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 formal charge and is the conjugate base of the hydrogensulfate ion, HSO4−, which is the conjugate base of H2SO4, sulfuric acid.
Sulfate compounds arise when cations combine with the anion SO42−. Usually this combination results in an ionic compound. Occasionally, however, sulfate compounds are not ionic, an example being the lipophilic complex PtSO4(P(C6H5)3)2. Almost all sulfates are highly soluble in water at standard temperature and pressure. 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, say, barium chloride to a solution containing sulfate ions. The weight of the resulting colorless precipitate is often taken as an indication of the sulfate content of the solution.
Uses
Sulfates, also known as sulfur oxides, are important in both the chemical industry and biological systems:
Oxoanion sulfides
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 Earths 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 more smaller droplets of water. More smaller droplets can diffuse light more efficiently than 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.