Sulfide
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The term '''sulfide''' (also spelled '''sulphide''' in [[British English]]) refers to several types of [[chemical compounds]] containing [[sulfur]] in its lowest [[oxidation number]] of −2.
Formally, "sulfide" is the di[[anion]], S<sup>2−</sup>, which exists in strongly [[alkaline]] aqueous solutions formed from H<sub>2</sub>S or alkali metal salts such as Li<sub>2</sub>S, Na<sub>2</sub>S, and K<sub>2</sub>S. Sulfide is exceptionally basic and, with a pK<sub>a</sub> > 14, it does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M [[Sodium hydroxide|NaOH]]). Instead, sulfide combines with electrons in hydrogen to form HS<sup>−</sup>, which is variously called '''hydrogen sulfide''' ion, '''hydrosulfide''' ion, '''sulfhydryl''' ion, or '''bisulfide''' ion. At still lower pH's (<7), HS<sup>−</sup> converts to H<sub>2</sub>S, [[hydrogen sulfide]]. Thus, the exact sulfur species obtained upon dissolving sulfide salts depends on the pH of the final solution.
Aqueous solutions of [[transition metals]] [[cations]] react with sulfide sources (H<sub>2</sub>S, NaSH, Na<sub>2</sub>S) to precipitate solid sulfides. Such [[inorganic]] sulfides typically have very low solubility in water and many are related to minerals. One famous example is the bright yellow species CdS or "[[cadmium yellow]]". The black tarnish formed on sterling silver is Ag<sub>2</sub>S. Such species are sometimes referred to as salts. In fact the bonding in transition metal sulfides is highly covalent, which gives rise to their [[semiconductor]] properties, which in turn is related to the practical applications of many sulfide materials.
In [[organic chemistry]], "sulfide" usually refers to the linkage C-S-C, although the term [[thioether]] is less ambiguous. For example, the thioether [[dimethyl sulfide]] is CH<sub>3</sub>-S-CH<sub>3</sub>. [[Polyphenylene sulfide]] (see below) has the empirical formula C<sub>6</sub>H<sub>4</sub>S. Occasionally, the term sulfide refers to molecules containing the -SH [[functional group]]. For example, methyl sulfide can mean CH<sub>3</sub>-SH. The preferred descriptor for such SH-containing compounds is [[thiol]] or mercaptan, i.e. methanethiol or methyl mercaptan.
Confusion arises from the different meanings of the term "[[disulfide]]". Molybdenum disulfide consists of separated sulfide centers, in association with molybdenum in the formal 4+ oxidation state. Iron disulfide on the other hand consists of S<sub>2</sub><sup>2−</sup>, or S<sup>−</sup>-S<sup>−</sup>, in association with iron in the formal 2+ oxidation state. Dimethyldisulfide has the connectivity CH<sub>3</sub>-S-S-CH<sub>3</sub>, whereas [[carbon disulfide]] has no S-S linkages, being S=C=S.
==Examples==
* [[hydrogen sulfide]] (H<sub>2</sub>S)
==Uses==
*[[Cadmium]] sulfide (CdS) can be used in [[photocell]]s.
*[[Calcium]] polysulfide ("[[lime sulfur]]") is a traditional [[fungicide]] in [[gardening]].
*[[Carbon disulfide]] (CS<sub>2</sub>) is sometimes used as a [[solvent]] in industrial chemistry.
*[[Lead sulfide]] (PbS) is used in [[infra-red]] sensors.
*[[Molybdenum disulfide]] (MoS<sub>2</sub>), the mineral [[molybdenite]], is used as a catalyst to remove sulfur from fossil fuels.
* [[Mustard Gas]] (Cl-CH<sub>2</sub>CH<sub>2</sub>-S-CH<sub>2</sub>CH<sub>2</sub>-Cl), is a sulfide that has been used as a chemical weapon, the chloride on the molecule acts as a leaving group when in the presence of water and forms a sulfide-alcohol and HCl.
*[[Silver sulfide]] (Ag<sub>2</sub>S) is formed on [[silver]] [[electrical contacts]] operating in an atmosphere rich in [[hydrogen sulfide]].
* [[Sodium]] sulfide (Na<sub>2</sub>S) is an important industrial chemical, used in manufacture of [[Kraft process|kraft]] [[paper]], [[dye]]s, [[leather tanning]], [[crude petroleum]] processing, treatment of [[heavy metals|heavy metal]] [[pollution]], and others.
