The sulfate ion is a polyatomic
anionwith the empirical formulaSO42− and a molecular mass of 96.06 daltons; it consists of a central sulfur atomsurrounded by four equivalent oxygen atoms in a tetrahedral arrangement. The sulfate ion carries a negative two chargeand is the conjugate base of the bisulfate(or hydrogen sulfate) ion, HSO4−, which is the conjugate base of H2SO4, sulfuric acid. Organic sulfates, such as dimethyl sulfate, are covalent compounds and esters of sulfuric acid.
Methods of preparing ionic sulfates include:Greenwood&Earnshaw]
*dissolving a metal in
sulfuric acidwith a metal hydroxide or oxide
sulfides or sulfites
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. Radium sulfateis the most insoluble sulfate known. The barium derivative is useful in the gravimetric analysisof sulfate: one adds a solution of, perhaps, barium chlorideto 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
chelateor a bridge. An example is the neutral metal complex PtSO4P(C6H5)32 where the sulfate ion is acting as a bidentate ligand. The metal-oxygen bonds in sulfate complexes can have significant covalent character.
tructure and bonding
The S-O bond length of 149 pm is shorter than expected for a S-O single bond. For example, the bond lengths in
sulfuric acidare 157 pm for S-OH. The tetrahedral geometry of the sulfate ion is as predicted by VSEPR theory.
The first description of the bonding in modern terms was by
Gilbert Lewisin his groundbreaking paper of 1916 where he described the bonding in terms of electron octets around each atom, i.e. no double bonds and a formal chargeof 2+ on the sulfur atom. ["The Atom and the Molecule by Gilbert N. Lewis Journal of the American Chemical Society Volume 38, 1916, pages 762-786]
Later, Linus Pauling used
valence bond theoryto 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 modern theory of valency" Linus Pauling J. Chem. Soc., 1948, 1461 - 1467, doi|10.1039/JR9480001461] 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. [C. A. Coulson, Nature, 221, 1106 (1969)] [K. A. R. Mitchell, Chem. Rev., 69, 157 (1969)] 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 "d""z"2 and "d""x"2-"y"2). [ Cotton, F. Albert; Wilkinson, Geoffrey (1966). "Advanced Inorganic Chemistry (2d Edn.). New York:Wiley. ] In this description, while there is some π character to the S-O bonds, the bond has significant ionic character. This explanation is quoted in some current textbooks.Cotton&Wilkinson6th] Greenwood&Earnshaw] The Pauling bonding representation for sulfate and other main group compounds with oxygen is a common way of representing the bonding in many textbooks.
Sulfates are important in both the chemical industry and biological systems:
lead-acid batterytypically uses sulfuric acid.
* Some anaerobic microorganisms, such as those living near deep sea thermal vents use sulfates as electron acceptors.
Copper sulfateis a common algaecide.
Magnesium sulfate, commonly known as Epsom salts, is used in therapeutic baths.
Gypsum, the natural mineralform of hydrated calcium sulfate, is used to produce plaster.
* The sulfate ion is used as
counter ionfor some cationic drugs.
Some sulfates were known to alchemists. The vitriol salts, from the Latin "vitreolum", glassy, were so-called because they were some of the first transparent crystals known. ["Inorganic and Theoretical Chemistry" F.Sherwood Taylor 6th Edition (1942) William Heinemann]
Green vitriolis ferrous sulfate heptahydrate, FeSO4 ·7H2O; blue vitriolis copper sulfate pentahydrate, CuSO4 ·5H2O and white vitriolis zinc sulfate heptahydrate, ZnSO4 ·7H2O. Alum, a double sulfate with the formula K2Al2(SO4)4 ·24H2O, figured in the development of the chemical industry.
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 forcingof about 0.5 W/m2 relative to pre-industrial values, [ [http://www.grida.no/climate/ipcc_tar/wg1/figspm-3.htm Figure 3: The global mean radiative forcing of the climate system for the year 2000, relative to 1750] . Climate Change 2001: Working Group I: The Scientific Basis. IPCC.] 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 nucleiand 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
cloudformation 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. [ [http://nenes.eas.gatech.edu/Preprints/Sensitivity_JASPP.pdf Chemical Amplification (or dampening) of the Twomey Effect: Conditions derived from droplet activation theory] . T.A. Rissman, A. Nenes, J.H. Seinfeld.]
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.
Other sulfur oxoanions
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