Silver chloride is an inorganic chemical compound with the chemical formula SilverChlorine. This white solid is well known for its low solubility in water and its Photosensitivity. Upon illumination or heating, silver chloride converts to silver (and chlorine), which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as the mineral chlorargyrite.
It is produced by a metathesis reaction for use in photography and in as electrodes.
Preparation
Silver chloride is unusual in that, unlike most
chloride salts, it has very low solubility. It is easily synthesized by metathesis: combining an
aqueous solution of
silver nitrate (which is soluble) with a soluble chloride salt, such as
sodium chloride (which is used industrially as a method of producing AgCl), or cobalt(II) chloride. The silver chloride that forms will precipitate immediately.
It can also be produced by the reaction of silver metal and
aqua regia; however, the insolubility of silver chloride decelerates the reaction. Silver chloride is also a
by-product of the
Miller process, where silver metal is reacted with chlorine gas at elevated temperatures.
History
Silver chloride has been known since ancient times. Ancient Egyptians produced it as a method of refining silver, which was done by roasting silver ores with salt to produce silver chloride, which was subsequently decomposed to silver and chlorine.
However, it was later identified as a distinct compound of silver in 1565 by
Georg Fabricius.
[Potonniée, Georges (1973). The history of the discovery of photography. Arno Press. p. 50. ] Silver chloride, historically known as
luna cornea (which could be translated as "horn silver" as the moon was an alchemic codename for silver),
has also been an intermediate in other historical silver refining processes. One such example is the
Augustin process developed in 1843, wherein copper ore containing small amounts of silver is roasted in chloridizing conditions and the silver chloride produced is leached by
brine, where it is more soluble.
Silver-based photographic films were first made in 1727 by Johann Heinrich Schulze with silver nitrate. However, he was not successful in making permanent images, as they faded away.[ Niépce House Museum: Invention of Photography: 1816-1818, Niépce's first tries (retrieved 2024-02-23)]
Structure
The solid adopts the
fcc Sodium chloride structure, in which each Ag
+ ion is surrounded by an
octahedron of six chloride ligands. AgF and
Silver bromide crystallize similarly.
[Wells, A.F. (1984) Structural Inorganic Chemistry, Oxford: Clarendon Press. . p. 349] However, the crystallography depends on the condition of crystallization, primarily free silver ion concentration, as is shown in the picture to the left (greyish tint and metallic lustre are due to partially
redox silver).
Above 7.5 gigapascal, silver chloride transitions into a monoclinic KOH phase. Then at 11 GPa, it undergoes another phase change to an orthorhombic TlI phase.
Reactions
AgCl dissolves in solutions containing
such as
chloride,
cyanide, triphenylphosphine,
thiosulfate,
thiocyanate and
ammonia. Silver chloride reacts with these ligands according to the following illustrative equations:
Of these reactions used to leach silver chloride from silver ores, cyanidation is the most commonly used. Cyanidation produces the soluble dicyanoargentate complex, which is later turned back to silver by reduction.
Silver chloride does not react with nitric acid, but instead reacts with sulfuric acid to produce silver sulfate.
Most complexes derived from AgCl are two-, three-, and, in rare cases, four-coordinate, adopting linear, trigonal planar, and tetrahedral coordination geometries, respectively.
These two reactions are particularly important in the qualitative analysis of AgCl in labs as AgCl is white, which changes to (silver arsenite) which is yellow, or (silver arsenate) which is reddish brown.S.M. Godfrey (1998). 075140389X, Blackie Academic and Professional. 075140389X
Chemistry
In one of the most famous reactions in chemistry, the addition of colorless aqueous
silver nitrate to an equally colorless solution of sodium chloride produces an opaque white precipitate of AgCl:
This conversion is a common test for the presence of
chloride in solution. Due to its conspicuousness, it is easily used in titration, which gives the typical case of
argentometry.
The solubility product, Ksp, for AgCl in water is at room temperature, which indicates that only 1.9 mg (that is, ) of AgCl will dissolve per liter of water.
For AgBr and AgI, the Ksp values are 5.2 x 10−13 and 8.3 x 10−17, respectively. Silver bromide (slightly yellowish white) and silver iodide (bright yellow) are also significantly more photosensitive than is AgCl.
AgCl quickly darkens on exposure to light by disintegrating into elemental chlorine and metallic silver. This reaction is used in photography and film and is the following:
- Cl− + hν → Cl + e− (excitation of the chloride ion, which gives up its extra electron into the conduction band)
- Ag+ + e− → Ag (liberation of a silver ion, which gains an electron to become a silver atom)
The process is not reversible because the silver atom liberated is typically found at a
crystal defect or an impurity site so that the electron's energy is lowered enough that it is "trapped".
Uses
Silver chloride electrode
Silver chloride is a constituent of the silver chloride electrode which is a common reference electrode in
electrochemistry. The electrode functions as a reversible
redox electrode and the equilibrium is between the solid
silver metal and silver chloride in a chloride solution of a given concentration. It is usually the internal reference electrode in
and it is often used as a reference in reduction potential measurements. As an example of the latter, the silver chloride electrode is the most commonly used reference electrode for testing cathodic protection
corrosion control systems in
seawater environments.
[Bates, R.G. and MacAskill, J.B. (1978). "Standard potential of the silver-silver chloride electrode". Pure & Applied Chemistry, Vol. 50, pp. 1701–1706, http://www.iupac.org/publications/pac/1978/pdf/5011x1701.pdf]
Photography
Silver chloride and silver nitrate have been used in
photography since it began, and are well known for their light sensitivity.
It was also a vital part of the
Daguerreotype sensitization where silver plates were fumed with chlorine to produce a thin layer of silver chloride.
Another famous process that used silver chloride was the gelatin silver process where embedded silver chloride crystals in
gelatin were used to produce images.
However, with advances in color photography, these methods of black-and-white photography have dwindled. Even though color photography uses silver chloride, it only works as a mediator for transforming light into organic image dyes.
Other photographic uses include making photographic paper, since it reacts with photons to form latent image via photoreduction; and in photochromic lenses, taking advantage of its reversible conversion to Ag metal. Unlike photography, where the photoreduction is irreversible, the glass prevents the electron from being 'trapped'.
Antimicrobial agent
Silver chloride nanoparticles are widely sold commercially as an
antimicrobial agent.
The antimicrobial activity of silver chloride depends on the particle size, but are usually below 100
nanometer. In general, silver chloride is antimicrobial against various
bacteria, such as E. coli.
Silver chloride nanoparticles for use as a microbial agent can be produced by a metathesis reaction between aqueous silver and chloride ions or can be biogenically synthesized by fungi and plants.
Other uses
Silver chloride's low solubility makes it a useful addition to pottery glazes for the production of "Inglaze lustre".
Silver chloride has been used as an antidote for mercury poisoning, assisting in the elimination of mercury.
Other uses of AgCl include:
-
in and wound healing products,
-
to create yellow, amber, and brown shades in stained glass manufacture,
-
as an infrared transmissive optical component, as it can be hot-pressed into window and lens shapes.
Natural occurrence
Silver chloride occurs naturally as
chlorargyrite in the arid and oxidized zones in silver deposits. If some of the chloride ions are replaced by bromide or iodide ions, the words bromian and iodian are added before the name, respectively.
This mineral is a source of silver and is leached by cyanidation, where it will produce the soluble Ag(CN)
2– complex.
Safety
According to the ECHA, silver chloride may damage the unborn child, is very toxic to aquatic life with long lasting effects and may be
Corrosion to metals.
See also
