Silver bromide

Last updated
Silver Bromide
Bromid stribrny.PNG
Silver-bromide-3D-vdW.png
Names
Other names
bromargyrite
bromyrite
silver(I) bromide
Identifiers
3D model (JSmol)
ChemSpider
ECHA InfoCard 100.029.160 OOjs UI icon edit-ltr-progressive.svg
UNII
  • InChI=1S/Ag.BrH/h;1H/q+1;/p-1 Yes check.svgY
    Key: ADZWSOLPGZMUMY-UHFFFAOYSA-M Yes check.svgY
  • InChI=1/Ag.BrH/h;1H/q+1;/p-1
    Key: ADZWSOLPGZMUMY-REWHXWOFAK
  • [Ag]Br
Properties
AgBr
Molar mass 187.77 g/mol
AppearancePale yellow solid
photosensitive
Density 6.473 g/cm3, solid
Melting point 432 °C (810 °F; 705 K)
Boiling point 1,502 °C (2,736 °F; 1,775 K) (decomposes)
0.140 mg/L (20 °C)
5.4 × 10 −13
Solubility insoluble in alcohol, most acids
sparingly soluble in ammonia
soluble in alkali cyanide solutions
Band gap 2.5 eV
Electron mobility 4000 cm2/(V·s)
−59.7·10−6 cm3/mol
2.253
Thermochemistry
270 J/(kg·K)
Std molar
entropy (S⦵298)
107 J·mol−1·K−1 [1]
−100 kJ·mol−1 [1]
Hazards
NFPA 704 (fire diamond)
NFPA 704.svgHealth 0: Exposure under fire conditions would offer no hazard beyond that of ordinary combustible material. E.g. sodium chlorideFlammability 0: Will not burn. E.g. waterInstability 0: Normally stable, even under fire exposure conditions, and is not reactive with water. E.g. liquid nitrogenSpecial hazards (white): no code
0
0
0
Related compounds
Other anions
Silver(I) fluoride
Silver chloride
Silver iodide
Other cations
Copper(I) bromide
Mercury(I) bromide
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
Yes check.svgY  verify  (what is  Yes check.svgYX mark.svgN ?)

Silver bromide (AgBr), a soft, pale-yellow, water-insoluble salt well known (along with other silver halides) for its unusual sensitivity to light. This property has allowed silver halides to become the basis of modern photographic materials. [2] AgBr is widely used in photographic films and is believed by some to have been used for making the Shroud of Turin. [3] The salt can be found naturally as the mineral bromargyrite (bromyrite).

Contents

Preparation

Although the compound can be found in mineral form, AgBr is typically prepared by the reaction of silver nitrate with an alkali bromide, typically potassium bromide: [2]

AgNO3(aq) + KBr(aq) → AgBr(s)+ KNO3(aq)

Although less convenient, the salt can also be prepared directly from its elements.

Modern preparation of a simple, light-sensitive surface involves forming an emulsion of silver halide crystals in a gelatine, which is then coated onto a film or other support. The crystals are formed by precipitation in a controlled environment to produce small, uniform crystals (typically < 1 μm in diameter and containing ~1012 Ag atoms) called grains. [2]

Reactions

Silver bromide reacts readily with liquid ammonia to generate a variety of ammine complexes, like Ag(NH
3)
2Br and Ag(NH
3)
2Br−
2. In general: [4]

AgBr + m NH3 + (n - 1) Br−
 → Ag(NH
3)
mBr1-n
n


Silver bromide reacts with triphenylphosphine to give a tris(triphenylphosphine) product: [5]

AgBr triphenylphosphine.png

Physical properties

Crystal structure

AgF, AgCl, and AgBr all have face-centered cubic (fcc) rock-salt (NaCl) lattice structure with the following lattice parameters: [6]

Silver halide lattice properties
CompoundCrystalStructureLattice, a /Å
AgFfccrock-salt, NaCl4.936
AgCl, Chlorargyritefccrock-salt, NaCl5.5491
AgBr, Bromargyritefccrock-salt, NaCl5.7745
Unit cell structure
Lattice face centered cubic.svg Sodium chloride crystal.png
face-centered cubicrock-salt structure

The larger halide ions are arranged in a cubic close-packing, while the smaller silver ions fill the octahedral gaps between them, giving a 6-coordinate structure where a silver ion Ag+ is surrounded by 6 Br− ions, and vice versa. The coordination geometry for AgBr in the NaCl structure is unexpected for Ag(I) which typically forms linear, trigonal (3-coordinated Ag) or tetrahedral (4-coordinated Ag) complexes.

