Overview

The sulfate ion, written SO42−, is the doubly charged anion that forms when sulfuric acid loses two protons. Known as sulphate in British English and sulfate in American English, it is a fundamental inorganic anion encountered widely in chemistry, geology, biology and industry. In simple terms a sulfate is any salt or ester containing this SO4 group; common salt examples include sodium sulfate and magnesium sulfate.

Structure and bonding

At the molecular level the sulfate ion consists of one central sulfur atom bonded to four oxygen atoms in a roughly tetrahedral arrangement. The four S–O interactions are equivalent through resonance, which distributes the negative charge over the oxygen atoms. Sulfur is formally in a high oxidation state (+6) in SO42−. For a concise chemical definition and context see chemical descriptions and references on ionic species such as the sulfate ion. The group of atoms that makes up the anion is often described as a polyatomic ion or an oxyanion; for a simple list of related compounds see atomic and molecular listings.

Sulfate is the result of sulfuric acid (H2SO4) being deprotonated twice. If only one proton is removed the intermediate hydrogen sulfate or bisulfate ion (HSO4−) is formed; this species is commonly written as hydrogen sulfate and is discussed in many acid–base resources (deprotonation processes, hydrogen sulfate). Sulfate also forms salts by combining with metal cations; typical examples include:

  • Na2SO4 (sodium sulfate)
  • MgSO4·7H2O (magnesium sulfate, Epsom salt)
  • CaSO4·2H2O (gypsum)
  • BaSO4 (barite, noted for its very low solubility)

Occurrence and uses

Sulfates occur naturally in minerals, in seawater, soils and in the atmosphere as aerosols. They are produced by weathering of rocks, oxidation of sulfur compounds, and biological processes. Industrially, sulfate salts and derived chemicals are important in agriculture (for example ammonium sulfate fertilizer), construction (plaster and cement additives), pharmaceuticals and bath salts, the paper and textile industries, and water treatment. The insoluble barium sulfate is widely used diagnostically in medical imaging as a radiopaque contrast agent because it passes through the digestive tract but is not absorbed.

Environmental and analytical aspects

Sulfate plays a role in environmental chemistry. Oxidation of sulfur dioxide to sulfate contributes to acid deposition and forms fine particulate matter that affects climate and air quality. In water chemistry sulfate influences hardness and can be measured by precipitation with barium ions to give barium sulfate, a classical gravimetric test. For practical notes on salts and handling see general resources on salts and ionic compounds and analytical techniques (oxygen-containing anions).

Distinctions and notable facts

Sulfate should be distinguished from related oxyanions such as sulfite (SO32−). Sulfate esters — organic compounds containing the −OSO3 group — are another important class, especially in biochemistry where sulfation modifies carbohydrates and proteins. The chemistry of sulfate is dominated by its strong association with cations and by its stability as the conjugate base of a strong acid, sulfuric acid.