Sulfur Dioxide

OVERVIEW

Sulfur dioxide (SUL-fur dye-OK-side) is a colorless gas with a sharp, harsh odor similar to that of a burning match. It can act as both an oxidizing agent and a reducing agent. An oxidizing agent is a substance that provides oxygen to other substances or provides electrons to them. A reducing agent removes oxygen from other substances or removes electrons from them. Sulfur dioxide dissolves readily in water, forming sulfurous acid (H₂SO₃), which is readily converted to sulfuric acid (H₂SO₄).

KEY FACTS

OTHER NAMES:

Sulfurous oxide; sulfurous anhydride

FORMULA:

SO₂

ELEMENTS:

Sulfur, oxygen

COMPOUND TYPE:

Nonmetallic oxide (inorganic)

STATE:

Gas

MOLECULAR WEIGHT:

64.06 g/mol

MELTING POINT:

−75.5°C (−104°F)

BOILING POINT:

−10.05°C (−13.91°F)

SOLUBILITY:

Soluble in water, ethyl alcohol, ether, and chloroform

Sulfur dioxide is a natural component of air. It is produced when trees, brush, and other organic matter burn, and it is present in volcanic gases. It is also released during the normal metabolic reactions of some living organisms, especially marine plankton and bacteria. The compound is also produced in large amounts by human activities. All fossil fuels (coal, oil, and natural gas) contain small amounts of sulfur as impurities. When those fuels burn, the sulfur they contain is converted to sulfur dioxide, which becomes a component of air pollution.

HOW IT IS MADE

Sulfur dioxide can be prepared by several methods, the most common of which is the combustion of sulfur or pyrites (FeS₂). A variety of furnaces have been developed for carrying out this reaction. Each type of furnace produces sulfur dioxide of different purities. After production, the sulfur dioxide is normally cooled and compressed to convert it to liquid form. Liquid sulfur dioxide is more easily stored and transported than the gaseous form. Sulfur dioxide is also obtained as the byproduct of a number of industrial operations, especially the smelting of metallic ores. Smelting is the process by which a metal is extracted from its ore by heating in air. Since many ores are sulfides, this process often results in the formation of sulfur dioxide, which can be captured as a byproduct of the operation. Finally, sulfur dioxide can be produced by the direct combustion of sulfur itself:

S + O₂ → SO₂

COMMON USES AND POTENTIAL HAZARDS

About 45 percent of all the sulfur dioxide produced in the United States is used in the manufacture of other chemical compounds, the most important of which is sodium bisulfite (NaHSO₃). Other compounds made from sulfur dioxide include sulfuric acid (H₂SO₄), chlorine dioxide (ClO₂), sodium dithionate (Na₂S₂O₆·2H₂O), and sodium thiosulfate (Na₂S₂O₃·5H₂O). Sulfur dioxide is also used as a bleaching agent for a number of products, including pulp and paper, textile fibers, straw, glue, gelatin, starches, grains, and various oils. The compound

Interesting Facts

• Sulfur dioxide was first studied in detail by the English physicist and chemist Joseph Priestley (1733–1804), who invented a method for collecting gases over water.
• The ancient Greeks and Romans fumigated their homes by burning sulfur. The sulfur dioxide formed destroyed microorganisms that cause disease and rot.
• The concentration of sulfur dioxide in clean air above the continents is less than one part per billion. Volcanic eruptions account for about half of all the gas produced by natural sources.

has a number of agricultural uses, especially in the treatment of soybeans and corn to destroy molds and preserve the product from decay. It is also used in the processing and refining of metal ores and petroleum. For example, it is added to some petroleum products to remove dissolved oxygen that would cause rust of pipes through which the products are distributed. Some other applications of the compound include:

• As a preservative for certain dried fruits and vegetables, such as cherries and apricots;
• As an additive in beers to prevent the formation of harmful products known as nitrosamines;
• As an additive in wines to prevent the growth of undesirable molds and other fungi;
• In the production of high-fructose corn syrups (HFCS) used as sweeteners in commercial food and drink products;
• As an antichlor in water purification systems;
• In the refining of sugar;
• In the manufacture of certain clay products to counteract the presence of compounds or iron and other metals that would impart color to the final product;

• In the molding and casting of magnesium parts and products; and
• In the sulfonation of oils, an important chemical process by which a sulfate group (−SO₃) is added to a compound.

