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The sulfate or sulphate ion is a polyatomic
anion An ion () is an atom or molecule with a net electrical charge. The charge of an electron is considered to be negative by convention and this charge is equal and opposite to the charge of a proton, which is considered to be positive by co ...
with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are
salt Salt is a mineral composed primarily of sodium chloride (NaCl), a chemical compound belonging to the larger class of salts; salt in the form of a natural crystalline mineral is known as rock salt or halite. Salt is present in vast qua ...
s of
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen Oxygen is the chemical ele ...
and many are prepared from that acid.


Spelling

"Sulfate" is the spelling recommended by IUPAC, but "sulphate" was traditionally used in
British English British English (BrE, en-GB, or BE) is, according to Oxford Dictionaries, " English as used in Great Britain Great Britain is an island in the North Atlantic Ocean The Atlantic Ocean is the second-largest of the world's five ...
.


Structure

The sulfate anion consists of a central
sulfur Sulfur (or sulphur in British English) is a chemical element with the Symbol (chemistry), symbol S and atomic number 16. It is abundance of the chemical elements, abundant, Polyvalency (chemistry), multivalent and nonmetallic. Under standard c ...
atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is made of one or more protons and a number of neutrons. Only the most common variety of hydrogen has no neutrons. Every solid, liquid, g ...
surrounded by four equivalent
oxygen Oxygen is the chemical element with the symbol O and atomic number 8. It is a member of the chalcogen group in the periodic table, a highly reactive nonmetal In chemistry, a nonmetal is a chemical element that generally la ...
atoms in a tetrahedral arrangement. The symmetry is the same as that of methane. The sulfur atom is in the +6 oxidation state while the four oxygen atoms are each in the −2 state. The sulfate ion carries an overall charge of −2 and it is the
conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atom Every atom is com ...
of the bisulfate (or hydrogensulfate) ion, , which is in turn the conjugate base of ,
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen Oxygen is the chemical ele ...
. Organic sulfate esters, such as dimethyl sulfate, are covalent compounds and
ester In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, ...
s of sulfuric acid. The tetrahedral molecular geometry of the sulfate ion is as predicted by VSEPR theory.


Bonding

The first description of the bonding in modern terms was by Gilbert Lewis in his groundbreaking paper of 1916 where he described the bonding in terms of electron octets around each atom, that is no double bonds and a formal charge of +2 on the sulfur atom. Later,
Linus Pauling Linus Carl Pauling (; February 28, 1901August 19, 1994) was an American chemist, biochemist, chemical engineer, peace activist, author, and educator. He published more than 1,200 papers and books, of which about 850 dealt with scientific ...
used
valence bond theory In chemistry Chemistry is the scientific study of the properties and behavior of matter. It is a natural science that covers the elements that make up matter to the compounds made of atoms, molecules and ions: their composition, s ...
to propose that the most significant resonance canonicals had two pi bonds involving d orbitals. His reasoning was that the charge on sulfur was thus reduced, in accordance with his principle of electroneutrality. The S−O bond length of 149 pm is shorter than the bond lengths in
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen Oxygen is the chemical ele ...
of 157 pm for S−OH. The double bonding was taken by Pauling to account for the shortness of the S−O bond. Pauling's use of d orbitals provoked a debate on the relative importance of pi 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 involving pπ – dπ bonding was initially proposed by Durward William John Cruickshank. In this model, fully occupied p orbitals on oxygen overlap with empty sulfur d orbitals (principally the d''z''2 and d''x''2–''y''2). However, in this description, despite there being some π character to the S−O bonds, the bond has significant ionic character. For sulfuric acid, computational analysis (with natural bond orbitals) confirms a clear positive charge on sulfur (theoretically +2.45) and a low 3d occupancy. Therefore, the representation with four single bonds is the optimal Lewis structure rather than the one with two double bonds (thus the Lewis model, not the Pauling model). In this model, the structure obeys the
octet rule The octet rule is a chemical rule of thumb that reflects the theory that main-group elements tend to bond in such a way that each atom Every atom is composed of a nucleus and one or more electrons bound to the nucleus. The nucleus is ...
and the charge distribution is in agreement with the electronegativity of the atoms. The discrepancy between the S−O bond length in the sulfate ion and the S−OH bond length in sulfuric acid is explained by donation of p-orbital electrons from the terminal S=O bonds in sulfuric acid into the antibonding S−OH orbitals, weakening them resulting in the longer bond length of the latter. However, the bonding representation of Pauling for sulfate and other main group compounds with oxygen is still a common way of representing the bonding in many textbooks. The apparent contradiction can be cleared if one realizes that the covalent double bonds in the Lewis structure in reality represent bonds that are strongly polarized by more than 90% towards the oxygen atom. On the other hand, in the structure with a dipolar bond, the charge is localized as a lone pair on the oxygen.


