# Silver iodide

Crystal structure
__ Ag +      __ I -
General
Surname Silver iodide
other names
Ratio formula AgI
Brief description

yellowish, odorless powder

External identifiers / databases
 CAS number 7783-96-2 EC number 232-038-0 ECHA InfoCard 100.029.125 PubChem 24563 Wikidata Q19052
properties
Molar mass 234.77 g mol −1
Physical state

firmly

density

5.67 g cm −3

Melting point

552 ° C

solubility

practically insoluble in water (0.03 mg l −1 )

safety instructions
GHS labeling of hazardous substances

Caution

H and P phrases H: 410
P: 273-391-501
MAK

0.01 mg m −3 (based on the inhalable fraction)

Thermodynamic properties
ΔH f 0

−61.8 kJ / mol

As far as possible and customary, SI units are used. Unless otherwise noted, the data given apply to standard conditions .

Silver iodide (also: silver iodide ) is a chemical compound of silver and iodine . It is a yellowish salt that is insoluble in water .

## Occurrence

Silver iodide occurs naturally as a mineral, iodine argyrite .

## Extraction and presentation

Silver iodide
without and with ammonia water

Silver iodide is obtained by precipitation from a silver nitrate solution with the help of potassium iodide .

${\ displaystyle \ mathrm {AgNO_ {3} + KI \ longrightarrow AgI \ downarrow + \ KNO_ {3}}}$

This reaction is also used in chemical analysis as evidence of iodide ions, because the AgI produced forms a slightly soluble, yellowish precipitate. In contrast to silver iodide, silver chloride (AgCl) and silver bromide (AgBr) , which are also sparingly soluble, can be dissolved in ammonia ( complex formation reaction ). Silver iodide can also be complexed or dissolved with sodium thiosulfate . Chloride, bromide and iodide ions can be distinguished from one another with the help of sodium thiosulphate and ammonia solution.

## properties

### Physical Properties

Several modifications of silver iodide are known. At room temperature, the β-AgI is thermodynamically stable, which crystallizes in the wurtzite structure. There is also a metastable modification, γ-AgI, which has a zinc blende structure.

The α-AgI, which is one of the solid ion conductors due to its high silver ion conductivity, is stable above about 147 ° C. Its ionic conductivity is in the order of 1 to 2  S / cm and is comparable to that of liquid electrolytes . The α-AgI has a body-centered cubic iodide sublattice and a structurally disordered silver ion sublattice. The silver ions can move freely between the larger iodide ions. By incorporating rubidium ions into rubidium silver iodide (Ag 4 RbI 5 ), the temperature of the α / β phase transition can be reduced to below room temperature. This also extends the range of ion conduction up to room temperature.

The electronic conductivity of α-AgI is based on electron hole conduction and is proportional to the I 2 partial pressure. Compared to the conductivity based on the silver ions, it is about a factor of 10 10 smaller. This makes silver iodide particularly suitable as a solid electrolyte.

### Chemical properties

Silver iodide

Silver iodide is sensitive to light and slowly breaks down into its elements. It turns green-gray in sunlight. AgI dissolves in strong complexing agents such as cyanides or thiocyanates.

## use

Silver iodide is mixed with acetone and sprayed from hailstones in order to generate tiny condensation nuclei in the atmosphere for targeted rain or hail formation .

On the one hand, it serves to prevent or mitigate harmful storms. This can prevent the formation of too large hailstones. In the USA attempts were made in the 1940s and 1950s to prematurely mitigate hurricanes with silver iodide; however, the effect was limited. In Germany in 1958, an organized hail defense system was set up in the Rosenheim district , which shot the silver iodide from over 100 launch points into the clouds with rockets. Since 1975 this task has been done by two anti-hail aircraft. In southern Germany, Austria and Switzerland there are other hail brigades organized as associations.

On the other hand, an attempt is made to supply certain areas with precipitation in a targeted manner: By inoculating the clouds with silver iodide fine dust in the updraft channel of a cloud front from an aircraft, since the 1980s (according to an unconfirmed claim by Russian major Alexei Gruschin also in 1986 at Chernobyl to prevent radioactive clouds over large Russian cities) in the mid-west of the USA and Russia, but also on a test basis in Bavaria, tried to let the clouds rain down in a targeted manner at a defined location. The effectiveness of this method has been statistically investigated, but the success is low (approx. 10% more precipitation). The silver iodide is analytically detectable in very small amounts in the snow that has fallen. These quantities are harmless to humans.

With the contrary goal, a certain area is kept rain-free on individual dates by letting the showers fall in front of it. So in Moscow there is sunshine on May 9, Victory Day , and June 12, Russia's day . At the 2008 Summer Olympics in Beijing, silver iodide was rocketed into rain clouds to avoid disrupting the opening ceremony.

In the early days of photography in the 19th century, silver iodide was used for various fine printing processes such as calotype or argyrotype because of its sensitivity to light . It was later replaced with more suitable substances like silver bromide .

## Individual evidence

1. Entry on silver iodide in the GESTIS substance database of the IFA , accessed on March 11, 2020(JavaScript required) .
2. David R. Lide (Ed.): CRC Handbook of Chemistry and Physics . 90th edition. (Internet version: 2010), CRC Press / Taylor and Francis, Boca Raton, FL, Standard Thermodynamic Properties of Chemical Substances, pp. 5-4.
3. James Huheey, Ellen Keiter, Richard Keiter: Inorganic Chemistry: Principles of Structure and Reactivity . Gruyter, Germany 2003, ISBN 978-3110179033 , p. 150.
4. ^ Halide separation
5. JGP Binner, G. Dimitrakis, DM Price, M. Reading, B. Vaidhyanathan: "Hysteresis in the β – α Phase Transition in Silver Iodide", Journal of Thermal Analysis and Calorimetry , 2006 , 84 , pp. 409–412 ( PDF )
6. ^ W. Biermann, W. Jost: Z. Phys. Chem. NF , 1960 , 25 , p. 139.
7. B. Ilschner: J. Chem. Phys. , 1958 , 28 , p. 1109.
8. a b c Mara Schneider: The weather can only be controlled to a limited extent. (News article) (No longer available online.) News.de, February 19, 2009, formerly in the original ; accessed on February 21, 2009 (German).  ( Page no longer available , search in web archivesInfo: The link was automatically marked as defective. Please check the link according to the instructions and then remove this notice.
9. Telepolis: Why it rained over Belarus after Chernobyl | Telepolis , accessed August 18, 2018
10. ^ BR.de: Wettermanipulation: Die Hagelflieger von Rosenheim | Knowledge | Topics | BR.de , accessed on August 18, 2018
11. WELT: Rain at the push of a button - WELT , accessed on August 18, 2018
12. ^ Anne Gellinek , Janin Renner, Kay Siering: Die Wolkenschieber ( Memento from March 5, 2011 in the Internet Archive ).
13. SPIEGEL ONLINE: Olympic weather: China shoots rain clouds