*[[Zinc]] sulfide (ZnS) is used for [[lens (optics)|lenses]] and other [[optics|optical]] devices in the [[infrared]] part of the [[Electromagnetic spectrum|spectrum]].
*[[Zinc]] sulfide with a trace of copper is used for [[photoluminescence|photoluminescent]] strips for emergency lighting and luminous watch dials.
*Several metal sulfides are used as [[pigment]]s in art, although their use has declined somewhat due to their toxicity. Sulfide pigments include cadmium, mercury, and arsenic.
*[[Polyphenylene sulfide]] is a polymer commonly called "Sulfar". Its repeating units are bonded together by sulfide (thioether) linkages.
==Natural occurrence==
Many important [[metal]] [[ore]]s are sulfides. Significant [[sulfide mineral]]s include:
* [[arsenopyrite]] ([[arsenic]] and [[iron]])
* [[argentite]] ([[silver]])
* [[chalcopyrite]] ([[iron]] and [[copper]])
* [[cinnabar]] ([[mercury (element)|mercury]])
* [[galena]] ([[lead]])
* [[molybdenite]] ([[molybdenum]])
* [[pentlandite]] ([[nickel]])
* [[pyrite]] ([[iron]])
* [[realgar]] ([[arsenic]])
* [[sphalerite]] ([[zinc]]) and
* [[stibnite]] ([[antimony]]).
==Safety==
Many metal sulfides are so insoluble in water that they are probably not very toxic. Some metal sulfides, when exposed to a strong mineral [[acid]], including [[gastric acid]]s, will release toxic hydrogen sulfide.
Organic sulfides are highly flammable. When a sulfide burns, the fumes usually include toxic [[sulfur dioxide]] (SO<sub>2</sub>) gas.
Hydrogen sulfide, some of its salts, and almost all organic sulfides have a strong and putrid stench; rotting biomass releases these. [[Thiol|Mercaptans]], in particular, are the strongest-smelling substances known.
==Corrosion induced by sulfide==
Dissolved free sulfides (H<sub>2</sub>S, HS<sup>-</sup> and S<sup>2-</sup>) are very aggressive species for the corrosion of many metals such as, ''e.g.'', steel, stainless steel, and copper. Sulfides present in aqueous solution are responsible for [[stress corrosion cracking]] (SCC) of steel, and is also known as [[sulfide stress cracking]]. Corrosion is a major concern in many industrial installations processing sulfides: sulfide ore mills, deep [[oil well]]s, pipeline transporting soured [[oil]], [[Kraft paper]] factories. [[Microbial corrosion|Microbially-induced corrosion]] ([[MIC]]) or [[biogenic sulfide corrosion]] are also caused by [[sulfate reducing bacteria]] producing sulfide.
Oxidation of sulfide can also form [[thiosulfate]] (S<sub>2</sub>O<sub>3</sub><sup>2-</sup>) an intermediate species responsible for severe problems of [[pitting corrosion]] of steel and stainless steel while the medium is also acidified by the production of sulfuric acid when oxidation is more advanced.
==References==
* Meyer B, Ward K, Koshlap K, Peter L (1983). Second dissociation constant of hydrogen sulfide. ''Inorg Chem, 22:2345''.
== See also ==
*[[Biogenic sulfide corrosion]]
*[[Sulfide stress cracking]]
*[[Sulfate]]
*[[Sulfidation]]
*[[Sulfite]]
*[[Thioether]]
*[[Thiosulfate]]
*[[Dithionite]]
[[Category:Anions]]
[[Category:Corrosion]]
[[Category:Sulfides|*]]
[[Category:Functional groups]]
[[bg:Сулфид]]
[[ca:Sulfur]]
[[cs:Sulfid]]
[[da:Sulfid]]
[[de:Sulfide]]
[[et:Sulfiidid]]
[[es:Sulfuro]]
[[eo:Sulfido]]
[[fr:Sulfure]]
[[he:סולפיד]]
[[it:Solfuro]]
[[ja:硫化物]]
[[nl:Sulfide]]
[[no:Sulfid]]
[[pl:Siarczki]]
[[pt:Sulfeto]]
[[ru:Сульфиды]]
[[sr:Сулфид]]
[[fi:Sulfidi]]
[[sv:Sulfider]]
[[zh:硫化物]]