Preparation

Methods of preparing metal sulfates include: *treating metal, metal hydroxide, metal carbonate or metal oxide with
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen Oxygen is the chemical ele ...
: : : *oxidation of metal sulfides or sulfites


Properties

There are numerous examples of ionic sulfates, many of which are highly soluble in
water Water (chemical formula ) is an inorganic, transparent, tasteless, odorless, and nearly colorless chemical substance, which is the main constituent of Earth's hydrosphere and the fluids of all known living organisms (in which it acts a ...
. 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: if one adds a solution of most barium salts, for instance barium chloride, to a solution containing sulfate ions, barium sulfate will precipitate out of solution as a whitish powder. This is a common laboratory test to determine if 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. An example is the complex or the neutral metal complex where the sulfate ion is acting as a bidentate ligand. The metal–oxygen bonds in sulfate complexes can have significant covalent character.


Uses and occurrence


Commercial applications

Sulfates are widely used industrially. Major compounds include: * Gypsum, the natural mineral form of hydrated calcium sulfate, is used to produce
plaster Plaster is a building material Building material is material used for construction Construction is a general term meaning the art and science Science is a systematic endeavor that builds and organizes knowledge in the f ...
. About 100 million tonnes per year are used by the construction industry. *
Copper sulfate Copper sulfate may refer to: * Copper(II) sulfate, CuSO4, a common compound used as a fungicide and herbicide * Copper(I) sulfate, Cu2SO4, which is uncommonly used * Copper(II) sulfate, CuSO4 is greenish blue Copper compounds {{Chem-st ...
, a common algaecide, the more stable form ( ) is used for galvanic cells as electrolyte * Iron(II) sulfate, a common form of iron in mineral supplements for humans, animals, and soil for plants * Magnesium sulfate (commonly known as Epsom salts), used in therapeutic baths * Lead(II) sulfate, produced on both plates during the discharge of a lead–acid battery * Sodium laureth sulfate, or SLES, a common detergent in shampoo formulations * Polyhalite, , used as fertiliser.


Occurrence in nature

Sulfate-reducing bacteria, some anaerobic microorganisms, such as those living in sediment or near deep sea thermal vents, use the reduction of sulfates coupled with the oxidation of organic compounds or hydrogen as an energy source for chemosynthesis.


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. Green vitriol is
iron Iron () is a chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that species. Unlike chemical compounds, chemical elements ca ...
(II) sulfate heptahydrate, ; blue vitriol is
copper Copper is a chemical element with the Symbol (chemistry), symbol Cu (from la, cuprum) and atomic number 29. It is a soft, malleable, and ductility, ductile metal with very high thermal conductivity, thermal and electrical conductivity. A fre ...
(II) sulfate pentahydrate, and white vitriol is zinc sulfate heptahydrate, . Alum, a double sulfate of
potassium Potassium is the chemical element with the symbol K (from Neo-Latin '' kalium'') and atomic number19. Potassium is a silvery-white metal that is soft enough to be cut with a knife with little force. Potassium metal reacts rapidly with atmo ...
and
aluminium Aluminium (aluminum in American and Canadian English) is a chemical element A chemical element is a species of atoms that have a given number of protons in their nuclei, including the pure substance consisting only of that speci ...
with the formula , 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 An atmosphere () is a layer of gas or layers of gases that envelop a planet, and is held in place by the gravity of the planetary body. A planet retains an atmosphere when the gravity is great and the temperature of the atmosphere is low. ...
and form acid rain. The anaerobic sulfate-reducing bacteria '' Desulfovibrio desulfuricans'' and '' D. vulgaris'' can remove the black sulfate crust that often tarnishes buildings.


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 0.4 W/m2 relative to pre-industrial values, 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 greater numbers of smaller droplets of water. Many 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, 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/m2, although the uncertainty is very large. Sulfates are therefore implicated in global dimming. Sulfate is also the major contributor to stratospheric aerosol formed by oxidation of sulfur dioxide injected into the stratosphere by impulsive volcanoes such as the 1991 eruption of Mount Pinatubo in the
Philippines The Philippines (; fil, Pilipinas, links=no), officially the Republic of the Philippines ( fil, Republika ng Pilipinas, links=no), * bik, Republika kan Filipinas * ceb, Republika sa Pilipinas * cbk, República de Filipinas * hil, Republ ...
. This aerosol exerts a cooling effect on climate during its 1-2 year lifetime in the stratosphere.


Hydrogensulfate (bisulfate)

The hydrogensulfate ion (), also called the bisulfate ion, is the
conjugate base A conjugate acid, within the Brønsted–Lowry acid–base theory, is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entities) containing atom Every atom is com ...
of
sulfuric acid Sulfuric acid ( American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen Oxygen is the chemical ele ...
(). Sulfuric acid is classified as a strong acid; in aqueous solutions it ionizes completely to form hydronium () and hydrogensulfate () ions. In other words, the sulfuric acid behaves as a Brønsted–Lowry acid and is deprotonated to form hydrogensulfate ion. Hydrogensulfate has a valency of 1. An example of a salt containing the ion is sodium bisulfate, . In dilute solutions the hydrogensulfate ions also dissociate, forming more hydronium ions and sulfate ions ().


Other sulfur oxyanions


See also

* Sulfonate * Sulfation and desulfation of lead–acid batteries * Sulfate-reducing microorganisms


Notes


References

{{Sulfates Particulates Sulfur oxyanions