Sulfur Dioxide

Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO₂. This important gas is the main product from the combustion of sulfur compounds and is of significant environmental concern. SO₂ is often described as the "smell of burning sulfur" but is not responsible for the smell of rotten eggs.

SO₂ is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO₂, usually in the presence of a catalyst such as NO₂, forms H₂SO₄, and thus acid rain.^[2]

Preparation

Sulfur dioxide can be prepared by burning sulfur:

S₈ + 8 O₂ → 8 SO₂

The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly.

2 H₂S(g) + 3 O₂(g) → 2 H₂O(g) + 2 SO₂(g)

The roasting of sulfide ores such as iron pyrites, sphalerite (zinc blende) and cinnabar (mercury sulfide) also releases SO₂:

4 FeS₂(s) + 11 O₂(g) → 2 Fe₂O₃(s) + 8 SO₂(g)

2 ZnS(s) + 3 O₂(g) → 2 ZnO(s) + 2 SO₂(g)

HgS(s) + O₂(g) → Hg(g) + SO₂(g)

Sulfur dioxide is a by-product in the manufacture of cement: CaSiO₃ and CaSO₄ is heated with coke and sand in this process:

2 CaSO₄(s) + 2SiO₂(s) + C(s) → 2 CaSiO₃(s) + 2 SO₂(g) + CO₂(g)

Action of hot sulfuric acid on copper turnings produces sulfur dioxide.

Cu(s) + 2H₂SO₄(aq) → CuSO₄(aq) + SO₂(g) + 2H₂O(l)

Structure and bonding

SO₂ is a bent molecule with C_2v symmetry point group. In terms of electron-counting formalisms, the sulfur atom has an oxidation state of +4, a formal charge of 0, and is surrounded by 5 electron pairs and can be described as a hypervalent molecule. From the perspective of molecular orbital theory, most of these valence electrons are engaged in S-O bonding.

Reactions

Treatment of basic solutions with sulfur dioxide affords sulfite salts:

SO₂ + 2 NaOH → Na₂SO₃ + H₂O

Featuring sulfur in the +4 oxidation state, sulfur doxide is a reducing agent. It is oxidized by halogens such as chlorine to give the sulfuryl halides:

SO₂ + Cl₂ → SO₂Cl₂

However, on rare occasions, it can also act as an oxidising agent: in the Claus process, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:

SO₂ + 2 H₂S → 3 S + 2 H₂O

Uses

As a preservative

Sulfur dioxide is sometimes used as a preservative for dried apricots and other dried fruits due to its antimicrobial properties. The preservative is used to maintain the appearance of the fruit rather than to prevent rotting. Its presence can give fruit a distinctive chemical taste.

In winemaking

Sulfur dioxide is a very important element in winemaking, and is designated as parts per million in wine."E number: E220^[3] It is present even in so-called unsulphurated wine at concentrations of up to 10 milligrams per litre.Sulphites in wine, MoreThanOrganic.com. It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation. It also helps to keep volatile acidity at desirable levels. Sulfur dioxide is responsible for the words "contains sulfites" found on wine labels. Wines with SO₂ concentrations below 10ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of SO₂ allowed in wine is 350ppm in US, in the EU is 160 ppm for red wines and 210 ppm for white and rosé wines. In low concentrations SO₂ is mostly undetectable in wine, but at over 50ppm, SO₂ becomes evident in the nose and taste of wine.

SO₂ is also a very important element in winery sanitation. Wineries and equipment must be kept very clean, and because bleach cannot be used in a winery, a mixture of SO₂, water, and citric acid is commonly used to clean hoses, tanks, and other equipment to keep it clean and free of bacteria.

As a reducing bleach

Sulfur dioxide is also a good reductant. In the presence of water, sulfur dioxide is able to decolorize substances. Specifically it is a useful reducing bleach for papers and delicate materials such as clothes. This bleaching effect normally does not last very long. Oxygen in the atmosphere reoxidizes the reduced dyes, restoring the color.

Precursor to sulfuric acid

Sulfur dioxide is also used to make sulfuric acid, being converted to sulfur trioxide, and then to oleum, which is made into sulfuric acid. Sulfur dioxide for this purpose is made when sulfur combines with oxygen. The method of converting sulfur doxide to sulfuric acid is called the contact process.

Biochemical and biomedical roles

Sulfur dioxide is toxic but not extremely so. It or its conjugate base bisulfite is produced biologically as an intermediate in both sulfate-reducing organisms and in sulfur oxidizing bacteria as well. Sulfur dioxide has no role in mammalian biology. Sulfur dioxide blocks nerve signals from the pulmonary stretch receptors (PSR's) and abolishes the Hering-Breuer inflation reflex.

As a refrigerant

Being easily condensed and with a high heat of evaporartion, sulfur dioxide is a candidate material for refrigerants. Prior to the development of freons, sulfur dioxide was used as a refrigerant in home refrigerators.

