Sulfate

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In inorganic chemistry, a sulfate (IUPAC-recommended spelling; also sulphate in British English) is a salt of sulfuric acid.

Contents

1 Chemical properties
2 Uses

2.1 Oxoanion sulfides

3 Environmental effects

3.1 Two main effects on climate

Chemical properties

The sulfate ion is a polyatomic anion with the empirical formula S O₄²⁻ 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:

The Lead-acid battery typically uses sulfuric acid.
Some anaerobic microorganisms, such as those living near deep sea thermal vents utilize sulfates as electron acceptors.
Copper sulfate is a common algaecide.
Magnesium sulfate, commonly known as Epsom salts, is used in therapeutic baths.

Oxoanion sulfides

S O₅²⁻ persulfate ion
S O₄²⁻ sulfate ion
S O₃²⁻ sulfite ion
S O₂²⁻ hyposulfite ion
S₂O₈²⁻ peroxydisulfate ion

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.

Two 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 [link], 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 second (indirect) effect is that sulfate aerosols can act as cloud condensation nuclei. This effect is proposed to facilitate cloud formation at low humidities. Sulfate may also result in longer lasting clouds, or changes in the particle size distribution, which can affect the clouds radiative properties in ways that are not fully understood. This probably has 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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