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[[Image:Sulfate-ion-2D-dimensions.png|thumb|The structure and bonding of the sulfate ion]]
[[Image:Sulfate-3D-vdW.png|thumb|Space-filling model of the sulfate ion]]
In [[inorganic chemistry]], a '''sulfate''' ([[International Union of Pure and Applied Chemistry|IUPAC]]-recommended spelling; also '''sulphate''' in [[British English]]) is a [[salt (chemistry)|salt]] of [[sulfuric acid]].

==Chemical properties==
The sulfate ion is a [[polyatomic ion|polyatomic]] [[anion]] with the [[empirical formula]] {{chem|SO|4|2-}} and a [[molecular mass]] of 96.06 [[Atomic mass unit|daltons]]; it consists of a central [[sulfur]] [[atom]] surrounded by four equivalent [[oxygen]] atoms in a [[tetrahedron|tetrahedral]] arrangement, symmetry of sulfate is Td.The symmetry is very similar to that of methane, CH<sub>4</sub>.  The sulfur atom is in the +6 [[oxidation state]] while the four oxygen atoms are each in the −2 state. The sulfate ion carries a negative two [[charge]] and is the [[conjugate acid|conjugate base]] of the bisulfate (or hydrogen sulfate) ion, {{chem|HSO|4|-}}, which is the conjugate base of {{chem|H|2|SO|4}}, [[sulfuric acid]]. Organic sulfates, such as [[dimethyl sulfate]], are covalent compounds and [[ester]]s of sulfuric acid.

===Preparation===
Methods of preparing ionic sulfates include:<ref name = greenwood>{{Greenwood&Earnshaw}}</ref>
*dissolving a metal in [[sulfuric acid]]
*reacting sulfuric acid with a metal hydroxide or oxide
*oxidizing metal [[sulfide]]s or [[sulfite]]s

===Properties===
Many examples of ionic sulfates are known, and many of these are highly [[solubility|soluble]] in [[water]]. Exceptions include [[calcium sulfate]], [[strontium sulfate]], lead (II) sulfate, and [[barium sulfate]], which are poorly soluble. [[Radium sulfate]] is the most insoluble sulfate known. 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.<ref name = greenwood/> An example is the neutral metal complex PtSO<sub>4</sub>(P(C<sub>6</sub>H<sub>5</sub>)<sub>3</sub>)<sub>2</sub> where the sulfate ion is acting as a bidentate ligand. The metal-oxygen bonds in sulfate complexes can have significant covalent character.

===Structure and bonding===
The S-O bond length of 149&nbsp;pm is shorter than expected for a S-O single bond. For example, the bond lengths in [[sulfuric acid]] are  157&nbsp;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 N. Lewis|Gilbert Lewis]] in 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 charge]] of 2+ on the sulfur atom.<ref>''The Atom and the Molecule by Gilbert N. Lewis Journal of the American Chemical Society Volume 38, 1916, pages 762-786</ref>

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 [[Paulings electroneutrality principle|principle of electroneutrality]].<ref>''The modern theory of valency'' Linus Pauling J. Chem. Soc., 1948, 1461 - 1467, {{doi|10.1039/JR9480001461}}</ref> 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.<ref>C. A. Coulson, Nature, 221, 1106 (1969)</ref><ref>K. A. R. Mitchell, Chem. Rev., 69, 157 (1969)</ref> 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''<sub>''z''<sup>2</sup></sub> and ''d''<sub>''x''<sup>2</sup>-''y''<sup>2</sup></sub>).<ref> [[F. Albert Cotton|Cotton, F. Albert]]; [[Geoffrey Wilkinson|Wilkinson, Geoffrey]] (1966). ''Advanced Inorganic Chemistry (2d Edn.). New York:Wiley. </ref> 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.<ref name = greenwood>{{Greenwood&Earnshaw}}</ref><ref name = cotton>{{Cotton&Wilkinson6th}}</ref> The Pauling bonding representation for sulfate and other main group compounds with oxygen is a common way of representing the bonding in many textbooks.<ref name = greenwood/><ref name = cotton/>

==Uses==
Sulfates 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 [[black smokers|thermal vent]]s use sulfates as electron acceptors.
* [[Copper sulfate]] is a common [[algae]]cide.
* [[Magnesium sulfate]], commonly known as [[Epsom salts]], is used in therapeutic baths.
* [[Gypsum]], the natural [[mineral]] form of hydrated [[calcium sulfate]], is used to produce [[plaster]].
* The sulfate ion is used as [[counter ion]] for some [[cation]]ic drugs.

==History==
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.<ref>''Inorganic and Theoretical Chemistry'' F.Sherwood Taylor 6th Edition (1942) William Heinemann</ref> [[Green vitriol]] is ferrous sulfate heptahydrate, FeSO<sub>4</sub><nowiki>&middot;</nowiki>7H<sub>2</sub>O; [[blue vitriol]] is copper sulfate pentahydrate, CuSO<sub>4</sub><nowiki>&middot;</nowiki>5H<sub>2</sub>O  and [[white vitriol]] is zinc sulfate heptahydrate, ZnSO<sub>4</sub><nowiki>&middot;</nowiki>7H<sub>2</sub>O. [[Alum]], a double sulfate with the formula K<sub>2</sub>Al<sub>2</sub>(SO<sub>4</sub>)<sub>4</sub><nowiki>&middot;</nowiki>24H<sub>2</sub>O, figured in the development of the chemical industry.

==Environmental effects==
Sulfates occur as microscopic particles ([[Particulate|aerosols]]) resulting from [[fossil fuel]] and [[biomass]] combustion. They increase the acidity of the [[Earth's atmosphere|atmosphere]] and form [[acid rain]]. <!-- do sulfate aerosols per se comprise "acid rain" vs. aerobic oxidation of SO2 and SO3 to give H2SO4-->

===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<sup>2</sup> relative to pre-industrial values,<ref>[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. [[Intergovernmental Panel on Climate Change|IPCC]].</ref> partially offsetting the larger (about 2.4 W/m<sup>2</sup>) warming effect of [[greenhouse gas]]es. 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, <ref>Pincus & Baker 1994</ref> to facilitate [[cloud]] formation at low [[humidity|humidities]] and longer cloud lifetime.<ref>Albrecht 1989</ref>  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.<ref>[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.</ref>

The indirect effects probably have a cooling effect, perhaps up to 2 W/m<sup>2</sup>, 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 oxyanions ==
{|class="wikitable"
|-
! Molecular formula
! Name
|-
|{{chem|SO|5|2-}}|| [[Peroxomonosulfate]]
|-
|{{chem|SO|4|2-}} || Sulfate
|-
|{{chem|SO|3|2-}} || [[Sulfite]]
|-
|{{chem|S|2|O|8|2-}} || [[Peroxodisulfate]]
|-
|{{chem|S|2|O|7|2-}} || [[Pyrosulfate]]
|-
|{{chem|S|2|O|6|2-}} || [[Dithionate]]
|-
|{{chem|S|2|O|5|2-}} || [[Metabisulfite]]
|-
|{{chem|S|2|O|4|2-}} || [[Dithionite]]
|-
|{{chem|S|2|O|3|2-}} || [[Thiosulfate]]
|-
|{{chem|S|4|O|6|2-}} || [[Tetrathionate]]
|}

== See also ==
{{commons|Sulfates}}
* [[Sulfonate]]

==References==
{{reflist}}

[[Category:Sulfates|*]]
[[Category:Oxoanions]]

{{Imported from Wikipedia|name=Sulfate|id=326968114}}