Silver halide

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A silver halide (or silver salt) is one of the chemical compounds that can form between the element silver (Ag) and one of the halogens. In particular, bromine (Br), chlorine (Cl), iodine (I) and fluorine (F) may each combine with silver to produce silver bromide (AgBr), silver chloride (AgCl), silver iodide (AgI), and four forms of silver fluoride, respectively.

Contents

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 states 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.

Silver halides are light-sensitive chemicals, and are commonly used in photographic film and paper.

Applications

Light sensitivity

Silver halides are used in photographic film and photographic paper, including 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. The 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 a silver halide crystal is exposed to light, a sensitivity speck on the surface of the crystal is turned into a 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.

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 some historical processes such as the collodion wet plate and daguerreotype, in which the iodide is sometimes used alone (generally regarded as necessary if a daguerreotype is to be developed by the Becquerel method, in which exposure to strong red light, which affects only the crystals bearing latent image specks, is substituted for exposure to mercury fumes). Silver fluoride is not used in photography.

When absorbed by an AgX crystal, photons 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 a silver metal speck. [1]

Silver halides are also used to make corrective lenses darken when exposed to ultraviolet light (see photochromism).

Chemistry

The three common silver halide precipitates: AgI, AgBr & AgCl (left to right) Common Silver Halide Precipitates.jpg
The three common silver halide precipitates: AgI, AgBr & AgCl (left to right)

Silver halides, except for silver fluoride, are very insoluble in water. Silver nitrate can be used to precipitate halides; this application is useful in quantitative analysis of halides. [2] 689-703 The three main silver halide compounds have distinctive colours that can be used to quickly identify halide ions in a solution. The silver chloride compound forms a white precipitate, silver bromide a creamy coloured precipitate and silver iodide a yellow coloured precipitate.

Some compounds can considerably increase or decrease the solubility of AgX. [3] 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.

Archival use

Silver halide can be used to deposit fine details of metallic silver on surfaces, such as film. Because of the chemical stability of metallic silver, this film can be used for archival purposes. For example, the Arctic World Archive uses film developed with silver halides [4] to store data of historical and cultural interest, such as a snapshot of the Open Source code in all active GitHub repositories as of 2020.

Related Research Articles

<span class="mw-page-title-main">Halogen</span> Group of chemical elements

The halogens are a group in the periodic table consisting of six chemically related elements: fluorine (F), chlorine (Cl), bromine (Br), iodine (I), and the radioactive elements astatine (At) and tennessine (Ts), though some authors would exclude tennessine as its chemistry is unknown and is theoretically expected to be more like that of gallium. In the modern IUPAC nomenclature, this group is known as group 17.

Photographic processing or photographic development is the chemical means by which photographic film or paper is treated after photographic exposure to produce a negative or positive image. Photographic processing transforms the latent image into a visible image, makes this permanent and renders it insensitive to light.

<span class="mw-page-title-main">Photographic paper</span> Light-sensitive paper used to make photographic prints

Photographic paper is a paper coated with a light-sensitive chemical formula, like photographic film, used for making photographic prints. When photographic paper is exposed to light, it captures a latent image that is then developed to form a visible image; with most papers the image density from exposure can be sufficient to not require further development, aside from fixing and clearing, though latent exposure is also usually present. The light-sensitive layer of the paper is called the emulsion. The most common chemistry was based on silver halide but other alternatives have also been used.

<span class="mw-page-title-main">Silver nitrate</span> Chemical compound

Silver nitrate is an inorganic compound with chemical formula AgNO
3
. It 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. It was once called lunar caustic because silver was called luna by ancient alchemists who associated silver with the moon. In solid silver nitrate, the silver ions are three-coordinated in a trigonal planar arrangement.

In chemistry, a halide is a binary chemical 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, astatide, or theoretically tennesside compound. The alkali metals combine directly with halogens under appropriate conditions forming halides of the general formula, MX. Many salts are halides; the hal- syllable in halide and halite reflects this correlation. All Group 1 metals form halides that are white solids at room temperature.