Unlike the other silver halides, iodargyrite (AgI) contains a hexagonal zincite lattice structure.

Solubility

The silver halides have a wide range of solubilities. The solubility of AgF is about 6 × 107 times that of AgI. These differences are attributed to the relative solvation enthalpies of the halide ions; the enthalpy of solvation of fluoride is anomalously large. [7]

Silver halide solubilities
CompoundSolubility (g / 100 g H2O)
AgF172
AgCl0.00019
AgBr0.000014
AgI0.000003

Photosensitivity

Although photographic processes have been in development since the mid-1800s, there were no suitable theoretical explanations until 1938 with the publication of a paper by R.W. Gurney and N.F. Mott. [8] This paper triggered a large amount of research in fields of solid-state chemistry and physics, as well more specifically in silver halide photosensitivity phenomena. [2]

Further research into this mechanism revealed that the photographic properties of silver halides (in particular AgBr) were a result of deviations from an ideal crystal structure. Factors such as crystal growth, impurities, and surface defects all affect concentrations of point ionic defects and electronic traps, which affect the sensitivity to light and allow for the formation of a latent image. [3]

Frenkel defects and quadropolar deformation

The major defect in silver halides is the Frenkel defect, where silver ions are located interstitially (Agi+) in high concentration with their corresponding negatively charged silver-ion vacancies (Agv−). What is unique about AgBr Frenkel pairs is that the interstitial Agi+ are exceptionally mobile, and that its concentration in the layer below the grain surface (called the space-charge layer) far exceeds that of the intrinsic bulk. [3] [9] The formation energy of the Frenkel pair is low at 1.16 eV, and the migration activation energy is unusually low at 0.05 eV (compare to NaCl: 2.18 eV for the formation of a Schottky pair and 0.75 eV for cationic migration). These low energies result in large defect concentrations, which can reach near 1% near the melting point. [9]

The low activation energy in silver bromide can be attributed the silver ions’ high quadrupolar polarizability; that is, it can easily deform from a sphere into an ellipsoid. This property, a result of the d9 electronic configuration of the silver ion, facilitates migration in both the silver ion and in silver-ion vacancies, thus giving the unusually low migration energy (for Agv−: 0.29–0.33 eV, compared to 0.65 eV for NaCl). [9]

Studies have demonstrated that the defect concentrations are strongly affected (up to several powers of 10) by crystal size. Most defects, such as interstitial silver ion concentration and surface kinks, are inversely proportional to crystal size, although vacancy defects are directly proportional. This phenomenon is attributed to changes in the surface chemistry equilibrium, and thus affects each defect concentration differently. [3]

Impurity concentrations can be controlled by crystal growth or direct addition of impurities to the crystal solutions. Although impurities in the silver bromide lattice are necessary to encourage Frenkel defect formation, studies by Hamilton have shown that above a specific concentration of impurities, the numbers of defects of interstitial silver ions and positive kinks reduce sharply by several orders of magnitude. After this point, only silver-ion vacancy defects, which actually increase by several orders of magnitude, are prominent. [3]

Electron traps and hole traps

When light is incident on the silver halide grain surface, a photoelectron is generated when a halide loses its electron to the conduction band: [2] [3] [10]

X− + hν → X + e−

After the electron is released, it will combine with an interstitial Agi+ to create a silver metal atom Agi0: [2] [3] [10]

e− + Agi+ → Agi0

Through the defects in the crystal, the electron is able to reduce its energy and become trapped in the atom. [2] The extent of grain boundaries and defects in the crystal affect the lifetime of the photoelectron, where crystals with a large concentration of defects will trap an electron much faster than a purer crystal. [10]

When a photoelectron is mobilized, a photohole h• is also formed, which also needs to be neutralized. The lifetime of a photohole, however, does not correlate with that of a photoelectron. This detail suggests a different trapping mechanism; Malinowski suggests that the hole traps may be related to defects as a result of impurities. [10] Once trapped, the holes attract mobile, negatively charged defects in the lattice: the interstitial silver vacancy Agv−: [10]

h• + Agv−⇌ h.Agv

The formation of the h.Agv lowers its energy sufficiently to stabilize the complex and reduce the probability of ejection of the hole back into the valance band (the equilibrium constant for hole-complex in the interior of the crystal is estimated at 10−4. [10]

