iodine is used in redox titrations. Aqueous KI3, "Lugol's solution," are used as disinfectants and as etchants for gold surfaces.

Potassium iodide is the precursor to silver(I) iodide, which are used for high speed photographic film:

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

Organic chemistry

KI serves as a source of iodide in organic synthesis. A useful application is in the preparation of aryl iodides from arenediazonium salts. For example:

Applications

The major uses of KI include nutritional supplement in animal feeds, a precursor to AgI, and as a component in disinfectants. Potassium iodide is also added to table salt in small quantities to make it "iodized." KI is also used as a fluorescence quenching agent in biomedical research because of collisional quenching by its iodide ion. A saturated solution of potassium iodide, abbreviated SSKI, is also used as treatment for sporotrichosis, a fungal infection.

Radiation protection

Following the Chernobyl nuclear reactor disaster in April, 1986, a saturated solution of potassium iodide (SSKI) was administered to 10.5 million children and 7 million adults in Poland as a prophylactic measure against accumulation of radioactive iodine-131 in the thyroid gland.

Potassium iodide was also approved in 1982 by the US FDA to protect the thyroid glands from radioactive iodine. In the event of an accident or attack at a nuclear power plant, or fallout from a nuclear bomb, several volatile fission product radionuclides may be released. 131I is a common fission by-product and is particularly dangerous as the body concentrates it in the thyroid gland, which may lead to thyroid cancer. By saturating the body with a source of stable iodide prior to exposure, inhaled or ingested 131I tends to be excreted. Potassium iodide cannot protect against any other causes of radiation poisoning, however, nor can it provide any degree of protection against most forms of "dirty bombs." In case of a nuclear emergency, iodine used for the cleaning of wounds should not be ingested, as it is poisonous.

Recommended Dosage for Radiological Emergencies involving radioactive iodine
Age KI in mg
Over 12 years old 130
3 - 12 years old 65
1 - 36 months old 32
< 1 month old 16

See fission products and the external links for more details.

Potassium iodide’s (KI) value as a radiation protective (thyroid blocking) agent was demonstrated at the time of the Chernobyl nuclear accident when Soviet authorities distributed it in a 30 km zone around the plant.  The purpose was to protect residents from radioactive iodine, a highly carcinogenic material found in nuclear reactors which had been released by the damaged reactor. Unfortunately, only a limited amount of KI was available, but those who received it were protected.  Later, the US Nuclear Regulatory Commission (NRC) reported, “thousands of measurements of I-131 (radioactive iodine) activity…suggest that the observed levels were lower than would have been expected had this prophylactic measure not been taken.  The use of KI…was credited with permissible iodine content in 97% of the evacuees tested.”

Poland, 300 miles from Chernobyl, also gave out  KI to protect its population.  Approximately 18 million doses were distributed, with follow-up studies showing no known thyroid cancer among KI recipients. But time has shown that people living in irradiated areas where KI was not available have developed thyroid cancer at epidemic levels, which is why the US Food and Drug Administration (FDA) reported “The data clearly demonstrate the risks of thyroid radiation…KI can be used [to] provide safe and effective protection against thyroid cancer caused by irradiation.

Chernobyl also demonstrated that the need to protect the thyroid from radiation was greater than expected.  Within ten years of the accident, it became clear that thyroid damage caused by released radioactive iodine was virtually the only adverse health effect that could be measured.  As reported by the NRC, studies after the accident showed, that “As of 1996, except for thyroid cancer, there has been no confirmed increase in the rates of other cancers, including leukemia, among the…public, that have been attributed to releases from the accident.”

But equally important to the question of KI is the fact that radiation releases are not “local” events.  Researchers at the World Health Organization accurately located and counted the cancer victims from Chernobyl and were startled to find that “the increase in incidence [of thyroid cancer] has been documented up to 500 km from the accident site…significant doses from radioactive iodine can occur hundreds of kilometers from the site, beyond emergency planning zones." Consequently, far more people than anticipated were affected by the radiation, which caused the United Nations to report in 2002 that “The number of people with thyroid cancer…has exceeded expectations.  Over 11,000 cases have already been reported.”

These findings were consistent with studies of the effects of previous radiation releases.  In 1945, millions of Japanese were exposed to radiation from nuclear weapons, and the effects can still be measured. Today, nearly half (44.8%) of Japanese studied have identifiable thyroid disease, with the American Medical Association reporting “it is remarkable that a biological effect from a single brief environmental exposure nearly 60 years in the past is still present and can be detected.”

These events, as well as the development of thyroid cancer among residents in the South Pacific from radioactive fallout following weapons testing in the 1950’s (on islands nearly 200 miles downwind of the tests) were instrumental in the decision by the FDA in 1978 to issue a request for the availability of KI for thyroid protection in the event of a release from a commercial nuclear power plant or weapons-related nuclear incident. Noting that KI’s effectiveness was “virtually complete” and finding that iodine in the form of potassium iodide (KI) was substantially superior to other forms including iodate (KIO-3) in terms of safety, effectiveness, lack of side effects, and speed of onset, the FDA invited manufacturers to submit applications to produce and market KI. Today, three companies (Anbex, Inc., Fleming Co, and Recip of Sweden) have met the strict FDA requirements for manufacturing and testing of KI, and they offer products (IOSAT, ThyroShield, and Thyro-Safe, respectively) which are available for purchase on the internet.

Precautions

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

References

External links

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