<span class="mw-page-title-main">Photographic developer</span> Chemical(s) which convert a latent image on photographic film to a visible image

In the processing of photographic films, plates or papers, the photographic developer is one or more chemicals that convert the latent image to a visible image. Developing agents achieve this conversion by reducing the silver halides, which are pale-colored, into silver metal, which is black when in the form of fine particles. The conversion occurs within the gelatine matrix. The special feature of photography is that the developer acts more quickly on those particles of silver halide that have been exposed to light. When left in developer, all the silver halides will eventually be reduced and turn black. Generally, the longer a developer is allowed to work, the darker the image.

An iodide ion is the ion I. Compounds with iodine in formal oxidation state −1 are called iodides. In everyday life, iodide is most commonly encountered as a component of iodized salt, which many governments mandate. Worldwide, iodine deficiency affects two billion people and is the leading preventable cause of intellectual disability.

<span class="mw-page-title-main">Gelatin silver process</span> Photographic process

The gelatin silver process is the most commonly used chemical process in black-and-white photography, and is the fundamental chemical process for modern analog color photography. As such, films and printing papers available for analog photography rarely rely on any other chemical process to record an image. A suspension of silver salts in gelatin is coated onto a support such as glass, flexible plastic or film, baryta paper, or resin-coated paper. These light-sensitive materials are stable under normal keeping conditions and are able to be exposed and processed even many years after their manufacture. The "dry plate" gelatin process was an improvement on the collodion wet-plate process dominant from the 1850s–1880s, which had to be exposed and developed immediately after coating.

Photographic fixer is a mix of chemicals used in the final step in the photographic processing of film or paper. The fixer stabilises the image, removing the unexposed silver halide remaining on the photographic film or photographic paper, leaving behind the reduced metallic silver that forms the image. By fixation, the film or paper is insensitive to further action by light. Without fixing, the remaining silver halide would darken and cause fogging of the image. Fixation is commonly achieved by treating the film or paper with a solution of thiosulfate salt. Popular salts are sodium thiosulfate—commonly called hypo—and ammonium thiosulfate—commonly used in modern rapid fixer formulae. Fixation involves these chemical reactions (X = halide, typically Br):

<span class="mw-page-title-main">Silver bromide</span> Chemical compound

Silver bromide (AgBr) is 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 the Shroud of Turin. The salt can be found naturally as the mineral bromargyrite.

<span class="mw-page-title-main">Silver chloride</span> Chemical compound with the formula AgCl

Silver chloride is an inorganic chemical compound with the chemical formula AgCl. This white crystalline solid is well known for its low solubility in water and its sensitivity to light. Upon illumination or heating, silver chloride converts to silver, which is signaled by grey to black or purplish coloration in some samples. AgCl occurs naturally as the mineral chlorargyrite.

Pseudohalogens are polyatomic analogues of halogens, whose chemistry, resembling that of the true halogens, allows them to substitute for halogens in several classes of chemical compounds. Pseudohalogens occur in pseudohalogen molecules, inorganic molecules of the general forms PsPs or Ps–X, such as cyanogen; pseudohalide anions, such as cyanide ion; inorganic acids, such as hydrogen cyanide; as ligands in coordination complexes, such as ferricyanide; and as functional groups in organic molecules, such as the nitrile group. Well-known pseudohalogen functional groups include cyanide, cyanate, thiocyanate, and azide.

Classical qualitative inorganic analysis is a method of analytical chemistry which seeks to find the elemental composition of inorganic compounds. It is mainly focused on detecting ions in an aqueous solution, therefore materials in other forms may need to be brought to this state before using standard methods. The solution is then treated with various reagents to test for reactions characteristic of certain ions, which may cause color change, precipitation and other visible changes.

<span class="mw-page-title-main">Latent image</span> An invisible image produced by the exposure of a photosensitive material to light.

