Silver iodide is an inorganic compound with the formula Ag I. 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 Photosensitivity is the amount to which an object reacts upon receiving photons, especially visible light. In medicine, the term is principally used for abnormal reactions of the skin, and two types are distinguished, photoallergy and phototoxicit ...

. This property is exploited in silver-based

photography Photography is the art, application, and practice of creating durable images by recording light, either electronically by means of an image sensor, or chemically by means of a light-sensitive material such as photographic film. It is employe ...

. Silver iodide is also used as an

antiseptic An antiseptic (from Greek ἀντί ''anti'', "against" and σηπτικός ''sēptikos'', "putrefactive") is an antimicrobial substance or compound that is applied to living tissue/skin to reduce the possibility of infection, sepsis, or putre ...

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 Wurtzite is a zinc and iron sulfide mineral with the chemical formula , a less frequently encountered structural polymorph form of sphalerite. The iron content is variable up to eight percent.Palache, Charles, Harry Berman & Clifford Frondel (19 ...

structure, is most stable. This phase is encountered in nature as the mineral

iodargyrite Iodyrite or iodargyrite is a natural mineral form of silver iodide. Related minerals are chlorargyrite and bromargyrite Bromyrite or bromargyrite is a natural mineral form of silver bromide found mainly in Mexico and Chile. Hardness is 1.5 to ...

. *Above 420 K, the α phase becomes more stable. This motif is a

body-centered cubic In crystallography, the cubic (or isometric) crystal system is a crystal system where the unit cell is in the shape of a cube. This is one of the most common and simplest shapes found in crystals and minerals. There are three main varieties of ...

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 In thermodynamics, the entropy of fusion is the increase in entropy when melting a solid substance. This is almost always positive since the degree of disorder increases in the transition from an organized crystalline solid to the disorganized ...

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. image:Iodargyrite-263859.jpg, left, 240px, The golden-yellow crystals on this mineral sample are

, 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

precipitate In an aqueous solution, precipitation is the process of transforming a dissolved substance into an insoluble solid from a super-saturated solution. The solid formed is called the precipitate. In case of an inorganic chemical reaction leading ...

s. The solid is a mixture of the two principal phases. Dissolution of the AgI in

hydroiodic acid Hydroiodic acid (or hydriodic acid) is an aqueous solution of hydrogen iodide (HI). It is a strong acid, one that is ionized completely in an aqueous solution. It is colorless. Concentrated solutions are usually 48% to 57% HI. Reactions Hy ...

, 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 In crystallography, crystal structure is a description of the ordered arrangement of atoms, ions or molecules in a crystalline material. Ordered structures occur from the intrinsic nature of the constituent particles to form symmetric patterns t ...

of β-AgI is similar to that of

ice Ice is water frozen into a solid state, typically forming at or below temperatures of 0 degrees Celsius or Depending on the presence of impurities such as particles of soil or bubbles of air, it can appear transparent or a more or less opaqu ...

, allowing it to induce

freezing Freezing is a phase transition where a liquid turns into a solid when its temperature is lowered below its freezing point. In accordance with the internationally established definition, freezing means the solidification phase change of a liquid ...

by the process known as heterogeneous nucleation. Approximately 50,000 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, Wiley-VCH, Weinheim, 2005. (see also

Project Stormfury Project Stormfury was an attempt to weaken tropical cyclones by flying aircraft into them and seeding with silver iodide. The project was run by the United States Government from 1962 to 1983. The hypothesis was that the silver iodide would cause ...

Operation Popeye Operation Sober Popeye (Project Controlled Weather Popeye / Motorpool / Intermediary-Compatriot) was a military cloud-seeding project carried out by the U.S. Air Force during the Vietnam War in 1967–1972. The highly classified program attempted ...

)

Safety

Extreme exposure can lead to

argyria Argyria or argyrosis is a condition caused by excessive exposure to chemical compounds of the element silver, or to silver dust. The most dramatic symptom of argyria is that the skin turns blue or blue-grey. It may take the form of ''generalize ...

, characterized by localized discoloration of body tissue.

References

{{Authority control Metal halides Iodides Silver compounds Photographic chemicals Weather modification Antiseptics Light-sensitive chemicals Wurtzite structure type