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 dianion, S2−, which exists in strongly alkaline aqueous solutions formed from H2S or alkali metal salts such as Li2S, Na2S, and K2S. Sulfide is exceptionally basic and, with a pKa > 14, it does not exist in appreciable concentrations even in highly alkaline water, being undetectable at pH < ~15 (8 M NaOH). Instead, sulfide combines with electrons in hydrogen 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. 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 (H2S, NaSH, Na2S) 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 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 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 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.
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.
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[edit] Examples
- hydrogen sulfide (H2S)
[edit] Uses
- Cadmium sulfide (CdS) can be used in photocells.
- Calcium polysulfide ("lime sulfur") is a traditional fungicide in gardening.
- Carbon disulfide (CS2) is sometimes used as a solvent in industrial chemistry.
- Lead sulfide (PbS) is used in infra-red sensors.
- Molybdenum disulfide (MoS2), the mineral molybdenite, is used as a catalyst to remove sulfur from fossil fuels.
- Mustard Gas (Cl-CH2CH2-S-CH2CH2-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 (Ag2S) is formed on silver electrical contacts operating in an atmosphere rich in hydrogen sulfide.
- Sodium sulfide (Na2S) is an important industrial chemical, used in manufacture of kraft paper, dyes, leather tanning, crude petroleum processing, treatment of heavy metal pollution, and others.
- Zinc sulfide (ZnS) is used for lenses and other optical devices in the infrared part of the spectrum.
- 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.
- Polyphenylene sulfide is a polymer commonly called "Sulfar". Its repeating units are bonded together by sulfide (thioether) linkages.
[edit] Natural occurrence
Many important metal ores are sulfides. Significant sulfide minerals include:
- arsenopyrite (arsenic and iron)
- argentite (silver)
- chalcopyrite (iron and copper)
- cinnabar (mercury)
- galena (lead)
- molybdenite (molybdenum)
- pentlandite (nickel)
- pyrite (iron)
- realgar (arsenic)
- sphalerite (zinc) and
- stibnite (antimony).
[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, the fumes usually include toxic 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. Mercaptans, in particular, are the strongest-smelling substances known.
[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] References
- Meyer B, Ward K, Koshlap K, Peter L (1983). Second dissociation constant of hydrogen sulfide. Inorg Chem, 22:2345.
