Sulfide
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In chemistry, a sulfide (sulphide in British and Canadian English) is a chemical compound or combination of sulfur with an oxidation number of -2, with another chemical element or a radical thereof. Some covalent sulfur compounds, such as carbon disulfide (CS2) and hydrogen sulfide (H2S), are also considered to be sulfides. Thioethers are organic compounds of the form
R-S-R' (where R and R' are organic radicals), which are also referred to as sulfides.
The sulfide ion is S-2, an anion with a -2 charge on it. In aqueous solutions, sulfide ions are only present in large concentrations at alkaline pH (high pH), because at lower pH, H+ will combine with sulfide ions to form HS- or H2S. HS- is the hydrogen sulfide ion, hydrosulfide ion, or sulfhydryl ion. H2S is hydrogen sulfide, a water-soluble gas which is a weak diprotic acid. Ionically bonded sulfides can be thought of as salts of the acid hydrogen sulfide. Many inorganic sulfide salts are not very water-soluble and many have very low solubility in water. If an -SH functional group is covalently bonded to another atom or group such as an organic radical R in a thiol, then it is typically called a sulfhydryl group. Such sulfhydryl groups can also be weakly acidic, and can give up an H+ to form a substituted sulfide ion. For example, ethyl hydrosulfide, C2H5SH, can give up an H+ to form an ethyl sulfide ion,C2H5S-, although often other names are used for such compounds; see Thiol.
The sulfur in sulfides (or in the sulfide functional groups) is in its lowest oxidation state. In sulfides, this sulfur can often be oxidized to a higher oxidation state. For example, dimethyl sulfide (CH3-S-CH3) could be oxidized to dimethyl sulfoxide (CH3-SO-CH3), which could in turn be oxidized to dimethyl sulfone (CH3-SO2-CH3). Disulfides are similar compounds having two sulfur atoms covalently bonded together and covalently or ionically bonded to the rest of the molecule or compound.
Hydrogen sulfide gas has the odor of rotten eggs, and is also highly toxic. It is formed biologically in the sediments of swamps and in the treatment of sewage sludge by anaerobic digestion of sulfur containing proteins, or bacterial reduction of sulfates. It also occurs in some natural gas and in the emissions of some volcanoes, and as a byproduct of some industrial processes.
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Examples
- hydrogen sulfide (H2S)
- sodium sulfide (Na2S)
- carbon disulfide (CS2)
- tetraphosphorus decasulfide (P4S10)
Uses
- Cadmium disulfide (CdS2) 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.
- 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 material commonly called Sulfar. Its repeating units are bonded together by sulfide (thioether) linkages.
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).
Safety
Many sulfides are significantly toxic by inhalation or injection, especially if the metal ion is toxic. Additionally many sulfides, when exposed to a strong mineral acid, will release toxic hydrogen sulfide - and this includes your stomach acids!
Also, many sulfides are somewhat flammable, and a few are highly flammable. When a sulfide burns, the fumes usually include toxic sulfur dioxide (SO2) gas.