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Sulfur dioxide

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Sulfur dioxide
Sulfur dioxide bonding.pngSulfur dioxide.png
Systematic name Sulfur dioxide
Other names sulphur dioxide
sulfurous anhydride
sulphurous anhydride
sulfur(IV) oxide
Molecular formula SO2
Molar mass Molar mass::64.07 g/mol
Appearance colorless gas
CAS number CAS number::7446-09-5
Density and phase [[Density:: 2.551]] (gas)
1.46 g/cm3 (liquid, −10 °C)
Solubility in water 22.97 g/100 mL (0 °C)
11.58 g/100 mL (20 °C)
9.4 g/100 mL (25 °C))
Melting point Melting point::−75.5 °C
Boiling point Boiling point::−10.0 °C
Acidity 1.81pKa
Viscosity 0.403 cP at 0°C
Molecular shape Bent, C2v
Dipole moment 1.63 D
MSDS Material safety data sheet
Main hazards Inhalation
NFPA 704

NFPA 704 svg.png

Flash point Non-flammable
R/S statement R: R23 R34
S: S1/2 S9 S26 S36/37/39 S45
RTECS number WS4550000
Related compounds
Other cations Selenium dioxide
Tellurium dioxide
Related Sulfur oxide Sulfur monoxide
Sulfur trioxide
Related compounds Sulfurous acids
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Disclaimer and references

Sulfur dioxide is an inorganic colorless gas with the molecular formula SO2. It has other names like sulphur dioxide, sulfur (IV) oxide, sulfurous anhydride, and sulphurous anhydride. In high concentration, it becomes toxic and harmful to the the environment, resulting in acid rain. The molecules of sulfur dioxide combine with water molecules forming acid rain. Also, it causes health issues such as respiratory problems at low concentration. Therefore, people with asthma need to be careful when handling sulfur dioxide. To handle it, wearing safety glasses, gloves and well ventilated surrounding are preferred in the laboratory. If you get sulfur dioxide in eyes or skin, wash it off with lots of water and then get medical help.


  • Sulphur superoxide
  • Sulphur dioxide
  • Sulfur superoxide
  • Fermenicide liquid
  • Fermenicide powder
  • Sulfurous acid anhydride [1]


Sulfur dioxide is colorless gas with pungent odor in nature at atmospheric temperatures and pressure. It has a 64.07 g/mol of molar mass. It starts to boil at -10 °C and melts at -73 °C. Its density is 2.551 g/L at gas form. It exist as a vapor regardless of pressure only at its critical temperature, 157.12°C. However, sulfur dioxide turns into a liquid when pressure becomes stronger temperature becomes cooler. The liquid form of sulfur dioxide has a gravity of 1.436 at 32°F (0°C) and 1.46 g/cm3 at −10 °C. It dissolves in acetone, methyl isobutyl ketone, acetic acid, alcohol, and sulfuric acid. Temperature and solubility of sulfur oxide are directly proportionally related to each other. Its acidity is 1.81pKa and viscosity is 0.403 cP at 0 °C. Liquid form of sulfur dioxide is heavier than water and the vapor form of it is heavier than air. [2] [3]

Structure and bonding

Sulfur dioxide (SO2) is comprised of the sulfur atom and two oxygen atoms. According to electron-counting formalisms, both the sulfur atom and the oxygen atoms have oxidation number of -2. Therefore, it forms a bent molecule with C2v symmetry point group and also called as a hypervalent molecule because of surrounding 5 electron pairs.
Both sulfur and oxygen have 6 valence electrons and those valence electrons are engaged in bonding of sulfur and oxygens. Despite of same number of valence electrons, molecular bonds in SO2 are different from those in ozone. Sulfur dioxide bonding is shorter than sulfur monoxide bonding. The bonding of two oxygens in SO2 is longer than dioxygen (O2). Also SO2 bonding has 548 kJ/mol of the mean bond energy than that of SO, 524 kJ/mol. From these results, it is referred that sulfur dioxide has at least bond order of 2 while the bonding of O2 has 1.5 in ozone. [3]


Reduction of higher oxides

2 CaSO4 + 2 SiO2 + C → 2 CaSiO3 + 2 SO2 + CO2
Cu + 2 H2SO4 → CuSO4 + SO2 + 2 H2O

From sulfite

H2SO4 + Na2S2O5 → 2 SO2 + Na2SO4 + H2O

Combustion routes

S8 + 8 O2 → 8 SO2
2 H2S + 3 O2 → 2 H2O + 2 SO2
4 FeS2 + 11 O2 → 2 Fe2O3 + 8 SO2
2 ZnS + 3 O2 → 2 ZnO + 2 SO2
HgS + O2 → Hg + SO2



Emission of Sulfur dioxide from Mount Etna, a volcano in Sicily, erupting in 2002

