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Silver iodide (AgI): structure, properties, production, uses

It is a light yellow crystalline solid that darkens when exposed to light for a long time. It is almost insoluble in water, but dissolves in the presence of a high concentration of iodide ion (I  ).

Silver iodide. Leiem / CC BY-SA (https://creativecommons.org/licenses/by-sa/4.0). Source: Wikimedia Commons.

Because it has a crystalline structure similar to ice, it has been used as a seed to produce rain and change the climate. This use has been questioned due to the potential damage that AgI can cause when dissolved in water.

Since the 19th century it has been used in photography for its ability to darken with light. It is also used in antimicrobial therapies.

It is a toxic compound for humans, animals and plants .

Structure

It is an ionic compound formed by silver in its +1 oxidation state and iodine with -1 valence. The bond between the two ions is very strong and stable.

Structure of silver iodide. Blue = silver; violet = iodine. Benjah-bmm27 / Public domain. Source: Wikimedia Commons.

Its crystalline structure depends on the temperature. Below 137 ° C it is in the cubic or gamma form (γ-AgI), between 137 and 145.8 ° C it is a greenish-yellow solid or beta form (β-AgI) and above 145.8 ° C It has a yellow color and is its alpha form (α-AgI).

Nomenclature

  • Silver iodide

Properties

Physical state

Light yellow solid, hexagonal or cubic crystals.

Molecular weight

234.773 g / mol

Melting point

558 ºC

Boiling point

1506 ºC

Density

5.68 g / cm 3

Solubility

Practically insoluble in water: 28 × 10-7 g / L at 25 ° C (0.0000028 g / L). Insoluble in acids except hydroiodic acid (solution of hydrogen iodide in water). Soluble in concentrated solutions of alkali bromides and alkaline chlorides.

Chemical properties

Acids concentrated at high temperatures (boiling) attack it slowly. However, hot alkali hydroxide solutions do not affect it.

It is sensitive to light, it darkens slowly as it forms metallic silver.

Obtaining

In nature it is found in the form of the mineral iodargyrite, which is the β-AgI form.

Yodargyrite, mineral of AgI. Rob Lavinsky, iRocks.com – CC-BY-SA-3.0 / CC BY-SA (https://creativecommons.org/licenses/by-sa/3.0). Source: Wikimedia Commons.

In the laboratory it can be prepared by heating a solution of silver nitrate (AgNO 3 ) with a solution of an alkaline iodide, such as potassium iodide (KI). AgI precipitates and is washed in the absence of light with boiling water.

AgNO 3 + KI → AgI ↓ + KNO 3

Use in weather modification

It is applied to clouds to change the amount or type of precipitation, trigger hail processes, disperse cold fogs, and weaken hurricanes.

It disperses as a seed in cold clouds containing supercooled liquid water (temperatures below 0 ° C). Its crystalline structure similar to ice favors the freezing of supercooled water.

Silver iodide is sometimes used to change the weather. Author: Tobias Hämmer. Source: Pixabay.

Adverse effects of this use

After dispersal as a seed in the clouds, the AgI is found within the interior of the cloud and is washed away by precipitation.

The presence of soluble silver iodide in rainwater is something to take into account, as it is a toxic compound for aquatic, terrestrial and human plants and animals.

The AgI used to generate precipitation can be toxic to natural environments. Author: Antonios Ntoumas. Source: Pixabay.

Repetitive cloud seeding over the same area can lead to a cumulative effect of this compound. Agency Protection of the Environment, or EPA (of English Environmental Protection Agency ) considers the AgI a polluting water and soil.

According to studies carried out in 2013, the concentration of silver iodide found in areas where this technique has been used is much higher than the limit above which it is toxic to some fish and lower organisms.

Use in photography

AgI is a material capable of reacting in the presence of light, which is why it is used in obtaining photosensitive materials such as photographic rolls, on which its crystals are applied.

The size of these crystals, also called grains, is what defines the magnitude of photosensitivity. The larger the grain size, the greater the sensitivity to light and therefore less of it is required to capture the image.

Photographic paper has a thin layer of gelatin in which the AgI grains are suspended.

Mechanism of action

The crystalline structure of this compound is such that it allows a certain movement of electrons, so when a photon hits the crystal, an electron is released, which combines with a nearby silver ion to form metallic silver (Ag 0 ).

AgI crystal + photon → e 

Ag + + e  → Ag 0

The effect of photons on the photographic emulsion is to reduce the silver ion to metallic silver, forming the latent image, which is invisible to the naked eye, but has hidden differences in brightness of the captured scene.

This photograph was obtained in 1862 using AgI. Colecao Militao Augusto de Azevedo / Treatment of two negatives and slides do Servico de Documentacao Textual e Iconografia – L3 Conservacao de Acervos S / S Ltda / Reproducao digital – Um Certo Olhar Imagens e Editora Ltda. / 2011. Militão Augusto de Azevedo / Public domain. Source: Wikimedia Commons.

That is, some areas of the photographic roll have received photons and others have not. To prevent the emulsion from continuing to react, the material is protected from the action of light and then chemical compounds are added to fix the image and make it visible.

Silver metal will create the dark areas due to its color.

Although today we can quickly obtain photographic images with our smartphones, chemical photography processes are still a fundamental part of cinematographic film and X-ray film, among other applications.

Use in the removal of radioactive iodine

Due to its insolubility, AgI has been proposed in a mechanism to remove iodine or radioactive iodide contained in aqueous waste generated by nuclear power stations.

According to studies carried out in 2019, silver nanoparticles with zeolite have the ability to remove iodine from water. In the presence of water, the Ag nanoparticles contained in the zeolite oxidize forming Ag 2 O, then the Ag + ion is generated, which binds to the iodide and precipitates the AgI on the surface of the zeolite.

The formation of silver iodide can be used to reduce radioactive iodine contamination from nuclear waste. Author: Dirk Rabe. Source: Pixabay.

Other uses

It has been used to treat infections of the mucous membranes of animals in the form of colloidal suspensions with 5-49% by weight . In situations of inflammation of the eyes, ears and nose, it is applied in the form of an ointment or ointment at 5%.

AgI nanoparticles have been used as agents for antimicrobial therapies. In chemical and biochemical laboratories it is used as a reagent and serves as an intermediate in the preparation of other silver and iodine compounds.

It has been studied by physicists for having a liquid lattice-type electricity conduction mechanism. It is used in optical fibers for infrared lasers as it is transparent in the middle and infrared region of the light spectrum.

Risks

It is toxic to humans through all routes, such as dermal contact, inhalation, and ingestion. It causes skin rashes, conjunctivitis, grayish discoloration of the skin, conjunctiva and internal organs, headache, fever, laryngitis and bronchitis.

Interaction with copper compounds can increase the mutagenic potential of AgI.

It is a very toxic compound for aquatic and terrestrial life, both animals and plants. Its damaging effects can linger in the environment.

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