The structure and bonding of the sulfate ion
In inorganic chemistry, a sulfate (IUPAC-recommended spelling; also sulphate in British English) is a salt of sulfuric acid.
Chemical properties
The sulfate ion is a polyatomic anion with the
empirical formula SO42− and a molecular mass of 96.06 daltons; it
consists of one central sulfur atom surrounded by four equivalent oxygen atoms in a
tetrahedral arrangement. The sulfate ion carries a negative two charge and is the conjugate base of the hydrogen sulfate
(also known as bisulfate) ion, HSO4−, which is the conjugate base of H2SO4,
sulfuric acid.
Sulfate compounds arise when cations combine with the anion
SO42−. Often this combination results in an ionic compound,
although sulfates can engage in covalent bonding with most elements. The metal complex
PtSO4P(C6H5)32 is clearly covalent Pt-O bonding. Dialkylsulfates, such as
dimethylsulfate are covalent, distillable species. Many sulfate salts are highly
soluble in water. Exceptions
include calcium sulfate, strontium sulfate,
and barium sulfate, which are poorly soluble. The barium derivative is useful in the
gravimetric analysis of sulfate: one adds a solution of, perhaps, barium chloride to a solution containing sulfate ions. The appearance of a white precipitate, which is
barium sulfate, indicates that sulfate anions are present.
Uses
Sulfates are important in both the chemical industry and biological systems:
Sulfur oxoanions
Environmental effects
Sulfates occur as microscopic particles (aerosols) resulting from fossil fuel and biomass combustion. They increase the acidity of the
atmosphere and form acid rain.
Main effects on climate
The first (direct) effect is to scatter light, effectively increasing the Earth's albedo. This
effect is moderately well understood and leads to a cooling from the negative radiative
forcing of about 0.5 W/m2 relative to pre-industrial values,[1] partially offsetting the larger (about 2.4 W/m2) warming effect of greenhouse gases. The effect is strongly spatially non-uniform, being largest downstream of large
industrial areas.
The first indirect effect is also known as the Twomey effect. Sulfate aerosols can act
as cloud condensation nuclei and this leads to greater numbers of smaller
droplets of water. Lots of smaller droplets can diffuse light more efficiently than just a few larger droplets.
The second indirect effect is the further knock-on effects of having more cloud condensation nuclei. It is proposed that these
include the suppression of drizzle, increased cloud height (Pincus & Baker 1994), to facilitate cloud formation at low humidities and longer cloud lifetime (Albrecht 1989).
Sulfate may also result in changes in the particle size distribution, which can affect the clouds radiative properties in ways
that are not fully understood. Chemical effects such as the dissolution of soluble gases and slightly soluble substances, surface
tension depression by organic substances and accommodation coefficient changes are also included in the second indirect effect
[2].
The indirect effects probably have a cooling effect, perhaps up to 2 W/m2, although the uncertainty is very
large.
Sulfates are therefore implicated in global dimming, which may have acted to offset
some of the effects of global warming.
See also
References
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