Natural Causes
Sulfur dioxides are made naturally through volcanic emissions, oxidized hydrogen sulfide, and ocean spray. Natural sulfur emissions are about one-third of the whole amount of sulfur emissions according to the scientists. For example, the Kilauea volcano on the island of Hawaii emits sulfur dioxides gas and other pollutants. Then the emitted sulfur dioxide gas combines with water, dust, sunlight and oxygen, resulting acid rain and volcanic smog around the area. Also, causing headache and respiratory problems to the residents, sulfur dioxide continuously has been emitted. The U.S Geological Survey reported the sulfur dioxide emission rates. [5]

Date SO2 (mt/d) Date SO2 (mt/d) Date SO2 (mt/d)
04/05/82 250 06/03/85 380 04/20/90 260
01/04/83 130 09/30/85 290 10/29/90 290
05/03/83 250 06/18/86 300 08/09/93 250
03/19/84 350 12/30/86 330 07/11/94 270
06/18/84 230 04/13/87 430 05/24/95 250
09/05/84 300 08/04/88 480 08/20/96 170
03/19/85 250 03/29/89 380 09/25/97 80

Human Causes
Sulfur dioxide is made not only by natural causes but also by human causes like oil, gas, or coal burnings. Residential heating, commercial heating, gasoline, burning of fuels by large ships and locomotives, fossil fuel combustion at industrial plants, and other fuels are examples of humans causing of sulfur dioxide emissions. However, metal smelters are other examples of non-energy sources of emission. According to the state and country emission summaries on U.S. Environmental Protection Agency, fossil fuel combustion takes a first place of causes of sulfur dioxide in Washington state in 2005. It rates 73% and other industrial processes about 20%. Estimately 15,584 tons of sulfur dioxide has been emitted from fossil fuel combustion, 14,906 tons from industrial process, 7,672 tons from non-road equipment, 3,418 tons from electricity generation, 2,823 tons from on road vehicles, 504 tons from fires, 486 tons from waste disposal, and 335 tons from residential wood combustion. [7] In 1971, the Environmental Protection Agency (EPA) made a standard on Sulfur dioxide to protect people from extensive exposure. A 24-hour primary standard is at 140 parts-per-billion (ppb) and a 3-hour average standard is at 500 ppb. Also, an annual average standard is at 30 ppb. [5]


Although sulfur dioxide is usually considered as a pesticide, it is used in food and drink industries as an antioxidant or preservative. To healthy people, sulfur dioxide does not harm them if it is used in some recommended concentrations. In the United Kingdom, there is regulation on how much foods contain sulfur dioxide. However, in U.S.A, recommendations of "good manufacturing practice" regulate the amount of it. [8]

Acid Rain
Sulfur dioxide forms sulfuric acid when it meets water molecule at atmosphere. Its process to produce sulfuric acid is called a contact process. Through this process, about several billion kilograms of sulfuric acids are made every year. [9]
Sulfur dioxide works as a preservative in dried fruits, especially dried apricots. When it is used as a preservative, it is called E220. It does not change the shape of fruits and prevents rotting. [9]
Sulfur dioxide is mainly used in winemaking as an antibiotic and antioxidant. There are unsulfured wines containing 10 milligrams of sulfur dioxide per liter. As a antioxidant and antibiotic, it prevents bacteria and oxidation to spoil wines and maintain volatile acidity. When a winemaker puts sulfur dioxide, he needs to label it "contains sulfites" if it contains above 10 parts-per-million (ppm). However, under 10 ppm, he does not need to label them.
Moreover, sulfur dioxide plays an important part in winery sanitation. Through its acidity, it is used to clean the equipments used in winemaking in the way not to affect the wine.[9]
Sulfur dioxide also works as a reductant. It has bleaching ability to reduce bleach for papers and clothes, and declines bleaches and mixed chlorine into chloride. [9]
Sulfur dioxide can carry out evaporation with pretty high heat, and then it can work as a refrigerants. Therefore, It is usually used in home refrigerators. [9]
Biochemical & Biomedical
With a little amount of sulfur dioxide, it can be used in biochemical and biomedical fields, excluding mammalian biology.[9]
Reagent and Solvent
Sulfur dioxide has been used to dissolve oxidizing salts because of its inert characters. [9]

Emission Fact

Sulfur dioxide emissions can be estimated through fuel use, sulfur content, and consumption rates. It is hard to get exact number for amount of sulfur emissions, since public data and inspecting infrastructure are not available. The equation is:

Sulfur Dioxide Estimate = E[FuelUse x Sulfur Contentx(1-f_ash)x(1-f_contorl)]

In the equation, 'f_ash' means the fraction of the sulfur reserved in ash, and 'f_control' means the fraction extracted by emission controls. Through the estimated amount of sulfur dioxide emissions, the declining of sulfur dioxide in U.S.A. and Western Europe have been detected while China continuously shows an incline due to its large amount of coal. [10]



  • Avoid exercise or play at high air pollution level.
  • People with asthma need to stay indoors.
  • People with asthma or respiratory problems should avoid foods containing sulfur dioxide or other sulfites.
  • Use proper fuels.
  • Use vented space heater.
  • Regularly check whether there is any leak of sulfur dioxide.
  • Use stoves that fit into Environment Protection Agency's standards.[11]

How to lessen emissions

  • Place the exhaust fan above the stove and use it frequently.
  • Open a flue while burning.
  • Do not waste engines.
  • Do not use gasoline-powered appliances.
  • Use adequate ventilation.[11]


  • Use ovens and heating equipments that run an electricity.
  • Use appliances that have high-leveled ventilation.
  • Try to use public transportation, or walk short distances if possible.
  • Eat foods with less sulfur dioxide.
  • Eat fresh, organic fruits and vegetables.



