Silver iodide
| Silver iodide | |
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Other names
Silver(I) iodide |
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| Identifiers | |
| CAS number | 7783-96-2  |
| ChemSpider | 22969 |
| UNII | 81M6Z3D1XE |
| Jmol-3D images | Image 1 |
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| 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) |
| Solubility product, Ksp | 8.52 × 10 −17 |
| Thermochemistry | |
| Std enthalpy of formation ΔfH |
-62.4 kJ/mol |
| Hazards | |
| EU classification | not listed |
| NFPA 704 |
 0
2
0
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 (verify) (what is:  / ?)Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa) |
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| Infobox references | |
Silver iodide (AgI) is a yellow, inorganic, photosensitive iodide of silver used in photography, in medicine as an antiseptic, and in rainmaking for cloud seeding.
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[edit] Properties
Silver iodide is notable as very insoluble in water.
[edit] Crystal structure
The crystalline structure adopted by silver iodide changes with temperature. The following phases are known:[1]
- Up to 420 K (147 °C), AgI exists in the β-phase, which has a wurtzite structure. It is known as the mineral iodargyrite.
- Above 420 K (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 420 K, which has a zinc blende structure.
[edit] Preparation
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 precipitate, darkening quickly, forms.
[edit] Applications
[edit] Rainmaking
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]
[edit] Fast ion conductor
Above 420 K, silver iodide becomes a fast ion conductor: Ag+ ions can move rapidly through the solid, making it a good conductor of electricity. 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 rationalized 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.
[edit] Safety
Under the guidelines of the United States Environmental Protection Agency Clean Water Act, silver iodide is considered a hazardous substance, a priority pollutant, and a toxic pollutant.
Chronic ingestion of iodides may produce “iodism”, which may be manifested by skin rash, running nose, headache, irritation of the mucous membranes, weakness, anemia, loss of weight and general depression. Chronic inhalation, ingestion or skin contact with silver compounds may cause argyria characterized by blue-gray discoloration of the eyes, skin and mucous membranes.[3]
[edit] References
- ^ Binner, J. G. P.; Dimitrakis, G.; Price, D. M.; Reading, M.; Vaidhyanathan, B. (2006). "Hysteresis in the β–α Phase Transition in Silver Iodide" (PDF). Journal of Thermal Analysis and Calorimetry 84 (2): 409–412. doi:10.1007/s10973-005-7154-1. http://www.sump4.com/publications/paper047.pdf.
- ^ Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
- ^ http://www.nature.nps.gov/hazardssafety/toxic//silver.pdf
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