Sodium iodide

Sodium iodide
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Template:Longitem	NaI
Molar mass	149.894
Appearance	white solid ; deliquescent
Odor	odorless
Density	3.67 g cm−3
Melting point	Template:Chembox CalcTemperatures
Boiling point	Template:Chembox CalcTemperatures
Solubility	ethanol, acetone
Band gap	5.89 eV
Template:Longitem	−57×10−6 cm3 mol−1
Template:Longitem	1.93 (300 nm); 1.774 (589 nm); 1.71 (10 μm)
Template:Longitem	Halite, cF8
Template:Longitem	Fm3m, No. 225
Template:Longitem	a = 0.6462 nm
Template:Longitem	Octahedral
Template:Longitem	52.1 J mol−1 K−1
Template:Longitem	98.5 J mol−1 K−1
Template:Longitem	−287.8 kJ mol−1
NFPA 704 (fire diamond)	Template:NFPA 704 diamond
Flash point	Template:Chembox CalcTemperatures
Template:Longitem	Sodium fluoride; Sodium chloride; Sodium bromide; Sodium astatide
Template:Longitem	Lithium iodide; Potassium iodide; Rubidium iodide; Caesium iodide; Francium iodide

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Sodium iodide (chemical formula NaI) is an ionic compound formed from the chemical reaction of sodium metal and iodine. Under standard conditions, it is a white, water-soluble solid comprising a 1:1 mix of sodium cations (Na⁺) and iodide anions (I⁻) in a crystal lattice. It is used mainly as a nutritional supplement and in organic chemistry. It is produced industrially as the salt formed when acidic iodides react with sodium hydroxide.^[9] It is a chaotropic salt.

Uses

Food supplement

Sodium iodide, as well as potassium iodide, is commonly used to treat and prevent iodine deficiency. Iodized table salt contains 10 ppm iodide.^[9]

Organic synthesis

File:NaI@DWNT.png

Monatomic NaI chains grown inside double-wall carbon nanotubes.^[10]

Sodium iodide is used for conversion of alkyl chlorides into alkyl iodides. This method, the Finkelstein reaction,^[11] relies on the insolubility of sodium chloride in acetone to drive the reaction:^[12]

R–Cl + NaI → R–I + NaCl

Nuclear medicine

Some radioactive iodide salts of sodium, including Na¹²⁵I and Na¹³¹I, have radiopharmaceutical uses for thyroid cancer and hyperthyroidism or as radioactive tracer in imaging (see Isotopes of iodine > Radioiodines I-123, I-124, I-125, and I-131 in medicine and biology).

Thallium-doped NaI(Tl) scintillators

Sodium iodide activated with thallium, NaI(Tl), when subjected to ionizing radiation, emits photons (i.e., scintillate) and is used in scintillation detectors, traditionally in nuclear medicine, geophysics, nuclear physics, and environmental measurements. NaI(Tl) is the most widely used scintillation material. The crystals are usually coupled with a photomultiplier tube, in a hermetically sealed assembly, as sodium iodide is hygroscopic. Fine-tuning of some parameters (i.e., radiation hardness, afterglow, transparency) can be achieved by varying the conditions of the crystal growth. Crystals with a higher level of doping are used in X-ray detectors with high spectrometric quality. Sodium iodide can be used both as single crystals and as polycrystals for this purpose. The wavelength of maximum emission is 415 nm.^[13]

Radiocontrast

António Egas Moniz searched for a radiocontrast agent for cerebral angiography.^[14] After experiments on rabbits and dogs he settled upon sodium iodide as the best medium.^[14]

Solubility data

Sodium iodide exhibits high solubility in some organic solvents, unlike sodium chloride or even bromide:

Solvent	Solubility of NaI (g NaI/kg of solvent at 25 °C)^[15]
H₂O	1842
Liquid ammonia	1620
Liquid sulfur dioxide	150
Methanol	625–830
Formic acid	618
Acetonitrile	249
Acetone	504^[16]
Formamide	570–850
Acetamide	323 (41.5 °C)
Dimethylformamide	37–64
Dichloromethane	0.09^[17]

Stability

Iodides (including sodium iodide) are detectably oxidized by atmospheric oxygen (O₂) to molecular iodine (I₂). I₂ and I⁻ complex to form the triiodide complex, which has a yellow color, unlike the white color of sodium iodide. Water accelerates the oxidation process, and iodide can also produce I₂ by photooxidation, therefore for maximum stability sodium iodide should be stored under dark, low temperature, low humidity conditions.