Additional investigations on electron- and hole-trapping demonstrated that impurities also can be a significant trapping system. Consequently, interstitial silver ions may not be reduced. Therefore, these traps are actually loss mechanisms, and are considered trapping inefficiencies. For example, atmospheric oxygen can interact with photoelectrons to form an O2− species, which can interact with a hole to reverse the complex and undergo recombination. Metal ion impurities such as copper(I), iron(II), and cadmium(II) have demonstrated hole-trapping in silver bromide. [3]

Crystal surface chemistry;

Once the hole-complexes are formed, they diffuse to the surface of the grain as a result of the formed concentration gradient. Studies demonstrated that the lifetimes of holes near the surface of the grain are much longer than those in the bulk, and that these holes are in equilibrium with adsorbed bromine. The net effect is an equilibrium push at the surface to form more holes. Therefore, as the hole-complexes reach the surface, they disassociate: [10]

h.Agv− → h• + Agv− → Br → FRACTION Br2

By this reaction equilibrium, the hole-complexes are constantly consumed at the surface, which acts as a sink, until removed from the crystal. This mechanism provides the counterpart to the reduction of the interstitial Agi+ to Agi0, giving an overall equation of: [10]

AgBr → Ag + FRACTION Br2
Latent image formation and photography

Now that some of the theory has been presented, the actual mechanism of the photographic process can be discussed. To summarize, as a photographic film is subjected to an image, photons incident on the grain produce electrons which interact to yield silver metal. More photons hitting a particular grain will produce a larger concentration of silver atoms, containing between 5 and 50 silver atoms (out of ~1012 atoms), depending on the sensitivity of the emulsion. The film now has a concentration gradient of silver atom specks based upon varying intensity light across its area, producing an invisible "latent image". [2] [10]

While this process is occurring, bromine atoms are being produced at the surface of the crystal. To collect the bromine, a layer on top of the emulsion, called a sensitizer, acts as a bromine acceptor. [10]

During film development the latent image is intensified by addition of a chemical, typically hydroquinone, that selectivity reduces those grains which contain atoms of silver. The process, which is sensitive to temperature and concentration, will completely reduce grains to silver metal, intensifying the latent image on the order of 1010 to 1011. This step demonstrates the advantage and superiority of silver halides over other systems: the latent image, which takes only milliseconds to form and is invisible, is sufficient to produce a full image from it. [2]

After development, the film is "fixed," during which the remaining silver salts are removed to prevent further reduction, leaving the "negative" image on the film. The agent used is sodium thiosulfate, and reacts according to the following equation: [2]

AgX(s) + 2 Na2S2O3(aq) → Na3[Ag(S2O3)2](aq) + NaX(aq)

An indefinite number of positive prints can be generated from the negative by passing light through it and undertaking the same steps outlined above. [2]

Semiconductor properties

As silver bromide is heated within 100 °C of its melting point, an Arrhenius plot of the ionic conductivity shows the value increasing and "upward-turning." Other physical properties such as elastic moduli, specific heat, and the electronic energy gap also increase, suggesting the crystal is approaching instability. [9] This behavior, typical of a semi-conductor, is attributed to a temperature-dependence of Frenkel defect formation, and, when normalized against the concentration of Frenkel defects, the Arrhenius plot linearizes. [9]

See also

Related Research Articles

<span class="mw-page-title-main">Crystallographic defect</span> Disruption of the periodicity of a crystal lattice

A crystallographic defect is an interruption of the regular patterns of arrangement of atoms or molecules in crystalline solids. The positions and orientations of particles, which are repeating at fixed distances determined by the unit cell parameters in crystals, exhibit a periodic crystal structure, but this is usually imperfect. Several types of defects are often characterized: point defects, line defects, planar defects, bulk defects. Topological homotopy establishes a mathematical method of characterization.

