potassium iodide

Potassium iodide
IUPAC name	Potassium iodide
Identifiers
CAS number	7681-11-0 Y
PubChem	4875
RTECS number	TT2975000
Properties
Molecular formula	KI
Molar mass	166.0028 g/mol
Appearance	white crystalline solid
Density	3.123 g/cm3
Melting point	681 °C, 954 K, 1258 °F
Boiling point	1330 °C, 1603 K, 2426 °F
Solubility in water	128 g/100 ml (6 °C) 140 g/100 mL (20 °C)
Solubility	2 g/100 mL (ethanol) soluble in acetone slightly soluble in ether, ammonia
Structure
Dipole moment	hi
Hazards
MSDS	External MSDS
EU Index	Not listed
NFPA 704	1 1 0
Related compounds
Other anions	Potassium fluoride Potassium chloride Potassium bromide
Other cations	Lithium iodide Sodium iodide Rubidium iodide Caesium iodide
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium iodide is an inorganic compound with formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons 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.^[1]

Potassium iodide is used medicinally in tablets, usually containing 130 mg of KI, or as a "saturated solution of potassium iodide" (SSKI) which in the U.S.P. generic formulation contains 1000 mg of KI per mL (or 500 mg KI in 10 drops, assumed to be 0.5 mL). ^[2]

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.^[1]

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.^[3][4][5]

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 is 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.^[6][7] 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 hair chemicals, and sometimes 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.

SSKI

A saturated solution of potassium iodide, abbreviated SSKI, is also used as treatment for sporotrichosis, a fungal infection. Since potassium iodide is highly soluble in water, SSKI contains 1 gram KI per milliliter (mL) of solution. This is less than 100% by weight, because SSKI is significantly more dense than pure water. Because KI is about 76.4% iodide by weight, SSKI contains about 764 mg iodide per mL.

Saturated solutions of potassium iodide can also be an emergency treatment for hyperthyroidism (so-called thyroid storm), as high amounts of iodide temporarily suppress secretion of thyroxine from the thyroid gland.^[8]

Iodide solutions made from a few drops of SSKI added to drinks have also been used as expectorants to increase the water content of respiratory secretions and encourage effective coughing.

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^[9] 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, nor can it provide any degree of protection against dirty bombs that produce radionuclides other than isotopes of iodine. In case of a nuclear emergency, iodine used for the cleaning of wounds should not be ingested, as it is poisonous.

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.” ^[11]

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. ^[12] 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. ^[13]

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.” ^[14]

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." ^[15] 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.” ^[16]

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%) the survivors of Nagasaki 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.” ^[17]

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. ^[18] 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.

Adverse reactions

There have been some reports of potassium iodide treatment causing swelling of the parotid gland (one of the three glands which secrete saliva), due to its stimulatory effects on saliva production. ^[19]

Adverse effects of SSKI, which is given orally in adult doses of about 250 mg KI several times a day (5 drops of SSKI several times a day, for an adult) for thryoid blockage and occassionally as an expectorant, include: acne, loss of appetite, or upset stomach during the first several days as the body adjusts to the medication. More severe side effects which required notification of a physician are: fever, weakness, unusual tiredness, swelling in the neck or throat, mouth sores, skin rash, nausea, vomiting, stomach pains, irregular heartbeat, numbness or tingling of the hands or feet, a metallic taste in the mouth. ^[20]

Precautions

Mild irritant, wear gloves. Chronic overexposure can have adverse effects on the thyroid. Potassium iodide is a possible teratogen (causing fetal goiter).

References

External links

• R05CA02
• World Health Organization's guidelines for iodine prophylaxis following a nuclear accident
• Potassium Iodide information from Ki4u.com
• Potassium Iodide information from Nukepills.com