Silver halide
Encyclopedia
A silver halide is one of the compounds
formed between silver
and one of the halogen
s — silver bromide
(AgBr), chloride
(AgCl), iodide
(AgI), and three forms of silver fluoride
s. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation state
s of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride
is the only known stable one.
The light-sensitive chemicals used in photographic film and paper are silver halides.
and photographic paper
, as well as graphic art film and paper, where silver halide crystals in gelatin
are coated on to a film base
, glass or paper substrate. The gelatin is a vital part of the emulsion as the protective colloid of appropriate physical and chemical properties. Gelatin may also contain trace elements (such as sulfur
) which increase the light sensitivity of the emulsion
, although modern practice uses gelatin without such components. When absorbed by an AgX crystal, photon
s cause electrons to be promoted to a conduction band
(de-localized electron orbital
with higher energy than a valence band
) which can be attracted by a sensitivity speck
, which is a shallow electron trap, which may be a crystalline defect or a cluster of silver sulfide, gold, other trace elements (dopant
), or combination thereof, and then combined with an interstitial silver ion to form silver metal speck.
Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride, except in the case of daguerreotype
production where a daguerreotype (one of the oldest photographic processes) is developed with pure red light instead of mercury vapors (a method known as the Bequerelle method, named for the inventor who discovered the phenomenon). Silver fluoride is not used in photography.
Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism
).
When a silver halide crystal is exposed to light, a sensitivity speck
on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image
). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light (reflected from a subject being photographed, for example) undergo the greatest development and therefore results in the highest optical density.
can be used to precipitate halide
s; this application is useful in quantitative analysis of halides. The precipitation of silver halides via silver nitrate is also useful for abstracting halide leaving group
s.
However, close attention is necessary for other compounds in the test solution. Some compounds can considerably increase or decrease the solubility of AgX. Examples of compounds that increase the solubility include: cyanide, thiocyanate, thiosulfate, thiourea, amines, ammonia, sulfite, thioether, crown ether. Examples of compounds that reduces the solubility include many organic thiols and nitrogen compounds that do not possess solubilizing group other than mercapto group or the nitrogen site, such as mercaptooxazoles, mercaptotetrazoles, especially 1-phenyl-5-mercaptotetrazole, benzimidazoles, especially 2-mercaptobenzimidazole, benzotriazole, and these compounds further substituted by hydrophobic groups. Compounds such as thiocyanate and thiosulfate enhance solubility when they are present in a sufficiently large quantity, due to formation of highly soluble complex ions, but they also significantly depress solubility when present in a very small quantity, due to formation of sparingly soluble complex ions.
s for transmitting mid-infrared light from carbon dioxide laser
s. The fibers allow laser welding of human tissue, as an alternative to traditional sutures.
Chemical compound
A chemical compound is a pure chemical substance consisting of two or more different chemical elements that can be separated into simpler substances by chemical reactions. Chemical compounds have a unique and defined chemical structure; they consist of a fixed ratio of atoms that are held together...
formed between silver
Silver
Silver is a metallic chemical element with the chemical symbol Ag and atomic number 47. A soft, white, lustrous transition metal, it has the highest electrical conductivity of any element and the highest thermal conductivity of any metal...
and one of the halogen
Halogen
The halogens or halogen elements are a series of nonmetal elements from Group 17 IUPAC Style of the periodic table, comprising fluorine , chlorine , bromine , iodine , and astatine...
s — silver bromide
Silver bromide
Silver bromide , a soft, pale-yellow, water insoluble salt well known for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. AgBr is widely used in photographic films and is believed by some to have been used for making...
(AgBr), chloride
Silver chloride
Silver chloride is a chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water . Upon illumination or heating, silver chloride converts to silver , which is signalled by greyish or purplish coloration to some samples...
(AgCl), iodide
Silver iodide
Silver iodide is a yellow, inorganic, photosensitive iodide of silver used in photography, in medicine as an antiseptic, and in rainmaking for cloud seeding.-Crystal structure:...