As a reagent and solvent

Sulfur dioxide is a versatile inert solvent that has been widely used for dissolving highly oxidizing salts. It is also used occasionally as a source of the sulfonyl group in organic synthesis. Treatment of aryldiazonium salts with sulfur dioxide affords the corresponding aryl sulfonyl chloride.^[4]

Emissions

According to the U.S. EPA (as presented by the 2002 World Almanac or in chart form^[5]), the following amount of sulfur dioxide was released in the U.S. per year, measured in thousands of short tons:

*1999	18,867
*1998	19,491
*1997	19,363
*1996	18,859
*1990	23,678
*1980	25,905
*1970	31,161

Due largely to the US EPA’s Acid Rain Program, the U.S. has witnessed a 33 percent decrease in emissions between 1983 and 2002. This improvement resulted from flue gas desulfurization, a technology that enables SO₂ to be chemically bound in power plants burning sulfur-containing coal or oil. In particular, calcium oxide (lime) reacts with sulfur dioxide to form calcium sulfite:

CaO + SO₂ → CaSO₃

Aerobic oxidation converts this CaSO₃ into CaSO₄, gypsum. Most gypsum sold in Europe comes from flue gas desulfurization.

New fuel additive catalysts, such as ferox, are being used in gasoline and diesel engines in order to lower the emission of sulfur oxide gases into the atmosphere. This is also done by forcing the sulfur into stable mineral salts and mixed mineral sulfates as opposed to sulfuric acid and sulfur oxides.

As of 2006, China is the world's largest sulfur dioxide polluter, with 2005 emissions estimated to be 25.49 million tons. This amount represents a 27% increase since 2000, and is roughly comparable with U.S. emissions in 1980^[6].

Al-Mishraq, an Iraqi sulfur plant, was the site of a 2003 disaster resulting in the release of massive amounts of sulfur dioxide into the atmosphere.

Temperature dependence of aqueous solubility

22 g/100ml (0 °C)	15 g/100ml (10 °C)
11 g/100ml (20 °C)	9.4 g/100 ml (25 °C)
8 g/100ml (30 °C)	6.5 g/100ml (40 °C)
5 g/100ml (50 °C)	4 g/100ml (60 °C)
3.5 g/100ml (70 °C)	3.4 g/100ml (80 °C)
3.5 g/100ml (90 °C)	3.7 g/100ml (100 °C)

The values are tabulated for 101.3 kPa partial pressure of SO₂. Solubility of gas in a liquid depends on the gas partial pressure according to Henry's law.
The solublity is given for "pure water", i.e., water that contains only SO₂ in the amount at equilibrium with the gas phase. This "pure water" is going to be acidic. The solublity of SO₂ in neutral (or alkaline) water is generally going to be higher because of the pH-dependent speciation of SO₂ in the solution with the production of bisulfite and some sulfite ions.

Safety

According to Claude Ribbe in The Crime of Napoleon, sulfur dioxide gas was used as an execution poison by the French emperor to suppress a slave revolt in Haiti early in the 19th century.

References

1. ^ ttp://chemed.chem.purdue.edu/genchem/topicreview/bp/ch8/table.php
2. ^ Holleman, A. F.; Wiberg, E. "Inorganic Chemistry" Academic Press: San Diego, 2001. ISBN 0-12-352651-5.
3. ^ Current EU approved additives and their E Numbers, The Food Standards Agency website.
4. ^ R. V. Hoffman “m-Trifluoromethylbenzenesulfonyl chloride” Organic Syntheses, Collected Volume 7, p.508 (1990). [1]
5. ^ National Trends in Sulfur Dioxide Levels, United States Environmental Protection Agency.
6. ^ China has its worst spell of acid rain, United Press International.

External links

CAS registry numbers are unique numerical identifiers for chemical compounds, polymers, biological sequences, mixtures and alloys. They are also referred to as CAS numbers, CAS RNs or CAS #s.
..... Click the link for more information.

A chemical formula is a concise way of expressing information about the atoms that constitute a particular chemical compound. A chemical formula is also a short way of showing how a chemical reaction occurs.
..... Click the link for more information.

6
(strongly acidic oxide)
Electronegativity 2.58 (Pauling scale)
Ionization energies
(more) 1st: 999.6 kJmol⁻¹
2nd: 2252 kJmol⁻¹
3rd: 3357 kJmol⁻¹

Atomic radius 100 pm
Atomic radius (calc.
..... Click the link for more information.

2, −1
(neutral oxide)
Electronegativity 3.44 (Pauling scale)
Ionization energies
(more) 1st: 1313.9 kJmol⁻¹
2nd: 3388.3 kJmol⁻¹
3rd: 5300.5 kJmol⁻¹

Atomic radius 60 pm
Atomic radius (calc.
..... Click the link for more information.

Molar mass, symbol M,^[1] is the mass of one mole of a substance (chemical element or chemical compound).^[2] It is a physical property which is characteristic of each pure substance.
..... Click the link for more information.

In physics, density is mass m per unit volume V—how heavy something is compared to its size. A small, heavy object, such as a rock or a lump of lead, is denser than a lighter object of the same size or a larger object of the same weight, such as pieces of
..... Click the link for more information.

