A sulfide (sulphide in British English) is an anion of sulfur in its lowest oxidation number of −2. Sulfide is also a slightly archaic term for thioethers, a common type of organosulfur compound that are well known for their bad odors.

Contents 1 Properties 2 Metal derivatives 3 Geology 4 Corrosion induced by sulfide 5 Organic chemistry 6 Disulfides 7 Examples 8 Safety 9 See also 10 References

[edit] Properties

The dianion S²⁻ exists only in strongly alkaline aqueous solutions. Such solutions can formed by dissolution of H₂S or alkali metal salts such as Li₂S, Na₂S, and K₂S in the presence of extra hydroxide. The ion S²⁻ is exceptionally basic with a pK_a > 14. It does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH).^[1]

Instead, sulfide combines with protons to form HS⁻, which is variously called hydrogen sulfide ion, hydrosulfide ion, sulfhydryl ion, or bisulfide ion. At still lower pH's (<7), HS⁻ converts to H₂S, hydrogen sulfide.^[2]

[edit] Metal derivatives

Aqueous solutions of transition metals cations react with sulfide sources (H₂S, NaSH, Na₂S) to precipitate solid sulfides. Such inorganic sulfides typically have very low solubility in water, and many are related to minerals with the same composition (see below). One famous example is the bright yellow species CdS or "cadmium yellow". The black tarnish formed on sterling silver is Ag₂S. 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 deep colors. Several have practical applications as pigments, in solar cells, and as catalysts.

[edit] Geology

Many important metal ores are sulfides.^[3] Significant examples include: argentite (silver sulfide), cinnabar (mercury, galena (lead sulfide), molybdenite (molybdenum sulfide), pentlandite ([[nickel] sulfide]), realgar (arsenic sulfide), and stibnite (antimony), sphalerite (zinc sulfide), and pyrite (iron disulfide), and chalcopyrite (iron-copper sulfide).

[edit] Corrosion induced by sulfide

Dissolved free sulfides (H₂S, HS⁻ and S²⁻) 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 wells, pipeline transporting soured oil, Kraft paper factories. Microbially-induced corrosion (MIC) or biogenic sulfide corrosion are also caused by sulfate reducing bacteria producing sulfide.

Oxidation of sulfide can also form thiosulfate (S₂O₃²⁻) 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.

[edit] Organic chemistry

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 CH₃-S-CH₃. Polyphenylene sulfide (see below) has the empirical formula C₆H₄S. Occasionally, the term sulfide refers to molecules containing the -SH functional group. For example, methyl sulfide can mean CH₃-SH. The preferred descriptor for such SH-containing compounds is thiol or mercaptan, i.e. methanethiol or methyl mercaptan.

[edit] Disulfides

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 S₂²⁻, or S⁻-S⁻, in association with iron in the formal 2+ oxidation state. Dimethyldisulfide has the connectivity CH₃-S-S-CH₃, whereas carbon disulfide has no S-S linkages, being S=C=S.

[edit] Examples

Formula		Melting point (°C)	Boiling point (°C)	CAS number
H2S	hydrogen sulfide	-85,7	-60,20	7783-06-4
CdS	Cadmium sulfide can be used in photocells.	1750		1306-23-6
	Calcium polysulfide ("lime sulfur") is a traditional fungicide in gardening.
CS2	Carbon disulfide is sometimes used as a solvent in industrial chemistry.	-111.6	46	75-15-0
PbS	Lead sulfide is used in infra-red sensors.	1114		1314-87-0
MoS2	Molybdenum disulfide, the mineral molybdenite, is used as a catalyst to remove sulfur from fossil fuels; also as lubricant for high temperatures and high pressures applications.			1317-33-5
Cl-CH2CH2-S-CH2CH2-Cl	Mustard Gas, 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.	13 - 14	217	505-60-2
Ag2S	Silver sulfide is formed on silver electrical contacts operating in an atmosphere rich in hydrogen sulfide.			21548-73-2
Na2S	Sodium sulfide is an important industrial chemical, used in manufacture of kraft paper, dyes, leather tanning, crude petroleum processing, treatment of heavy metal pollution, and others.	920	1180	1313-82-2
ZnS	Zinc sulfide is used for lenses and other optical devices in the infrared part of the spectrum.		1185	1314-98-3
ZnS	Zinc sulfide with a trace of copper is used for photoluminescent strips for emergency lighting and luminous watch dials.
	Several metal sulfides are used as pigments in art, although their use has declined somewhat due to their toxicity. Sulfide pigments include cadmium, mercury, and arsenic.
C6H4S	Polyphenylene sulfide is a polymer commonly called "Sulfar". Its repeating units are bonded together by sulfide (thioether) linkages.			26125-40-6 25212-74-2
SeS2	Selenium sulfide is an antifungal used in anti-dandruff preparations, such as Selsun Blue.	<100		7488-56-4
FeS2	Iron disulfide occurs in pyrite	600		1317-66-4

[edit] 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 acids, will release toxic hydrogen sulfide.

Organic sulfides are highly flammable. When a sulfide burns to give sulfur dioxide (SO₂) gas.

Hydrogen sulfide, some of its salts, and almost all organic sulfides have a strong and putrid stench; rotting biomass releases these.

[edit] See also

• Biogenic sulfide corrosion
• Sulfide stress cracking
• Sulfate
• Sulfite
• Thioether
• Thiosulfate
• Dithionite

[edit] References

1. ^ Meyer B, Ward K, Koshlap K, Peter L "Second dissociation constant of hydrogen sulfide" Inorganic Chemistry 1983, volume 22, pp. 2345. doi:10.1021/ic00158a027.
2. ^ Greenwood, N. N.; & Earnshaw, A. (1997). Chemistry of the Elements (2nd Edn.), Oxford:Butterworth-Heinemann. ISBN 0-7506-3365-4.
3. ^ Vaughan, D. J.; Craig, J. R. “Mineral Chemistry of Metal Sulfides" Cambridge University Press, Cambridge: 1978. ISBN 0521214890.