(AgI), and three forms of silver fluoride
Silver fluoride
Silver fluoride can refer to:* Silver fluoride , AgF* Silver difluoride , AgF2* Silver subfluoride , Ag2F...
s. As a group, they are often referred to as the silver halides, and are often given the pseudo-chemical notation AgX. Although most silver halides involve silver atoms with oxidation state
Oxidation state
In chemistry, the oxidation state is an indicator of the degree of oxidation of an atom in a chemical compound. The formal oxidation state is the hypothetical charge that an atom would have if all bonds to atoms of different elements were 100% ionic. Oxidation states are typically represented by...
s of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of which silver(II) fluoride
Silver(II) fluoride
Silver fluoride is a chemical compound with the formula AgF2. It is a rare example of a silver compound. Silver is usually present in its +1 oxidation state. It is used as a fluorinating agent.-Preparation:...
is the only known stable one.
The light-sensitive chemicals used in photographic film and paper are silver halides.
Light sensitivity
Silver halides are used in photographic filmPhotographic film
Photographic film is a sheet of plastic coated with an emulsion containing light-sensitive silver halide salts with variable crystal sizes that determine the sensitivity, contrast and resolution of the film...
and photographic paper
Photographic paper
Photographic paper is paper coated with light-sensitive chemicals, used for making photographic prints.Photographic paper is exposed to light in a controlled manner, either by placing a negative in contact with the paper directly to produce a contact print, by using an enlarger in order to create a...
, as well as graphic art film and paper, where silver halide crystals in gelatin
Gelatin
Gelatin is a translucent, colorless, brittle , flavorless solid substance, derived from the collagen inside animals' skin and bones. It is commonly used as a gelling agent in food, pharmaceuticals, photography, and cosmetic manufacturing. Substances containing gelatin or functioning in a similar...
are coated on to a film base
Film base
A film base is a transparent substrate which acts as a support medium for the photosensitive emulsion that lies atop it. Despite the numerous layers and coatings associated with the emulsion layer, the base generally accounts for the vast majority of the thickness of any given film stock...
, glass or paper substrate. The gelatin is a vital part of the emulsion as the protective colloid of appropriate physical and chemical properties. Gelatin may also contain trace elements (such as sulfur
Sulfur
Sulfur or sulphur is the chemical element with atomic number 16. In the periodic table it is represented by the symbol S. It is an abundant, multivalent non-metal. Under normal conditions, sulfur atoms form cyclic octatomic molecules with chemical formula S8. Elemental sulfur is a bright yellow...
) which increase the light sensitivity of the emulsion
Emulsion
An emulsion is a mixture of two or more liquids that are normally immiscible . Emulsions are part of a more general class of two-phase systems of matter called colloids. Although the terms colloid and emulsion are sometimes used interchangeably, emulsion is used when both the dispersed and the...
, although modern practice uses gelatin without such components. When absorbed by an AgX crystal, photon
Photon
In physics, a photon is an elementary particle, the quantum of the electromagnetic interaction and the basic unit of light and all other forms of electromagnetic radiation. It is also the force carrier for the electromagnetic force...
s cause electrons to be promoted to a conduction band
Conduction band
In the solid-state physics field of semiconductors and insulators, the conduction band is the range of electron energies, higher than that of the valence band, sufficient to free an electron from binding with its individual atom and allow it to move freely within the atomic lattice of the material...
(de-localized electron orbital
Atomic orbital
An atomic orbital is a mathematical function that describes the wave-like behavior of either one electron or a pair of electrons in an atom. This function can be used to calculate the probability of finding any electron of an atom in any specific region around the atom's nucleus...
with higher energy than a valence band
Valence band
In solids, the valence band is the highest range of electron energies in which electrons are normally present at absolute zero temperature....
) which can be attracted by a sensitivity speck
Sensitivity speck
A sensitivity speck is a place in silver halide crystal where latent image is preferentially formed. This is very often the site of shallow electron traps, such as crystalline defect and silver sulfide specks created by sulfur sensitization process.When a photon is absorbed by a silver halide...
, which is a shallow electron trap, which may be a crystalline defect or a cluster of silver sulfide, gold, other trace elements (dopant
Dopant
A dopant, also called a doping agent, is a trace impurity element that is inserted into a substance in order to alter the electrical properties or the optical properties of the substance. In the case of crystalline substances, the atoms of the dopant very commonly take the place of elements that...
