CID 4875

Potassium iodide

2 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; potassium iodide has no molecular shape.

IUPAC name: potassium iodide
Molar mass: 165.998(11) g/mol
Atoms: 2
SMILES: [K+].[I-]
InChIKey: NLKNQRATVPKPDG-UHFFFAOYSA-M
PubChem CID: 4875

Made of

Potassium iodide belongs to two quite different settings. One is the salt shaker, where it is commonly added to make table salt iodised; the other is emergency planning around nuclear power plants. Haz-Map sums up its uses as a medicine for thyroid disorders, an expectorant and an antifungal agent, and as a way of adding iodine to salt, water and animal feed.

It is a salt of potassium and iodine, built from K⁺ and I⁻ ions in a one-to-one ratio, with a molar mass of 166.00 g/mol. CAMEO Chemicals describes an odourless white solid, and HSDB, the Hazardous Substances Data Bank, records a strong, bitter, salty taste.

Water takes it up readily. HSDB gives 148 g dissolving in 100 g of water at 25 °C, about one and a half times the water’s own mass. Ethanol dissolves only a little.

Structure and bonding

The drawing sets one potassium ion, K⁺, beside one iodide ion, I⁻, and draws no bond between them. None belongs there, since ionic bonding, the attraction between oppositely charged ions, mainly holds the solid together. The pair stands only for the crystal’s one-to-one ratio; the chemical bonding article explains why such a solid has no molecules.

What the drawing cannot show is how the ions sit in the solid. Each potassium ion touches its iodide neighbours, Chem1 explains, while the iodide ions keep apart. In lithium iodide, by contrast, the iodide ions are the ones in contact. That potassium–iodide contact lets the size of the K⁺ ion be estimated from this crystal.

Iodide is the large partner. According to Chem1, the larger an ion, the more loosely it holds its outer electrons and the more easily its electron cloud is distorted, a property called polarisability. In the alkali halides, plain attraction between charges accounts for less of the bonding as ions grow larger and more polarisable, anions most of all. HSDB describes the crystals as cubical, a shape the formula-unit drawing cannot convey.

Where it turns up

In US food law, iodised salt is table salt with iodide added in one of two forms, cuprous iodide or potassium iodide. Its label has to tell buyers that this salt supplies iodide, a necessary nutrient. HSDB also lists potassium iodide as an iodine source in some drinking water. The federal feed rule 21 CFR 582.80 names it among the iodine sources added to animal feed as trace minerals. Mixed with iodine in alcohol, Los Alamos notes, it gives a solution used on external wounds.

Seawater and salt deposits hold it naturally, according to its US food-additive rule, 21 CFR 184.1634. Most US production absorbs iodine in potassium hydroxide, HSDB notes, and evaporating natural brines is another route. For 2023, companies reported a combined US volume of 1 million to under 2.5 million pounds to the US Environmental Protection Agency.

Why it matters

Its place in a nuclear emergency rests on how the thyroid gland handles iodine. The gland cannot tell stable iodine from radioactive forms, most of them by-products of nuclear fission. It therefore absorbs both, CDC explains. Potassium iodide floods the gland with stable iodine until it is full and takes up no more. That may lower the thyroid’s uptake of radioactive iodine, and with it the added risk of thyroid cancer.

In hyperthyroidism, an overactive thyroid, potassium iodide relieves symptoms by holding back the release of thyroid hormone into the blood, HSDB records. MeSH, the Medical Subject Headings index, lists it for preparing such patients for surgical removal of the gland. It also serves as an expectorant, a medicine meant to help clear mucus; HSDB suggests it works by raising the airways’ secretions, which thins the mucus. The World Health Organization also classes it as an eye medicine and an antidote, and includes it in its Model List of Essential Medicines.

Properties

Properties of potassium iodide Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 4875.
Property	Value	Unit	Source
Formula (Hill order)	IK
Molar mass	165.998(11) g/mol	g/mol	IUPAC/CIAAW 2024
Potassium atoms	1
Iodine atoms	1
PubChem molecular weight	166.0028 g/mol	g/mol	PubChem

Safety

Potassium iodide is affirmed as generally recognised as safe when added to food, but the pure chemical carries the signal word Danger. Organ damage through prolonged or repeated exposure is the most notified hazard, in 43.6% of reports to the European Chemicals Agency. Skin and serious eye irritation, allergic skin reactions and harm if swallowed follow, and workplace exposure limits exist. The PubChem Laboratory Chemical Safety Summary for potassium iodide sets out the full classification.

Questions

What does air do to potassium iodide?

Given time, it gives up iodine. HSDB, the Hazardous Substances Data Bank, records that the solid yellows after long exposure to air, and that light and moisture hasten the breakdown. A solution in water also yellows with time as it oxidises, a change a little alkali prevents. CAMEO Chemicals calls the salt a weak reducing agent, a substance that gives up electrons, able to react with strong or weak oxidising agents.

Does potassium iodide protect against radiation?

Only for one gland. The US Nuclear Regulatory Commission (NRC) says it guards the thyroid alone, and only against radioactive iodine absorbed into the body. It neither keeps radioactive iodine out of the body nor undoes harm to a damaged gland, CDC adds. In emergency planning the NRC treats it as a supplement to sheltering and evacuation, the measures that protect the whole body.

What else is potassium iodide used for?

HSDB lists photography, where it precipitates silver, and scintillation, the flashes of light that ionising radiation produces in certain materials. It also lists spectroscopy, the study of how matter absorbs and gives off radiation, and infrared transmission. Los Alamos notes that its reaction with copper sulfate yields ultrapure iodine. Haz-Map adds a deodorant for manure. ChEBI names it a radical scavenger, a substance that removes reactive free radicals.

Is potassium iodide FDA approved?

Yes. The US Food and Drug Administration (FDA) has approved it as an over-the-counter medicine, sold without a prescription. The agency's December 2001 guidance on using it to shield the thyroid during radiation emergencies updated recommendations from 1982. A US Nuclear Regulatory Commission rule of 19 January 2001 requires states with residents near a commercial nuclear power plant to consider it in their emergency plans. The rule stops short of making it compulsory.