Silver iodide is an inorganic compound with the formula SilverIodide. The compound is a bright yellow salt, but samples almost always contain impurities of metallic silver that give a grey colouration. The silver contamination arises because some samples of AgI can be highly Photosensitivity. This property is exploited in silver-based photography. Silver iodide is also used as an antiseptic and in cloud seeding.
Structure
The structure adopted by silver iodide is temperature dependent:
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Below 420 K, the β phase of AgI, with the wurtzite structure, is most stable. This phase is encountered in nature as the mineral iodargyrite.
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Above 420 K, the α phase becomes more stable. This motif is a body-centered cubic structure which has the silver centers distributed randomly between 6 octahedral, 12 tetrahedral and 24 trigonal sites.
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A metastable γ phase also exists below 420 K with the 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
. 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.
[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.] Unless the preparation is conducted in dark conditions, the solid darkens rapidly, the light causing the reduction of ionic silver to metallic. The photosensitivity varies with sample purity.
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 11,000 kg are used for
cloud seeding annually, each seeding experiment consuming 10–50 grams.
[Phyllis A. Lyday "Iodine and Iodine Compounds" in Ullmann's Encyclopedia of Industrial Chemistry, p. 441. Wiley-VCH, Weinheim, 2005. ]
Safety
Extreme exposure can lead to
argyria, characterized by localized discolouration of body tissue.
Cited sources
