Potassium Iodide

Potassium iodide is an inorganic compound with the chemical formula KI. This white salt is the most commercially significant iodide compound, with approximately 37,000 tons produced in 1985. It is less hygroscopic (absorbs water less readily) than sodium iodide, making it easier to work with. Aged and impure samples are yellow because of aerial oxidation of the iodide to elemental iodine.

4 KI + 2 CO₂ + O₂ → 2 K₂CO₃ + 2 I₂

Potassium iodide is medicinally supplied in 130 mg tablets (each containing 100 mg iodine as iodide) for emergency purposes related to blockade of radioiodine uptake. Potassium iodide may also be administered pharmaceutically for thyroid storm or as an expectorant, as a "saturated solution of potassium iodide" (SSKI) which in the U.S.P. generic formulation contains 1000 mg of KI per mL of solution. This represents 333 mg KI and about 250 mg iodide (I -) in a typical adult dose of 5 drops, assumed to be ⅓ mL. Because SSKI is a viscous liquid, it is normally assumed to contain 15 drops/milliliter, not 20 drops/milliliter as is often assumed for water. Thus, each drop of U.S.P. SSKI is assumed to contain about 50 mg iodine as iodide, I -. Thus, two (2) drops of U.S.P. SSKI solution is equivalent to one 130 mg KI tablet (100 mg iodide).

SSKI can also be prepared by truly saturating water with KI. This preparation can be made without a measuring scale. Since the solubility of KI in water at room temperature is about 1.40 to 1.48 grams per mL pure water, and the resulting solution has a density of about 1.72 g/mL, this process also results in a final concentration of KI of about 1000 mg KI per mL of saturated KI solution, and also contains essentially the same concentration of iodide per drop as does the U.S.P. formulation. Due to its high potassium content, SSKI is extremely bitter, and if possible it is administered in a sugar cube or small ball of bread. It may also be mixed into much larger volumes of juices.

Neither SSKI or KI tablets are used as nutritional supplements, since the nutritional requirement for iodine is only 150 micrograms or 0.15 mg of iodide per day. Thus, a drop of SSKI provides 50/0.15 = 333 times the daily iodine requirement, and a standard KI tablet provides twice this much.

Kelp is a natural KI source. The iodide content can range from 89 µg/g to 8165 µg/g in Asian varieties, making prepared foods content difficult to estimate. Eating 3-5 grams of most dried, unrinsed seaweeds will provide the 100-150 micrograms iodide recommended daily allowance for nutritional purposes.