), or combination thereof, and then combined with an interstitial silver ion to form silver metal speck.
Silver bromide and silver chloride may be used separately or combined, depending on the sensitivity and tonal qualities desired in the product. Silver iodide is always combined with silver bromide or silver chloride, except in the case of daguerreotype
Daguerreotype
The daguerreotype was the first commercially successful photographic process. The image is a direct positive made in the camera on a silvered copper plate....
production where a daguerreotype (one of the oldest photographic processes) is developed with pure red light instead of mercury vapors (a method known as the Bequerelle method, named for the inventor who discovered the phenomenon). Silver fluoride is not used in photography.
Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism
Photochromism
Photochromism is the reversible transformation of a chemical species between two forms by the absorption of electromagnetic radiation, where the two forms have different absorption spectra. Trivially, this can be described as a reversible change of colour upon exposure to light...
).
When a silver halide crystal is exposed to light, a sensitivity speck
Sensitivity speck
A sensitivity speck is a place in silver halide crystal where latent image is preferentially formed. This is very often the site of shallow electron traps, such as crystalline defect and silver sulfide specks created by sulfur sensitization process.When a photon is absorbed by a silver halide...
on the surface of the crystal is turned into a small speck of metallic silver (these comprise the invisible or latent image
Latent image
A latent image on photographic film is an invisible image produced by the exposure of the film to light. When the film is developed, the area that was exposed darkens and forms a visible image...
). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergo development which turns the entire crystal into metallic silver. Areas of the emulsion receiving larger amounts of light (reflected from a subject being photographed, for example) undergo the greatest development and therefore results in the highest optical density.
Chemistry
Silver halides, except for silver fluoride, are extremely insoluble in water. Silver nitrateSilver nitrate
Silver nitrate is an inorganic compound with chemical formula . This compound is a versatile precursor to many other silver compounds, such as those used in photography. It is far less sensitive to light than the halides...
can be used to precipitate halide
Halide
A halide is a binary compound, of which one part is a halogen atom and the other part is an element or radical that is less electronegative than the halogen, to make a fluoride, chloride, bromide, iodide, or astatide compound. Many salts are halides...
s; this application is useful in quantitative analysis of halides. The precipitation of silver halides via silver nitrate is also useful for abstracting halide leaving group
Leaving group
In chemistry, a leaving group is a molecular fragment that departs with a pair of electrons in heterolytic bond cleavage. Leaving groups can be anions or neutral molecules. Common anionic leaving groups are halides such as Cl−, Br−, and I−, and sulfonate esters, such as para-toluenesulfonate...
s.
However, close attention is necessary for other compounds in the test solution. Some compounds can considerably increase or decrease the solubility of AgX. Examples of compounds that increase the solubility include: cyanide, thiocyanate, thiosulfate, thiourea, amines, ammonia, sulfite, thioether, crown ether. Examples of compounds that reduces the solubility include many organic thiols and nitrogen compounds that do not possess solubilizing group other than mercapto group or the nitrogen site, such as mercaptooxazoles, mercaptotetrazoles, especially 1-phenyl-5-mercaptotetrazole, benzimidazoles, especially 2-mercaptobenzimidazole, benzotriazole, and these compounds further substituted by hydrophobic groups. Compounds such as thiocyanate and thiosulfate enhance solubility when they are present in a sufficiently large quantity, due to formation of highly soluble complex ions, but they also significantly depress solubility when present in a very small quantity, due to formation of sparingly soluble complex ions.
Medical technology
Scientists from Tel Aviv University are experimenting with silver halide optical fiberOptical fiber
An optical fiber is a flexible, transparent fiber made of a pure glass not much wider than a human hair. It functions as a waveguide, or "light pipe", to transmit light between the two ends of the fiber. The field of applied science and engineering concerned with the design and application of...
s for transmitting mid-infrared light from carbon dioxide laser
Carbon dioxide laser
The carbon dioxide laser was one of the earliest gas lasers to be developed , and is still one of the most useful. Carbon dioxide lasers are the highest-power continuous wave lasers that are currently available...
s. The fibers allow laser welding of human tissue, as an alternative to traditional sutures.