<span class="mw-page-title-main">Exciton</span> Quasiparticle which is a bound state of an electron and an electron hole

An electron and an electron hole that are attracted to each other by the Coulomb force can form a bound state called an exciton. It is an electrically neutral quasiparticle that exists mainly in condensed matter, including insulators, semiconductors, some metals, but also in certain atoms, molecules and liquids. The exciton is regarded as an elementary excitation that can transport energy without transporting net electric charge.

<span class="mw-page-title-main">Crystal structure</span> Ordered arrangement of atoms, ions, or molecules in a crystalline material

In crystallography, crystal structure is a description of ordered arrangement of atoms, ions, or molecules in a crystalline material. Ordered structures occur from intrinsic nature of constituent particles to form symmetric patterns that repeat along the principal directions of three-dimensional space in matter.

<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.

<span class="mw-page-title-main">Potassium bromide</span> Ionic compound (KBr)

Potassium bromide (KBr) is a salt, widely used as an anticonvulsant and a sedative in the late 19th and early 20th centuries, with over-the-counter use extending to 1975 in the US. Its action is due to the bromide ion. Potassium bromide is used as a veterinary drug, in antiepileptic medication for dogs.

<span class="mw-page-title-main">F-center</span> Type of crystallographic defect

An F-center or color center or Farbe center is a type of crystallographic defect in which an anionic vacancy in a crystal lattice is occupied by one or more unpaired electrons. Electrons in such a vacancy in a crystal lattice tend to absorb light in the visible spectrum such that a material that is usually transparent becomes colored. The greater the number of F centers, the more intense the color of the compound. F centers are a type of color center.

<span class="mw-page-title-main">Gelatin silver print</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.

A silver halide 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.

<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.

<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">Crystallographic defects in diamond</span>

Imperfections in the crystal lattice of diamond are common. Such defects may be the result of lattice irregularities or extrinsic substitutional or interstitial impurities, introduced during or after the diamond growth. The defects affect the material properties of diamond and determine to which type a diamond is assigned; the most dramatic effects are on the diamond color and electrical conductivity, as explained by the electronic band structure.

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.

In crystallography, a Frenkel defect is a type of point defect in crystalline solids, named after its discoverer Yakov Frenkel. The defect forms when an atom or smaller ion leaves its place in the structure, creating a vacancy and becomes an interstitial by lodging in a nearby location. In elemental systems, they are primarily generated during particle irradiation, as their formation enthalpy is typically much higher than for other point defects, such as vacancies, and thus their equilibrium concentration according to the Boltzmann distribution is below the detection limit. In ionic crystals, which usually possess low coordination number or a considerable disparity in the sizes of the ions, this defect can be generated also spontaneously, where the smaller ion is dislocated. Similar to a Schottky defect the Frenkel defect is a stoichiometric defect. In ionic compounds, the vacancy and interstitial defect involved are oppositely charged and one might expect them to be located close to each other due to electrostatic attraction. However, this is not likely the case in real material due to smaller entropy of such a coupled defect, or because the two defects might collapse into each other. Also, because such coupled complex defects are stoichiometric, their concentration will be independent of chemical conditions.

Kröger–Vink notation is a set of conventions that are used to describe electric charges and lattice positions of point defect species in crystals. It is primarily used for ionic crystals and is particularly useful for describing various defect reactions. It was proposed by Ferdinand Anne Kröger and Hendrik Jan Vink.

<span class="mw-page-title-main">Interstitial defect</span> Crystallographic defect; atoms located in the gaps between atoms in the lattice

In materials science, an interstitial defect is a type of point crystallographic defect where an atom of the same or of a different type, occupies an interstitial site in the crystal structure. When the atom is of the same type as those already present they are known as a self-interstitial defect. Alternatively, small atoms in some crystals may occupy interstitial sites, such as hydrogen in palladium. Interstitials can be produced by bombarding a crystal with elementary particles having energy above the displacement threshold for that crystal, but they may also exist in small concentrations in thermodynamic equilibrium. The presence of interstitial defects can modify the physical and chemical properties of a material.

<span class="mw-page-title-main">Solid state ionics</span>

Solid-state ionics is the study of ionic-electronic mixed conductor and fully ionic conductors and their uses. Some materials that fall into this category include inorganic crystalline and polycrystalline solids, ceramics, glasses, polymers, and composites. Solid-state ionic devices, such as solid oxide fuel cells, can be much more reliable and long-lasting, especially under harsh conditions, than comparable devices with fluid electrolytes.

