Definitions

sulfide

sulfide

[suhl-fahyd, -fid]
sulfide, chemical compound containing sulfur and one other element or sulfur and a radical. Sulfides may be salts or esters of hydrogen sulfide, H2S, or may be formed directly, e.g., by heating a metal with sulfur. Hydrosulfides are formed when only one of the hydrogens in hydrogen sulfide is replaced with a metal or radical. Soluble metal sulfides are used in preparing dyes, in leather tanning, as depilatory compounds, and as pesticides. Sulfides of antimony, copper, lead, mercury, silver, and zinc are important as ores; the ores are often roasted, yielding sulfur dioxide and an oxide of the metal. Pyrite is iron disulfide; tarnish on silver is mostly silver sulfide. In chemical analysis, hydrogen sulfide is often used to precipitate from a solution of metal salts certain metal sulfides that have characteristic colors and solubilities. Carbon disulfide is an important solvent for organic compounds.
or sulphide mineral

Pyrite from Butte, Mont.

Any member of a group of compounds of sulfur with one or more metals. The metals that occur most commonly are iron, copper, nickel, lead, cobalt, silver, and zinc. They are the ore minerals of most metals used by industry (e.g., antimony, bismuth, copper, lead, nickel, and zinc). Other industrially important metals such as cadmium and selenium occur in trace amounts in numerous common sulfides and are recovered in refining processes.

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Any of three classes of chemical compounds containing the element sulfur. The three classes of sulfides include inorganic sulfides, organic sulfides (thioethers), and phosphine sulfides. Sulfides of many metals are naturally occurring minerals; for example, pyrite (fool's gold) is a sulfide of iron, FeS2. Sulfides are important components of lithium and sodium sulfide batteries, and phosphorous sulfides are used in the tips of strike-anywhere matches and in the preparation of industrial lubricant additives.

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The term sulfide (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.

Examples

Uses

Natural occurrence

Many important metal ores are sulfides. Significant sulfide minerals include:

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.

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.

References

  • Meyer B, Ward K, Koshlap K, Peter L (1983). Second dissociation constant of hydrogen sulfide. Inorg Chem, 22:2345.

See also

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