Sulfur dioxide is a very toxic gas that can be an irritant to the eyes, the respiratory system, and, in some cases, the skin. At concentrations normally found in ambient air (the typical atmospheric environment surrounding us), these effects are annoying, but not particularly dangerous. Such is not the case for individuals with respiratory disorders, the young, or the elderly. Such individuals may experience more serious breathing problems that require medical attention. Higher concentrations of the gas may cause more serious problems, such as coughing, headache, dizziness, feelings of suffocation, and nausea. These conditions are most likely to occur in areas where air pollution is a problem, as in urban or industrial areas. People who are constantly exposed to relatively high concentrations of sulfur dioxide (such as smelter workers) may experience more serious long-term health problems, such as asthma, chronic bronchitis, lung disease, or emphysema.

The health effects of sulfur dioxide are serious enough that the compound is not allowed as a food additive in meats and other food products that contain vitamin B₁. The reason is that sulfur dioxide reacts with and destroys the vitamin. The U.S. Environmental Protection Agency (EPA) has determined that humans should not be exposed to a concentration of more than 0.030 parts per million on average throughout the year, or more than 0.14 parts per million over any one 24-hour period.

In addition to its health effects on humans, sulfur dioxide has some important consequences for the physical and biological environment. Those effects occur because sulfur dioxide released to the atmosphere from electricity-generating plants and factories combines with moisture in the air to form sulfuric acid. Sulfuric acid then falls to earth in the form of acid rain, acid snow, or some other form of acid precipitation where it damages buildings and other structures, trees and other plant life, and fish and other aquatic organisms. Since 1995, the EPA has sponsored a variety of control programs designed to reduce the release of sulfur dioxide into the atmosphere to prevent such problems.

Words to Know

ANTICHLOR

A chemical that reacts with excess chlorine used for purification, disinfecting, or some other purpose.

METABOLISM

All of the chemical reactions that occur in cells by which fats, carbohydrates, and other compounds are broken down to produce energy and the compounds needed to build new cells and tissues.

FOR FURTHER INFORMATION

"Acid Rain Program." U.S. Environmental Protection Agency. http://www.epa.gov/airmarkets/arp/ (accessed on November 15, 2005).

"SO₂—How Sulfur Dioxide Affects the Way We Live & Breathe." Office of Air Quality Planning and Standards, U.S. Environmental Protection Agency. November 2000. Also available online at http://www.epa.gov/air/urbanair/so2. (accessed on November 15, 2005).

"Sulfur Dioxide." Air Liquide. http://www.airliquide.com/en/business/products/gases/gasdata/index.asp?GasID=27 (accessed on November 15, 2005).

"Sulfur Dioxide." New Jersey Department of Health and Senior Services. http://www.state.nj.us/health/eoh/rtkweb/1759.pdf (accessed on November 15, 2005).

"ToxFAQs for Sulfur Dioxide." Agency for Toxic Substances and Disease Registry. http://www.atsdr.cdc.gov/tfacts116.html (accessed on November 15, 2005).

See AlsoSodium Sulfite; Sulfuric Acid

Sulfur Dioxide

Sulfur dioxide, with chemical formula SO₂, is a colorless gas with an irritating, choking odor. Normally a gas, sulfur dioxide has a density of 2.551 g/L and a solubility in water of 9.4 g per 100 mL (at 25°C). It has a melting point of−98.3°F (−72.4°C) and a boiling point of 14°F (−10°C).

Sulfur dioxide is produced naturally in volcanoes and in geysers, and is produced in ever-increasing amounts by the burning of sulfur-containing coal. Sulfur dioxide is a key contributor to acid rain. It is used in the manufacturing of sulfuric acid, in preserving many foods, as a disinfectant, and in the wine industry.

Sulfur dioxide is produced naturally inside the Earth by the reaction of sulfur with oxygen gas. This accounts for the sulfur-like smell around active volcanoes and geysers. Sulfur is a natural component of most coal, and it is the burning of sulfur-containing coal that has contributed the most to the acid rain problem in the world. When burned, the sulfur in coal reacts with oxygen and makes sulfur dioxide gas. This gas may react with water to make sulfurous acid (H₂SO₃, a moderately strong acid), or it may react with oxygen to form sulfur trioxide, SO₃, which then reacts with water in the atmosphere to produce sulfuric acid, H₂SO₄, the major contributor to acid rain. This same series of reactions is used to make sulfuric acid, the most widely produced chemical in the world. Efforts to reduce the amounts of sulfur dioxide in the atmosphere use two approaches. Low sulfur coal can be used, reducing emissions. However, as human society uses up more and more of the low sulfur coal, people are forced to use the remaining higher sulfur content coal. Many factories now use scrubbers, which put sulfur dioxide gas into contact with calcium oxide (CaO) to produce solid calcium sulfite, thus preventing the sulfur dioxide from reaching the atmosphere. The reaction is shown below:

CaO + SO₂ (gas) → CaSO₃

One drawback to this approach is that for every one ton of sulfur dioxide that reacts, two tons of solid calcium sulfite are produced, which must be disposed of.