Sulfur dioxide has high solubility in water. The solubility of sulfur dioxide is about 200 g/so2/L at room temperature. That means that 76 mL of sulfur dioxide can be dissolved in 1mL of water. Sulfur dioxide shows the relationship between temperature and solubility. At 0°C, it has 228 g/L solubility and at 90°C, it is 5.8 g/L. Also at room temperature, it shows exothermic process of dissolving. [12]

SO2(g) ---> SO2(aq)  DHo = -95.6 kJ

Temperature dependence of aqueous solubility

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


Air pollution

Emission of sulfur dioxide from Halemaumau Crater near the summit of Kilauea Volcano, Hawaii

According to the United States Environmental Protection Agency (EPA), the emission of sulfur dioxide has been constantly occurring from 1970, leading not only to human health problems, but also endangering to the whole environment.

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

Sulfur dioxide is a main precursor of acid rain along nitrogen oxides. When it meets water molecules at the atmosphere, it forms acids which will later cause acid rains. Acid rain acidifies lakes, streams, soils, and does not offer a beneficial environment to the animals living in those areas. It also corrodes buildings, monuments, and statues especially made of limestone. It even discolors materials through corrosions. Fine particulate soot is another result from sulfur dioxides. It causes people some health problems.
The Environment Protection Agency has made the National Ambient Air Quality Standard of sulfur dioxide to monitor its emissions. It also points out sulfur dioxide as the world's longest concern of the sulfur oxides gases. In the extent of atmospheric concentration, sulfur dioxide gases exceed the amounts of sulfur monoxide and sulfur trioxides. Sulfur dioxide also brings tn the production of other sulfur oxides. The more sulfur dioxides is produced, the more other sulfur oxides will be made. [13]


Sulfur dioxide is simply toxic and corrosive, especially in high concentration of sulfur dioxide when it becomes a severe threat to the respiratory system at low concentration. Asthmogenic people need to be careful to treat sulfur dioxide since it may cause respiratory problems. If one experiments with sulfur dioxide, one should consider ventilation to make sure no direct exposure of it occurs. When sulfur dioxide combines with moisture, it becomes a corrosive and acidic solution which can result in burns if contact to skin or eyes occurs.
To deal with sulfur dioxide, one needs to wear safety glasses and to make sure that it does not leak into the air. Also, if needed, gloves made of neoprene, nitrile or natural rubber are good to use during experiments. If one gets sulfur dioxide on one's eyes, one should fully wash it out with lots of water and ask for a doctor's assistance. If it touches the skin, like the way how one treats eye contact, washes it out with water and gets medical help. However, if one inhales sulfur dioxide gas, one needs to get immediate medical help. One may show signs of distress.
One needs to be careful to throw sulfur dioxide away. Unless the local laws do not forbid, a little amount of sulfur dioxide solution with lots of water can be flushed down in a sink. More amounts of it must be neutralised to throw them away. [14]


  1. Safety data for sulfur dioxide Safety Officer in Physical Chemistry at Oxford University. Sep 1, 2006
  2. Sulfur dioxide and its properties TRAMFLOC, INC., Accessed on Feb 5.2011.
  3. 3.0 3.1 3.2 sulfur dioxide New World Encyclopedia contributors, New World Encyclopedia, November 1. 2008.
  4. Sulfur dioxide New world Encyclopedia. Accessed on Feb 1, 2011
  5. 5.0 5.1 Acid Rain Sources of Sulfate nitrate in the atmosphere Accessed on Feb.16 2011
  6. SUMMIT SO2 EMISSION RATES - VEHICLE-BASED 1982-1997 Accessed on Feb 16, 2011
  7. State and Country Emission Summaries U.S. Environmental Protection Agency. Accessed on Feb 17,2011
  8. Sulphur dioxide in foods and beverages: its use as a preservative and its effect on asthmaFreedman BJ. Br J Dis Chest. Apr, 1980
  9. 9.0 9.1 9.2 9.3 9.4 9.5 9.6 Sulfur dioxide Wikipedia. Accessed on Feb 17, 2011
  10. Sulfur Dioxide in China Science & Society. Xia Zhang Paster & Noemi Smith. Dec 03, 2007
  11. 11.0 11.1 11.2 sulfur dioxide Chemical Encyclopedia. Accessed on Feb 16, 2011
  12. Sulfur dioxide information Accessed on Feb 16, 2011
  13. Health US EPA. Accessed on Feb 16,2011
  14. Chemical Safety Data: Sulphur dioxide Accessed on Feb 16, 2011

Additional Information