Silver iodide is an inorganic compound with the formula AgI. 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 photosensitive. 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. 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. 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. At this temperature, Ag+ ions can move rapidly through the solid, allowing fast ion conduction. The transition between the β and α forms represents the melting of the silver (cation) sublattice. The entropy of fusion for α-AgI is approximately half that for sodium chloride (a typical ionic solid). This can be rationalized by considering the AgI crystalline lattice to have already "partly melted" in the transition between α and β polymorphs. 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 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. 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. AgI is yellow due to band theory, where the excitation of electrons across the band gap absorbs blue and violet light.
Cloud seeding
The use of silver iodide for cloud seeding was discovered by Bernard Vonnegut in 1946 . 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.
Safety Toxicological evidence specific to silver iodide is limited. Most human and animal studies of silver have examined elemental silver or other compounds, particularly silver nitrate, silver oxide and silver chloride; their results therefore do not establish a dose–response relationship specific to AgI. Prolonged exposure to sufficiently large amounts of silver or silver compounds can cause argyria, a generally permanent blue-grey pigmentation resulting from silver deposition in the skin and other tissues. Ocular deposition is often termed argyrosis. Reviews describe argyria as primarily a cosmetic condition, although the exposure required to cause it is poorly defined. Most documented cases involve medicinal silver preparations or mixed occupational exposures rather than exposure to AgI alone. Reported GHS classifications list silver iodide as Aquatic Chronic 1 and assign the hazard statement H410, "very toxic to aquatic life with long-lasting effects". Silver iodide is nearly insoluble in water; when it dissolves, it releases small amounts of silver ions, which can harm beneficial environmental bacteria at sufficiently high concentrations. For cloud seeding, a 2016 laboratory study tested AgI concentrations selected to represent accumulation after repeated seeding treatments. At the highest concentration tested, 12.5 μM, moderate effects were observed in selected microbial and algal assays, while the growth and survival of the nematode Caenorhabditis elegans were unaffected. A 2024 review by the U.S. Government Accountability Office found that the available literature was limited to a handful of recent studies and reported no identified environmental or public-health concern at current cloud-seeding exposure levels. It also concluded that the effects of substantially wider use remain unknown.
References
Cited sources
Haynes, William M., ed. (2016). CRC Handbook of Chemistry and Physics (97th ed.). CRC Press. ISBN 9781498754293.
