silver iodide

Silver iodide
Other names	Silver(I) iodide
Identifiers
CAS number	7783-96-2 Y
Properties
Molecular formula	AgI
Molar mass	234.77 g/mol
Appearance	yellow, crystalline solid
Density	5.675 g/cm3, solid
Melting point	558 °C
Boiling point	1506 °C
Solubility in water	3 × 10−7g/100mL (20 °C)
Thermochemistry
Std enthalpy of formation ΔfHo298	-62.4 kJ/mol
Hazards
EU classification	not listed
NFPA 704	0 2 0
Y (what is this?)  (verify) Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Silver iodide (AgI) is an inorganic compound. This yellow photosensitive solid is used in photography, as an antiseptic in medicine, and in rainmaking or cloud seeding. Silver iodide is highly insoluble in water.

The crystalline structure adopted by silver iodide changes with temperature. The following phases are known:^[1]

• Up to 420K (147 °C), AgI exists in the β-phase, which has a wurtzite structure. It is known as the mineral iodargyrite.
• Above 420K (147 °C), AgI undergoes a transition to the α-phase, which has a body-centered cubic structure and has the silver ions distributed randomly between 2-, 3-, and 4-coordinate sites.
• A metastable γ-phase also exists below 420K, which has a zinc blende structure.

Rainmaking

Cessna 210 equipped with a silver iodide generator for cloud seeding

The crystalline structure of AgI is similar to that of ice, allowing it to induce freezing (heterogeneous nucleation) in cloud seeding for the purpose of rainmaking. Approximately 50,000 kg/year are used for this purpose, each seeding experiment consuming 10-50 grams.^[2]

Silver iodide as a fast ion conductor

The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion (melting) for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalised by noting that the AgI crystalline lattice has essentially already partly melted in the transition between α and β forms. Adding the entropy of transition from α-AgI to β-AgI to the entropy of fusion gives a value that is much closer to the entropy of fusion for sodium chloride.

References

External links

Wikimedia Commons has media related to: Silver iodide

• A commercial site (Vista Chemicals) regarding silver iodide

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