A latent image is an invisible image produced by the exposure to light of a photosensitive material such as photographic film. When photographic film is developed, the area that was exposed darkens and forms a visible image. In the early days of photography, the nature of the invisible change in the silver halide crystals of the film's emulsion coating was unknown, so the image was said to be "latent" until the film was treated with photographic developer.

Ionic radius, rion, is the radius of a monatomic ion in an ionic crystal structure. Although neither atoms nor ions have sharp boundaries, they are treated as if they were hard spheres with radii such that the sum of ionic radii of the cation and anion gives the distance between the ions in a crystal lattice. Ionic radii are typically given in units of either picometers (pm) or angstroms (Å), with 1 Å = 100 pm. Typical values range from 31 pm (0.3 Å) to over 200 pm (2 Å).

<span class="mw-page-title-main">Mercury(II) thiocyanate</span> Chemical compound

Mercury(II) thiocyanate (Hg(SCN)2) is an inorganic chemical compound, the coordination complex of Hg2+ and the thiocyanate anion. It is a white powder. It will produce a large, winding "snake" when ignited, an effect known as the Pharaoh's serpent.

<span class="mw-page-title-main">Silver compounds</span> Chemical compounds containing silver

Silver is a relatively unreactive metal, although it can form several compounds. The common oxidation states of silver are (in order of commonness): +1 (the most stable state; for example, silver nitrate, AgNO3); +2 (highly oxidising; for example, silver(II) fluoride, AgF2); and even very rarely +3 (extreme oxidising; for example, potassium tetrafluoroargentate(III), KAgF4). The +3 state requires very strong oxidising agents to attain, such as fluorine or peroxodisulfate, and some silver(III) compounds react with atmospheric moisture and attack glass. Indeed, silver(III) fluoride is usually obtained by reacting silver or silver monofluoride with the strongest known oxidizing agent, krypton difluoride.

The thallium halides include monohalides, where thallium has oxidation state +1, trihalides in which thallium generally has oxidation state +3, and some intermediate halides containing thallium with mixed +1 and +3 oxidation states. These salts find use in specialized optical settings, such as focusing elements in research spectrophotometers. Compared to the more common zinc selenide-based optics, materials such as thallium bromoiodide enable transmission at longer wavelengths. In the infrared, this allows for measurements as low as 350 cm−1 (28 μm), whereas zinc selenide is opaque by 21.5 μm, and ZnSe optics are generally only usable to 650 cm−1 (15 μm).

Photographic emulsion is a light-sensitive colloid used in film-based photography. Most commonly, in silver-gelatin photography, it consists of silver halide crystals dispersed in gelatin. The emulsion is usually coated onto a substrate of glass, films, paper, or fabric. The substrate is often flexible and known as a film base.

<span class="mw-page-title-main">Lead compounds</span> Type of compound

Compounds of lead exist with lead in two main oxidation states: +2 and +4. The former is more common. Inorganic lead(IV) compounds are typically strong oxidants or exist only in highly acidic solutions.

References

  1. Myers, Dr. Drew. "Chemistry of Photography". Cheresources.com. GlobalSpec. Retrieved January 25, 2009.
  2. Burgot, Jean-Louis (2012), "Titrimetric Methods Involving a Precipitation", Ionic Equilibria in Analytical Chemistry, New York, NY: Springer New York, pp. 689–703, doi:10.1007/978-1-4419-8382-4_37, ISBN   978-1-4419-8381-7 , retrieved December 4, 2023
  3. Borresen, H. C. (July 1, 1963). "An Improvement of the Amperometric Titration of Thiols at the Rotating Platinum Electrode". Analytical Chemistry. 35 (8): 1096–1097. doi:10.1021/ac60201a056. ISSN   0003-2700.
  4. Sabliński, Jędrzej; Trujillo, Alfredo (2021). "Piql. Long-term preservation technology study". Archeion. 122: 13–32. doi: 10.4467/26581264ARC.21.011.14491 .