The melting point of a crystalline solid is the temperature range at which it changes state from solid to liquid. Although the phrase would suggest a specific temperature and is commonly and incorrectly used as such in most textbooks and literature, most crystalline compounds
..... Click the link for more information.

boiling point of a liquid is the temperature at which the vapor pressure of the liquid equals the environmental pressure surrounding the liquid.^[1]^[2]^[3]^[4]
..... Click the link for more information.

Solubility is a physical property referring to the ability for a given substance, the solute, to dissolve in a solvent.^[1] It is measured in terms of the maximum amount of solute dissolved in a solvent at equilibrium. The resulting solution is called a saturated solution.
..... Click the link for more information.

Water is a common chemical substance that is essential to all known forms of life.^[1] In typical usage, water refers only to its liquid form or state, but the substance also has a solid state, ice, and a gaseous state, water vapor.
..... Click the link for more information.

An acid dissociation constant, denoted by K_a, is an equilibrium constant for the dissociation of a weak acid. According to the Brønsted-Lowry theory of acids and bases an acid is only recognised by its reaction with a base.
..... Click the link for more information.

The debye (symbol: D) is a non-SI and non-CGS unit of electrical dipole moment. It is defined as 10^-18 statcoulomb centimeter (or 10^-20 esu m). In SI units, 1 D equals 3.33564*10^-30 coulomb meter. It is named after the physicist Peter J.
..... Click the link for more information.

Council Directive 67/548/EEC of 27 June 1967 on the approximation of laws, regulations and administrative provisions relating to the classification, packaging and labelling of dangerous substances (as amended) is the main European Union law concerning chemical safety.
..... Click the link for more information.

NFPA 704 is a standard maintained by the U.S. National Fire Protection Association. It defines the colloquial "fire diamond" used by emergency personnel to quickly and easily identify the risks posed by nearby hazardous materials.
..... Click the link for more information.

R-phrases (short for Risk Phrases) are defined in Annex III of European Union Directive 67/548/EEC: Nature of special risks attributed to dangerous substances and preparations.
..... Click the link for more information.

S-phrases are defined in Annex IV of European Union Directive 67/548/EEC: Safety advice concerning dangerous substances and preparations. The list was consolidated and republished in Directive 2001/59/EC , where translations into other EU languages may be found.
..... Click the link for more information.

The flash point of a flammable liquid is the lowest temperature at which it can form an ignitable mixture in air. At this temperature the vapor may cease to burn when the source of ignition is removed.
..... Click the link for more information.

Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO₃. In the gaseous form, this species is an important pollutant, being the primary agent in acid rain. It is prepared on massive scale as a precursor to sulfuric acid.
..... Click the link for more information.

Sulfuric (or sulphuric) acid, H₂SO₄, is a strong mineral acid. It is soluble in water at all concentrations. It was once known as oil of vitriol, coined by the 8th-century Arabian alchemist Jabir ibn Hayyan (Geber) after his discovery of the chemical.
..... Click the link for more information.

standard state of a material is its state at 1 bar (100 kilopascals exactly). This pressure was changed from 1 atm (101.325 kilopascals) by IUPAC in 1990.^[1] The standard state of a material can be defined at any given temperature, most commonly 25 degrees Celsius,
..... Click the link for more information.

Editing of this page by unregistered or newly registered users is currently disabled.
If you are prevented from editing this page, and you wish to make a change, please discuss changes on the talk page, request unprotection, log in, or .
..... Click the link for more information.

Hydrogen sulfide (hydrogen sulphide in British English) is the chemical compound with the formula H₂S. This colorless, toxic and flammable gas is responsible for the foul odor of rotten eggs and flatulence.
..... Click the link for more information.

Volcano:
1. Large magma chamber
2. Bedrock
3. Conduit (pipe)
4. Base
5. Sill
6. Branch pipe
7. Layers of ash emitted by the volcano
8. Flank 9. Layers of lava emitted by the volcano
10. Throat
11. Parasitic cone
12. Lava flow
13. Vent
14.
..... Click the link for more information.

Coal (IPA: /ˈkəʊl/) is a fossil fuel formed in swamp ecosystems where plant remains were saved by water and mud from oxidization and biodegradation.
..... Click the link for more information.

Petroleum (Latin Petroleum derived from Greek πέτρα (Latin petra) - rock + έλαιον (Latin oleum) - oil) or crude oil
..... Click the link for more information.

Nitrogen dioxide is the chemical compound NO₂. It is one of the several nitrogen oxides. This reddish-brown gas has a characteristic sharp, biting odor. NO₂ is one of the most prominent air pollutants and a poison by inhalation.
..... Click the link for more information.

The term "acid rain" is commonly used to mean the deposition of acidic components in rain, snow, fog, dew, or dry particles. The more accurate term is "acid precipitation." Distilled water, which contains no carbon dioxide, has a neutral pH of 7.
..... Click the link for more information.

This article is copied from an article on Wikipedia.org - the free encyclopedia created and edited by online user community. The text was not checked or edited by anyone on our staff. Although the vast majority of the wikipedia encyclopedia articles provide accurate and timely information please do not assume the accuracy of any particular article. This article is distributed under the terms of GNU Free Documentation License.

Herod_Archelaus

iPedia Free Information Center