|Jmol-3D images||Image 1|
|Molar mass||234.77 g/mol|
|Appearance||yellow, crystalline solid|
|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|
|Crystal structure||hexagonal (β-phase, < 147 °C)
cubic (α-phase, > 147 °C)
|Std enthalpy of
|EU classification||not listed|
|Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)|
|(what is: / ?)|
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.
The structure (or phase) adopted by silver iodide depends on the temperature of the solid:
- 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 a body-centered cubic structure which 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.
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. 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.
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.
Extreme exposure can lead to argyria characterized by localized discoloration of body tissue.
- Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN 0-618-94690-X.
- 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.
- 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.
- 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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