Silver iodide

• Iodsilber
• Jodargyrit ( mineral )

Yellow, odorless powder

Fixed

5.67 g · cm -3

552 ° C

Insoluble in water (30 ug · l -1)

Attention

0.01 mg · m-3 (based on the inhalable fraction )

-61.8 KJ / mol

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Silver (also of silver iodide ) is a chemical compound of silver and iodine. It is a yellowish, water-insoluble salt.

Occurrence

Of course silver iodide occurs as mineral Jodargyrit.

Production and representation

Silver iodide is recovered by precipitation of a silver nitrate solution using potassium iodide.

This reaction is also used in chemical analysis as proof of iodide ions, because the resulting AgI forms a sparingly soluble yellow precipitate. In contrast to the silver iodide also insoluble silver chloride ( AgCl) and silver can ( AgBr ) in sodium thiosulfate solve ( complex formation reaction). Chloride, bromide and iodide ions can be distinguished by means of sodium thiosulfate and ammonia solution from each other.

Properties

Physical Properties

From silver iodide several modifications are known. At room temperature, the β -AgI is thermodynamically stable, which crystallizes in the wurtzite structure. In addition, there is a metastable form, the γ -AgI, which has a zinc blende structure.

Above about 147 ° C, the α -AgI stable, which is part of the solid ion conductors due to its high silver ion conductivity. Its ionic conductivity is in the order of 1 to 2 S / cm, which is comparable to that of liquid electrolytes. The α -AgI has a cubic body- centered iodide sublattice and a structurally disordered silver ions sublattice. The silver ions are free to move between the larger iodide ion, therefore. By inclusion of rubidium ions to Rubidiumsilberiodid ( Ag4RbI5 ) the temperature of the α / β - phase transition can be reduced to below room temperature. Thereby, the area of the ion conduction is extended up to room temperature.

The electronic conductivity of α -AgI is based on electron-hole direction and is proportional to the partial pressure of I2. It is smaller compared to the conductivity based on the silver ions by about a factor of 1010. This makes silver iodide as a solid electrolyte especially suitable.

Chemical Properties

Silver is light sensitive and decomposes slowly in the elements. In sunlight it turns green - gray. AgI dissolves in strong complexing agents such as cyanides or thiocyanates.

Use

Silver iodide is mixed with acetone sprayed from planes hail to produce in the atmosphere smallest condensation nuclei for targeted rain or hail formation.

• On one hand, it serves to prevent the adverse storms to prevent or mitigate. It can thus be prevented the formation of large hailstones. In the U.S., was tried in the 1940s and 1950s to weaken prematurely with silver iodide hurricanes, the effect was limited. In Germany, was established in 1958 organized a hail suppression in the district of Rosenheim, who scored the silver iodide from over 100 launch sites by rockets into the clouds. Since 1975, this task is handled by two anti- hail aircraft. In southern Germany, Austria and Switzerland there are more organized as a club hail weirs.
• On the other hand, thereby attempting to provide specific areas targeted with Precipitation: By seeding the clouds with silver iodide fine dust in the ascendant channel of a cloud front of an aircraft is since the 1980s ( in 1986 at Chernobyl to prevent radioactively contaminated clouds) and in the Midwestern USA Russia also tried a test basis in Bavaria, to let the clouds rain down at a defined location targeted. The effectiveness of this method is indeed statistically analyzed, but the success is low ( about 10 % more precipitation ). The silver iodide is analytically detectable in by fallen snow in the smallest quantities. These amounts are not dangerous to humans.
• With the conflicting target a specific area will be rain kept free by letting fall the rain about to individual appointments. So there is in Moscow on 9 May, Victory Day, and on June 12, the day Russia, sunshine. At the Olympic Summer Games in Beijing 2008 silver iodide was introduced with the help of missiles in rain clouds to prevent disturbance of the opening ceremonies.

In the early days of photography in the 19th century it was used because of its sensitivity for different printing processes such as precious talbotype, calotype and Argyrotypie. Later it was replaced by more suitable substances such as silver.