Everything about Sulfate totally explained
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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 a 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
bisulfate (or hydrogen sulfate) ion, HSO
4−, which is the conjugate base of H
2SO
4,
sulfuric acid. Organic sulfates, such as
dimethyl sulfate, are covalent compounds and
esters of sulfuric acid.
Preparation
Methods of preparing ionic sulfates include:
Properties
Many examples of ionic sulfates are known, and many of these 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.
The sulfate ion can act as a ligand attaching either by one oxygen (monodentate) or by two oxygens as either a
chelate or a bridge.
Later, Linus Pauling used
valence bond theory to propose that the most significant resonance canonicals had two π bonds (see above) involving d orbitals. His reasoning was that the charge on sulfur was thus reduced, in accordance with his
principle of electroneutrality. The double bonding was taken by Pauling to account for the shortness of the S-O bond (149 pm).
Pauling's use of d orbitals provoked a debate on the relative importance of π bonding and bond polarity (electrostatic attraction) in causing the shortening of the S-O bond. The outcome was a broad consensus that d orbitals play a role, but are not as significant as Pauling had believed. A widely accepted description involves pπ - dπ bonding, initially proposed by D.W.J Cruickshank, where fully occupied p orbitals on oxygen overlap with empty sulfur d orbitals (principally the
dz2 and
dx2-y2). In this description, while there's some π character to the S-O bonds, the bond has significant ionic character. This explanation is quoted in some current textbooks. The Pauling bonding representation for sulfate and other main group compounds with oxygen is a common way of representing the bonding in many textbooks.
Green vitriol is ferrous sulfate heptahydrate, FeSO
4·7H
2O;
blue vitriol is copper sulfate pentahydrate, CuSO
4·5H
2O and
white vitriol is zinc sulfate heptahydrate, ZnSO
4·7H
2O.
Alum, a double sulfate with the formula K
2Al
2(SO
4)
4·24H
2O, figured in the development of the chemical industry.
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 main direct effect of sulfates on the climate involves the scattering of 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/m
2 relative to pre-industrial values, partially offsetting the larger (about 2.4 W/m
2) 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, to facilitate
cloud formation at low
humidities and longer cloud lifetime. 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/m
2, 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.
Other sulfur oxoanions
Further Information
Get more info on 'Sulfate'.
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