Silver iodide

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Silver iodide
Other names

Silver(I) iodide

Identifiers
CAS number 7783-96-2 YesY
PubChem 6432717
ChemSpider 22969 YesY
UNII 81M6Z3D1XE YesY
EC number 232-038-0
Jmol-3D images {{#if:[Ag]I|Image 1
Properties
Molecular formula AgI
Molar mass 234.77 g/mol
Appearance yellow, crystalline solid
Odor odorless
Density 5.675 g/cm3, solid
Melting point 558 °C; 1,036 °F; 831 K
Boiling point 1,506 °C; 2,743 °F; 1,779 K
Solubility in water 3×10−7g/100mL (20 °C)
Solubility product, Ksp 8.52 × 10 −17
Solubility soluble in acid
Structure
Crystal structure hexagonal (β-phase, > 147 °C)
cubic (α-phase, < 147 °C)
Thermochemistry
Std enthalpy of
formation ΔfHo298		 −62 kJ·mol−1[1]
Standard molar
entropy So298		 115 J·mol−1·K−1[1]
Hazards
MSDS Sigma-Aldrich
EU classification not listed
NFPA 704
0
2
0
Flash point Non-flammable
 YesY (verify) (what is: YesY/N?)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

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

Structure

The structure (or phase) adopted by silver iodide depends on temperature of the solid:[2]

  • Up to 420 K (147 °C), AgI is more stable in the β-phase, which has a wurtzite structure. It is known as the mineral iodargyrite. In this motif, the silver and iodide centers are tetrahedrally coordinated.
  • Above 420 K (147 °C), the α-phase becomes more stable. This motif is body-centered cubic structure with has the silver centers distributed randomly between 2-, 3-, and 4-coordinate sites. Above 420 K, Ag+ ions can move rapidly through the solid, making it 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 rationalized by noting that the AgI crystalline lattice has essentially already partly melted in the transition between α and β forms.
  • A metastable γ-phase also exists below 420 K, which has a zinc blende structure.
The golden-yellow crystals on this mineral sample are iodargyrite, a naturally occurring form of β-AgI.

Preparation and properties

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.[3] If the preparation is not conducted in the absence of sunlight, the solid darkens rapidly, the light causing the reduction of ionic silver to metallic. The photosensitivity varies with sample purity.

Cloud seeding

Cessna 210 equipped with a silver iodide generator for cloud seeding

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 50,000 kg are used for cloud seeding annually, each seeding experiment consuming 10-50 grams.[4]

Safety

Silver compounds are "much less toxic than other heavy metals".[4] Extreme exposure can lead to argyria characterized by localized discoloration of the tissue.

References

  1. ↑ 1.0 1.1 Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X. 
  2. ↑ 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. 
  3. ↑ O. Glemser, H. Saur "Silver Iodide" in Handbook of Preparative Inorganic Chemistry, 2nd Ed. Edited by G. Brauer, Academic Press, 1963, NY. Vol. 1. p. 1036-7.
  4. ↑ 4.0 4.1 Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, Wiley-VCH, Weinheim, 2005. doi:10.1002/14356007.a14_381
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