Silver iodide


Silver iodide
Silver iodide
Silver-iodide-3D-balls.png
other names
Silver(I) iodide
Identifiers
CAS number 7783-96-2 Yes check.svgY
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
NFPA 704.svg
0
2
0
 Yes check.svgY (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.

Contents

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.

Health Effects

Under the guidelines of the Clean Water Act by the EPA, silver iodide is considered a hazardous substance, a priority pollutant, and as a toxic pollutant.

Chronic Exposure/Target Organs: Chronic ingestion of iodides may produce “iodism”, which may be manifested by skin rash, running nose, headache and irritation of the mucous membranes. Weakness, anemia, loss of weight and general depression may also occur. Chronic inhalation or ingestion may cause argyria characterized by blue-gray discoloration of the eyes, skin and mucous membranes. Chronic skin contact may cause permanent discoloration of the skin.[3]

References

  1. ^ 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: 409–412. doi:10.1007/s10973-005-7154-1. http://www.sump4.com/publications/paper047.pdf. 
  2. ^ Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005.
  3. ^ http://www.nature.nps.gov/hazardssafety/toxic//silver.pdf

External links

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