Sulfur dioxide is used widely to prevent dried fruits (especially apricots and peaches) from becoming discolored. Sulfur dioxide is a strong reducing agent, and prevents the formation of discolored oxidation products in the fruits. It is also used to bleach vegetables and in the wine industry is used to prevent wines from discoloring and turning brown. In the food industry, sulfur dioxide is used as a disinfectant during the manufacturing process.

See also Volcano.

Sulfur dioxide

Sulfur dioxide, SO2, is a colorless gas with an irritating, choking odor. It is produced naturally in volcanoes and in geysers, and is produced in ever-increasing amounts by the burning of sulfur-containing coal . Sulfur dioxide is a key contributor to acid rain . It is used in the manufacturing of sulfuric acid , in preserving many foods, as a disinfectant, and in the wine industry.

Sulfur dioxide is produced naturally inside the earth by the reaction of sulfur with oxygen gas. This accounts for the "sulfur" smell around active volcanoes and geysers. Sulfur is a natural component of most coal, and it is the burning of sulfur-containing coal that has contributed the most to the acid rain problem in the world. When burned, the sulfur in coal reacts with oxygen and makes sulfur dioxide gas. This gas may react with water to make sulfurous acid (H2SO3, a moderately strong acid), or it may react with oxygen to form sulfur trioxide, SO3, which then reacts with water in the atmosphere to produce sulfuric acid, H2SO4, the major contributor to acid rain. This same series of reactions is used to make sulfuric acid, the most widely produced chemical in the world. Efforts to reduce the amounts of sulfur dioxide in the atmosphere use two approaches. Low sulfur coal can be used, reducing emissions. However, as we use up more and more of the low sulfur coal, we are forced to use the remaining higher sulfur content coal. Many factories now use scrubbers, which put sulfur dioxide gas into contact with calcium oxide (CaO) to produce solid calcium sulfite, thus preventing the sulfur dioxide from reaching the atmosphere. The reaction is shown below:

One drawback to this approach is that for every ton of sulfur dioxide that reacts, two tons of solid calcium sulfite are produced, and they must be disposed of.

Sulfur dioxide is used widely to prevent dried fruits (especially apricots and peaches) from becoming discolored. Sulfur dioxide is a strong reducing agent, and prevents the formation of discolored oxidation products in the fruits. It is also used to bleach vegetables and in the wine industry is used to prevent wines from discoloring and turning brown. In the food industry, sulfur dioxide is used as a disinfectant during the manufacturing process.

See also Volcano.

Sulfur Dioxide

Sulfur dioxide (SO2) is an air pollutant known primarily for its role in acid rain. SO2 is emitted naturally from volcanoes. Anthropogenic emissions arise largely from the production of electricity, particularly coal-fired power plants (65%). The sulfur in the coal reacts with oxygen during combustion, converting it to SO2. Scrubbers, using a slurry of limestone and water, are used to extract the SO2 before it exits the stack.

Once in the atmosphere, SO2 is converted to other compounds such as sulfuric acid (H2SO4), the primary contributor to acid rain. SO2 also reacts to form sulfate aerosols. These tiny airborne particles are the major cause of haze in U.S. national parks.

Both SO2 gas and sulfate aerosols cause breathing problems, particularly for people with existing respiratory illnesses such as asthma. For health reasons, to reduce acid rain, and to improve visibility, SO2 emissions are regulated by a market-based allowance trading system established by the U.S. Environmental Protection Agency (EPA).

see also Acid Rain; Coal; Electric Power; Scrubbers.

Bibliography

Turco, Richard P. (1997). Earth under Siege: From Air Pollution to Global Change. New York: Oxford University Press.

internet resource

Agency for Toxic Substances and Disease Registry (ATSDR). "ToxFAQ for Sulfur Dioxide." Available from http://www.atsdr.cdc.gov/tfacts116.html.

U.S. Environmental Protection Agency Web site. Available from http://epa.gov.

Marin Sands Robinson

Sulfur dioxide

Along with sulfur trioxide, one of the two common oxides of sulfur existing in the atmosphere . Sulfur dioxide is produced naturally in the atmosphere by the oxidation of hydrogen sulfide, a gas released from volcanoes and hot springs. About 22 million tons (20 million metric tons) of sulfur dioxide is released annually into the atmosphere by human activities, primarily during the combustion of fossil fuels . The amount of anthropogenic sulfur dioxide is roughly equal to that formed naturally from hydrogen sulfide. Sulfur dioxide reacts with water in clouds to form acid rain , causes damage in plants, and is responsible for a variety of respiratory problems in humans.