Potassium Iodide
Properties
| State | Solid at room temperature |
| Color | White or colorless crystals |
| Solubility | Very soluble in water (140 g/100 mL at 20 °C) |
| Melting Point | 681 °C |
| Boiling Point | 1330 °C |
About Potassium Iodide
Potassium iodide, KI at 166.003 g/mol, is a colorless cubic crystal that dissolves in water at roughly 140 g per 100 mL — one of the highest solubilities of any common salt. The famous emergency-preparedness use is thyroid blockade: a single 130 mg adult dose of KI floods the thyroid with stable I-127, saturating the sodium-iodide symporter so radioactive I-131 from a nuclear release can't be taken up. The Polish government distributed KI tablets nationwide after Chernobyl in 1986; FEMA stockpiles them around U.S. nuclear plants. The everyday public-health use is far more important by total impact: roughly 75% of household salt globally is iodized with 20-40 ppm KI or KIO3, and that one intervention has nearly eliminated endemic goiter and prevented countless cases of cretinism since the 1920s. In analytical chemistry, KI is the iodide source in iodometric titrations — react with Cu2+, H2O2, hypochlorite, or peroxydisulfate, then titrate the liberated I2 with thiosulfate to the starch-blue endpoint. KI also gives clear solutions a slight yellow tint over time as I- slowly oxidizes to I3- on exposure to air and light, which is why analytical-grade KI is bottled in amber glass.
Where you'll encounter it
If you've ever taken a multivitamin with iodine, sipped reconstituted iodized table salt, run an iodometric titration to check the chlorine in a swimming pool, or stocked a hurricane go-bag near a nuclear plant, KI has been part of the picture. In a working environmental lab, the standard chlorine assay is exactly this: spike a buffered KI solution with the sample, let the chlorine quantitatively oxidize I- to I2, add a few drops of starch indicator, and titrate with standardized 0.0250 N sodium thiosulfate to the disappearance of the blue starch-iodine complex. In organic synthesis, KI catalyzes Finkelstein reactions in acetone, swapping primary alkyl chlorides or bromides for the more reactive iodides — handy when an alkylation needs a better leaving group.
Common Uses
- Thyroid blockade tablets distributed for nuclear-emergency preparedness near reactors
- Iodine fortification additive at 20-40 ppm in household table salt globally
- Iodide source for iodometric titration of chlorine, peroxide, and copper(II)
- Finkelstein reaction catalyst for halide exchange in acetone solvent
- Saturated SSKI expectorant in respiratory medicine and sporotrichosis treatment
- X-ray contrast iodide for some thyroid uptake imaging studies
- Photographic emulsion component in silver iodide photographic film and paper
Safety Information
GHS: H319 eye irritation Cat 2A. Oral LD50 in rats around 1862 mg/kg, so acute toxicity is low. The chronic concerns matter more: doses above 1100 µg iodine per day can cause iodine-induced hyperthyroidism (Jod-Basedow phenomenon) in iodine-deficient adults or hypothyroidism (Wolff-Chaikoff effect) in susceptible individuals, especially those with autoimmune thyroid disease. Pregnant women take a special category — too little iodine causes cretinism, too much causes fetal goiter. Iodide allergy is rare but real and contraindicates KI tablets. Store in amber glass away from light; aged solutions develop yellow I3- and lose accurate iodide concentration for analytical use. Compatible with most plastics; avoid prolonged contact with steel.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.