Sodium iodide

Sodium iodide is a colorless-to-white ionic solid whose lattice contains Na⁺ and I⁻ ions. Its formula is NaI, and its relatively high molar mass reflects iodine’s large atomic mass.¹ The compound is highly soluble in water and also dissolves in several polar solvents. Aqueous sodium iodide conducts electricity because its ions are mobile in solution.

Solid NaI commonly adopts the rock-salt crystal structure, the same broad structural type found in sodium chloride. Iodide is readily oxidized to molecular iodine, so solutions can darken when exposed to oxygen, light, or oxidizing contaminants. Laboratory material is therefore generally stored tightly closed and protected from conditions that promote oxidation. Sodium iodide itself is not radioactive unless a radioactive iodine isotope is present.

In medicine, sodium iodide supplies iodide for nutritional or therapeutic purposes and serves as a chemically defined form of iodine in some pharmaceutical preparations.² Iodide is required for the synthesis of thyroid hormones, although routine dietary needs are usually met through iodized salt and other foods rather than medicinal sodium iodide.

The compound has also been used as a carrier for radioactive iodine, particularly sodium iodide containing iodine-131, in diagnostic and therapeutic nuclear medicine. The thyroid actively concentrates iodide, allowing radioactive iodine to image thyroid tissue or deliver targeted radiation to thyroid cells. This radioactive application is distinct from ordinary, nonradioactive NaI: isotope identity, activity, shielding, handling, and clinical indication determine the medical risk and use.

Sodium iodide activated with a small amount of thallium, written NaI(Tl), is one of the most widely used inorganic scintillators. A gamma ray or X-ray deposits energy in the crystal, creating electronic excitations; activator sites then help convert part of that energy into a pulse of visible light. A photomultiplier tube or solid-state photosensor converts the flash into an electrical signal.

NaI(Tl) offers high light output, useful gamma-ray stopping power, and a practical balance of cost and performance. It is used in gamma spectroscopy, nuclear medicine cameras, environmental monitoring, and radiation-survey instruments. Its energy resolution is generally poorer than that of high-purity germanium, while its hygroscopic nature requires sealed housings and careful protection from moisture. Crystal size, optical coupling, surface treatment, and temperature all influence detector performance.

NaI(Tl) crystals are hygroscopic: they absorb water from air, and moisture can cloud or damage an unsealed crystal. This practical vulnerability explains why commercial detectors normally enclose the scintillator behind a hermetic window, often with reflective and optical-coupling layers. The crystal’s useful wavelength is matched to the photosensor, so detector design involves more than the salt’s chemical purity.

Sodium iodide also appears in analytical chemistry as a source of iodide for precipitation, ion-exchange, and redox reactions. Iodide can reduce some oxidizing agents, producing iodine that is measured by titration or spectroscopic methods. Another subtle distinction concerns isotope composition: ordinary NaI contains stable iodine-127, whereas sodium iodide used in nuclear medicine may contain iodine-123 or iodine-131. The chemical compound is similar, but the isotope changes its radiation and clinical behavior.

Scintillator

A material that emits flashes of light when it absorbs ionizing radiation.

NaI(Tl)

Sodium iodide activated with a small concentration of thallium to improve scintillation light output.

Hygroscopic

Readily absorbing moisture from the surrounding air.

Iodide

The negatively charged iodine ion, I−, or a compound containing that ion.

Gamma spectroscopy

Measurement of gamma-ray energies to identify radionuclides and estimate their activities.