Sulfate salts have diverse applications. For example, magnesium sulfate (or Epsom salts) is used in therapeutic baths; gypsum, the mineral form of hydrated calcium sulfate, is used to produce plaster; and copper sulfate is an algaecide. Some microorganisms that live near deep-sea thermal vents utilize sulfates as electron acceptors.
The sulfate ion is the conjugate base of the hydrogen sulfate (also known as bisulfate) ion, HSO4−. The hydrogen sulfate ion in turn is the conjugate base of sulfuric acid, H2SO4.
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.
Sulfates are important in both the chemical industry and biological systems. Some uses are listed below.
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, 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.
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.
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