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Silver iodide

Names
IUPAC name Silver(I) iodide
Other names Argentous iodide
Identifiers
CAS Number	7783-96-2 Y
3D model (JSmol)	Interactive image
ChemSpider	22969 Y
ECHA InfoCard	100.029.125
EC Number	232-038-0
PubChem CID	6432717
UNII	81M6Z3D1XE Y
CompTox Dashboard (EPA)	DTXSID0064836
InChI InChI=1S/Ag.HI/h;1H/q+1;/p-1 Y Key: MSFPLIAKTHOCQP-UHFFFAOYSA-M Y InChI=1/Ag.HI/h;1H/q+1;/p-1 Key: MSFPLIAKTHOCQP-REWHXWOFAV
SMILES [Ag]I
Properties
Chemical formula	AgI
Molar mass	234.77 g/mol
Appearance	yellow, crystalline solid
Odor	odorless
Density	5.68 g/cm3, solid[1]
Melting point	558 °C (1,036 °F; 831 K)[1]
Boiling point	1,506 °C (2,743 °F; 1,779 K)[1]
Solubility in water	0.03 mg/L (20 °C)[1]
Solubility product (Ksp)	8.52 × 10 −17[2]
Solubility	very soluble in liquid ammonia
Magnetic susceptibility (χ)	−80.0·10−6 cm3/mol[3]
Structure[4]
Crystal structure	Hexagonal, hP4
Space group	P63mc, No. 186
Lattice constant	a = 0.4591 nm, c = 0.7508 nm α = 90°, β = 90°, γ = 120°
Formula units (Z)	2
Dipole moment	4.55 D[5]
Thermochemistry[6]
Heat capacity (C)	56.8 J·mol−1·K−1
Std molar entropy (S⦵298)	115.5 J·mol−1·K−1
Std enthalpy of formation (ΔfH⦵298)	−61.8 kJ·mol−1
Gibbs free energy (ΔfG⦵)	−66.2 kJ·mol−1
Hazards
GHS labelling:[7]
Pictograms
Signal word	Warning
Hazard statements	H410
NFPA 704 (fire diamond)	2 0 0
Flash point	Non-flammable
Safety data sheet (SDS)	Sigma-Aldrich
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

Silver iodide is an inorganic compound with the formula AgI. The compound is a bright yellow salt, but samples almost always contain impurities of metallic silver that give a grey colouration. The silver contamination arises because some samples of AgI can be highly photosensitive. This property is exploited in silver-based photography. Silver iodide is also used as an antiseptic and in cloud seeding.

The structure adopted by silver iodide is temperature dependent.^[8]

Below 420 K, the β phase of AgI, with the wurtzite structure, is most stable. This phase is encountered in nature as the mineral iodargyrite.

Above 420 K, the α phase becomes more stable. This motif is a body-centered cubic structure which has the silver centers distributed randomly between 6 octahedral, 12 tetrahedral and 24 trigonal sites.^[9] At this temperature, Ag⁺ ions can move rapidly through the solid, allowing fast ion conduction. The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalized by considering the AgI crystalline lattice to have already "partly melted" in the transition between α and β polymorphs.

A metastable γ phase also exists below 420 K with the zinc blende structure.

Silver iodide is prepared by reaction of an iodide solution (e.g., potassium iodide) with a solution of silver ions (e.g., silver nitrate). A yellowish solid quickly precipitates. The solid is a mixture of the two principal phases. Dissolution of the AgI in hydroiodic acid, followed by dilution with water, precipitates β-AgI. Alternatively, dissolution of AgI in a solution of concentrated silver nitrate followed by dilution affords α-AgI.^[10] Unless the preparation is conducted in dark conditions, the solid darkens rapidly, the light causing the reduction of ionic silver to metallic. The photosensitivity varies with sample purity.

The use of silver iodide for cloud seeding was discovered by Bernard Vonnegut in 1946 ^[11]. The crystalline structure of β-AgI is similar to that of ice, allowing it to induce freezing by the process known as heterogeneous nucleation. Approximately 11,000 kg are used for cloud seeding annually, each seeding experiment consuming 10–50 grams.^[12]

Extreme exposure can lead to argyria, characterized by localized discolouration of body tissue.^[13]

Wikimedia Commons has media related to Silver iodide.

• Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. ISBN 9781498754293.