Sodium Iodide
Properties
| State | Solid (white crystalline, deliquescent) |
| Color | White (turns yellow-brown on air exposure as I- oxidizes to I2) |
| Solubility | Highly soluble in water (1842 g/L at 25 °C); also soluble in acetone (28 g/100 mL) |
| Melting Point | 661 °C |
| Boiling Point | 1304 °C |
About Sodium Iodide
Sodium iodide (NaI, MW 149.894) is a face-centered cubic ionic salt with the same rock-salt structure as NaCl, but the polarizable iodide anion gives NaI an unusual portfolio of jobs across nuclear medicine, gamma-ray spectroscopy, and synthetic organic chemistry. The headline application is NaI(Tl), a single crystal of sodium iodide doped with about 0.1% thallium grown by the Bridgman or Czochralski process. When a gamma photon strikes the crystal it produces a cascade of secondary electrons that excite the Tl+ activator centers, and these emit a flash of 415 nm visible light proportional to the deposited energy. Coupled to a photomultiplier tube, an NaI(Tl) detector resolves gamma-ray energies to about 7% FWHM at 662 keV (the Cs-137 line), which is enough for radioisotope identification at airports, environmental monitoring stations, well-logging tools, and gamma cameras in nuclear medicine. The radioactive isotope I-131 (beta-emitter, t1/2 8.02 days) administered as oral Na-131-I is the gold-standard treatment for hyperthyroidism and follicular/papillary thyroid cancer; the thyroid scavenges iodide via the sodium-iodide symporter (NIS), so the radiation dose lands almost entirely in thyroid tissue. In synthesis, NaI in dry acetone runs the Finkelstein reaction (R-Cl or R-Br + NaI -> R-I + NaCl/NaBr), driven by the precipitation of the much less acetone-soluble NaCl/NaBr byproduct. The iodide ion is also the nucleophilic iodine source in Appel-type and Mitsunobu workups. NaI is sharply deliquescent and turns yellow-brown on air exposure as trace I- is air-oxidized to I2.
Where you'll encounter it
If you've ever stood near a 2x2 inch NaI(Tl) probe at a customs checkpoint, had a thyroid scan with technetium-99m or radioiodine therapy after a thyroidectomy, or run a Finkelstein reaction in undergraduate organic chemistry to convert an alkyl bromide into the more reactive alkyl iodide, you've used the iodide chemistry of NaI. In a radiochemistry lab, the standard 137Cs check source you calibrate against gives a clean 662 keV photopeak, a Compton edge near 477 keV, and a backscatter peak around 184 keV in an NaI(Tl) spectrum, and that recognizable shape is how everyone learns scintillator response. In a clinical nuclear medicine suite, the patient swallows a capsule of 30-200 mCi I-131 sodium iodide for ablation of residual thyroid tissue, then sleeps in a lead-shielded room for 48 hours while the radiation drops below release criteria. In a teaching lab, dissolving 1-bromobutane in acetone with a spoonful of NaI gives a creamy NaBr precipitate within minutes, the textbook visual cue that an SN2 displacement happened.
Common Uses
- NaI(Tl) scintillation crystals for gamma-ray spectroscopy (Cs-137, Co-60, K-40 ID)
- I-131 sodium iodide capsules for hyperthyroidism and thyroid cancer ablation
- Finkelstein reaction in acetone to convert alkyl chlorides/bromides to iodides
- Iodide source in the Krapcho decarboxylation and other SN2 nucleophilic substitutions
- Component of Lugol's iodine alternative formulations and iodized table salt blends
- Photographic emulsion sensitizer (legacy silver halide film chemistry)
- Reducing agent and iodide titrant in iodometric determinations of oxidizers
- Expectorant in older cough syrups (saturated SSKI-style preparations)
Safety Information
Low acute oral toxicity (rat oral LD50 about 4,340 mg/kg). Chronic excess intake causes iodism: metallic taste, salivation, coryza, acneiform rash, and at high doses thyroid suppression (Wolff-Chaikoff effect) or paradoxical hyperthyroidism in iodine-deficient individuals (Jod-Basedow phenomenon). Pregnancy category: high doses can cross the placenta and induce fetal goiter or hypothyroidism. No occupational exposure limit is set for NaI itself; the governing figure is the low ceiling published for iodine vapor. GHS classifications: H319 (causes serious eye irritation), H400 (very toxic to aquatic life). The radioiodine forms (Na-125-I, Na-131-I) require licensed handling under NRC 10 CFR 35 for therapeutic and diagnostic use, with patient release criteria, contamination surveys, and shielded storage. Store solid NaI in tightly closed amber bottles away from light, moisture, and oxidizers; the yellowing on the shoulder of an old bottle is liberated I2 and indicates the salt has degraded.
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. We deliberately do not publish occupational exposure limits or other regulatory thresholds: those values are revised over time and differ between jurisdictions, so the only correct source is the current SDS and the regulations that apply where you work.