In condensed-matter physics, a primary knock-on atom (PKA) is an atom that is displaced from its lattice site by irradiation; it is, by definition, the first atom that an incident particle encounters in the target. After it is displaced from its initial lattice site, the PKA can induce the subsequent lattice site displacements of other atoms if it possesses sufficient energy, or come to rest in the lattice at an interstitial site if it does not.

The strength of metal oxide adhesion effectively determines the wetting of the metal-oxide interface. The strength of this adhesion is important, for instance, in production of light bulbs and fiber-matrix composites that depend on the optimization of wetting to create metal-ceramic interfaces. The strength of adhesion also determines the extent of dispersion on catalytically active metal. Metal oxide adhesion is important for applications such as complementary metal oxide semiconductor devices. These devices make possible the high packing densities of modern integrated circuits.

<span class="mw-page-title-main">Solarization (photography)</span> Photographic tone reversal due to overexposure

In photography, solarization is the effect of tone reversal observed in cases of extreme overexposure of the photographic film in the camera. Most likely, the effect was first observed in scenery photographs including the sun. The sun, instead of being the whitest spot in the image, turned black or grey. For instance, Minor White's photograph of a winter scene, The Black Sun 1955, was a result of the shutter of his camera freezing in the open position, producing severe overexposure. Ansel Adams had also earlier created a solarized sun image, titled Black Sun, Owens Valley, California, 1939, by overexposure.

References

  1. 1 2 Zumdahl, Steven S. (2009). Chemical Principles 6th Ed. Houghton Mifflin Company. p. A23. ISBN   978-0-618-94690-7.
  2. 1 2 3 4 5 6 7 8 9 10 11 Greenwood, N.N., Earnshaw, A. (1984). Chemistry of the Elements. New York: Permagon Press. pp. 1185–87. ISBN   978-0-08-022057-4.{{cite book}}: CS1 maint: multiple names: authors list (link)
  3. 1 2 3 4 5 6 7 8 Hamilton, J.F. (1974). "Physical Properties of Silver Halide Microcrystals". Photographic Science and Engineering. 18 (5): 493–500.
  4. ↑ Leden, I., Persson, G.; Persson; Sjöberg; Dam; Sjöberg; Toft (1961). "The Solubility of Silver Chloride and Silver Bromide in Aqueous Ammonia and the Formation of Mixed Silver-Ammonia-Halide Complexes". Acta Chem. Scand. 15: 607–614. doi: 10.3891/acta.chem.scand.15-0607 .{{cite journal}}: CS1 maint: multiple names: authors list (link)
  5. ↑ Engelhardt, LM; Healy, PC; Patrick, VA; White, AH (1987). "Lewis-Base Adducts of Group-11 Metal(I) Compounds. XXX. 3:1 Complexes of Triphenylphosphine With Silver(I) Halides". Aust. J. Chem. 40 (11): 1873–1880. doi:10.1071/CH9871873.
  6. ↑ Glaus, S. & Calzaferri, G. (2003). "The band structures of the silver halides AgF, AgCl, and AgBr: A comparative study". Photochem. Photobiol. Sci. 2 (4): 398–401. Bibcode:2003PhPhS...2..398G. doi:10.1039/b211678b.
  7. ↑ Lide, David R. (ed). (2005)Handbook of Chemistry and Physics, 86th Edition, The Chemical Rubber Publishing Co., Cleveland.
  8. ↑ Gurney, R. W.; Mott, N. F. (1938). "The theory of the photolysis of silver bromide and the photographic latent image". Proc. Roy. Soc. A164 (917): 151–167. Bibcode:1938RSPSA.164..151G. doi: 10.1098/rspa.1938.0011 .
  9. 1 2 3 4 5 Slifkin, L. M. (1989). "The Physics of Lattice Defects in Silver Halides". Crystal Lattice Defects and Amorphous Materials. 18: 81–96.
  10. 1 2 3 4 5 6 7 8 9 10 Malinowski, J. (1968). "The Role of Holes in the Photographic Process". The Journal of Photographic Science. 16 (2): 57–62. doi:10.1080/00223638.1968.11737436.
This page is based on this Wikipedia article
Text is available under the CC BY-SA 4.0 license; additional terms may apply.
Images, videos and audio are available under their respective licenses.