Asilver halide (or silver salt) is one of thechemical compounds that can form between theelementsilver (Ag) and one of thehalogens. In particular,bromine (Br),chlorine (Cl),iodine (I) andfluorine (F) may each combine with silver to producesilver bromide (AgBr),silver chloride (AgCl),silver iodide (AgI), and four forms ofsilver fluoride, respectively.
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 withoxidation states of +1 (Ag+), silver halides in which the silver atoms have oxidation states of +2 (Ag2+) are known, of whichsilver(II) fluoride is the only known stable one.
Silver halides are light-sensitive chemicals, and are commonly used in photographic film and paper.
Silver halides are used inphotographic film andphotographic paper, including graphic art film and paper, where silver halidecrystals ingelatin are coated on to afilm base, glass or papersubstrate. The gelatin is a vital part of the emulsion as theprotective colloid of appropriate physical and chemical properties. The gelatin may also contain trace elements (such assulfur) which increase the light sensitivity of theemulsion, although modern practice uses gelatin without such components.
When a silver halide crystal is exposed to light, asensitivity speck on the surface of the crystal is turned into a speck of metallic silver (these comprise the invisible orlatent image). If the speck of silver contains approximately four or more atoms, it is rendered developable - meaning that it can undergodevelopment 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 thecollodion wet plate anddaguerreotype, in which the iodide is sometimes used alone (generally regarded as necessary if a daguerreotype is to be developed by theBecquerel 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 aconduction band (de-localizedelectron orbital with higher energy than avalence band) which can be attracted by asensitivity speck, which is a shallow electron trap, which may be a crystalline defect or a cluster ofsilver 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 makecorrective lenses darken when exposed toultraviolet light (seephotochromism).
Silver halides, except for silver fluoride, are very insoluble in water.Silver nitrate can be used toprecipitatehalides; this application is useful inquantitative 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.
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, theArctic 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 activeGitHub repositories as of 2020[update].