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Sulfur dioxide

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Sulfur dioxide Basic information

Product Name:
Sulfur dioxide
Mol File:

Sulfur dioxide Chemical Properties

Melting point:
−73 °C(lit.)
Boiling point:
−10 °C(lit.)
1.25 g/mL at 25 °C(lit.)
vapor density 
2.26 (21 °C, vs air)
vapor pressure 
1779 mm Hg ( 21 °C)
colorless gas
Pungent odor detectable at 0.3 to 5 ppm
Odor Threshold
Water Solubility 
%H2O: 17.7 (0°C), 11.9 (15°C), 8.5 (25°C), 6.4 (35°C); % other solvents: 25, alcohol; 32, methanol [MER06]
Exposure limits
TLV-TWA 2 ppm (~5 mg/m3) (ACGIH), 5 ppm (OSHA and MSHA); IDLH 100 ppm (NIOSH).
Stable. Non-corrosive when dry to common materials except zinc; corrosive when wet. Incompatible with strong reducing or oxidizing agents, moisture, zinc and its alloys.
CAS DataBase Reference
7446-09-5(CAS DataBase Reference)
NIST Chemistry Reference
Sulfur dioxide(7446-09-5)
EPA Substance Registry System
Sulfur dioxide (7446-09-5)

Safety Information

Hazard Codes 
Risk Statements 
Safety Statements 
UN 2037 2.3
WGK Germany 
Hazardous Substances Data
7446-09-5(Hazardous Substances Data)
LC50 inhal (rat) 2520 ppm (6590 mg/m3; 1 h)
LCLO inhal (human) 1000 ppm (2600 mg/m3; 10 min)
PEL (OSHA) 5 ppm (13 mg/m3)
TLV-TWA (ACGIH) 2 ppm (5.2 mg/m3)
STEL (ACGIH) 5 ppm (13 mg/m3)



Sulfur dioxide Usage And Synthesis

Physical Properties

Colorless gas; pungent suffocating odor; gas density 2.927 g/L at 20°C; heavier than air, vapor density 2.263 (air=1); condenses to a colorless liquid at –10°C; density of liquid SO2 1.434 g/mL; freezes at –72.7°C; critical temperature 157.65°C; critical pressure 77.78 atm; critical volume 122 cc/g; dielectric constant 17.27 at –16.5°C; dissolves in water forming sulfurous acid, solubility 22.97 g and 11.58 g/100mL water at 0° and 20°C, respectively, under atmospheric pressure; very soluble in acetone, methyl isobutyl ketone, acetic acid, and alcohol; soluble in sulfuric acid; liquid SO2 slightly miscible in water.


Sulfur dioxide has a wide range of industrial applications, the most notable being as an intermediate in producing sulfuric acid. It is used to produce chlorine dioxide, sodium bisulfite, and sodium dithionate, which are all bleaching agents. Also, sulfur dioxide itself is a bleaching agent used in bleaching textile fibers, straw, glue, gelatin, and other substances. Sulfur dioxide is used extensively in the food industry to preserve vegetables and dry fruits such as apricots and cherries; as a bleach and steeping agent for grains; to prevent growth of bacteria in processing soy proteins; in refining sugar; as an additive to wine to destroy molds, bacterias, and undesired wild yeast; to prevent formation of nitrosamines in beer in the malting process; and in producing highfructose corn syrups.
Sulfur dioxide is used in removing oxygen in petroleum recovery processes to prevent corrosion in piping and storage systems. Also, it is applied in water treatment to reduce residual chlorine. In clay processing it reduces iron compounds and other color-forming impurities. Other uses are extracting sulfide ores; casting magnesium; catalyst modifier in certain organic oxidation reactions; and colorimetric analysis of sulfite ion in aqueous samples. In the chemical industry, sulfur dioxide is used as a reducing agent in a number of preparative and analytical reactions. Liquid sulfur dioxide is used as a solvent for sulfur trioxide in sulfonation.
Sulfur dioxide occurs in nature in volcanic gases. It is found in the upper atmosphere at varying but trace concentrations resulting from natural processes and human activities. It occurs in the automotive exhausts and in many fire and stack emission gases. It is produced from burning sulfur–containing fuels, such as coal and oil. Also, it is produced in the petroleum refining process. Sulfur dioxide is partly responsible for causing acid rain.


Sulfur dioxide is manufactured mostly by combustion of sulfur or its iron sulfide mineral, pyrite, FeS2 , in air. The flame temperatures for such combustion of sulfur in the air are usually in the range 1,200 to 1,600°C. Many types of sulfur burners are available and are used to produce sulfur dioxide. They include rotary-kiln, spray, spinning-cup and air-atomizing sulfur burners. Selection and design of burners depend on quality of sulfur to be burned, and rate and concentration of sulfur dioxide to be generated. Pyrites or other metal sulfides may be burned in air in fluid-bed roasters to form sulfur dioxide. Other sources of sulfur dioxide are flue gases and spent sulfuric acid.
Sulfur dioxide may be recovered from stack gases in smelting or power plants. Similarly, SO2 can be generated from spent sulfuric acid recovered from oil refineries. The spent acid is burned in a high temperature furnace above 900°C to form sulfur dioxide, water, and gaseous products.
Liquid sulfur dioxide can be produced by passing a stream of sulfur trioxide, SO3, through molten sulfur:
2SO3 + S → 3SO2
Trace sulfur trioxide in the product may be removed by passing the product sulfur dioxide through sulfuric acid.


Sulfur dioxide is highly toxic. It causes severe irritation of eyes, skin, and respiratory tract. Effects are coughing, suffocation, and bronchial constriction.

Chemical Properties

Sulfur dioxide has a sharp, pungent odor. It has been employed in food processing as a sanitizing agent for food containers and fermentation equipment, as a preservative to reduce or prevent microbial spoilage of food, as a selective inhibitor of undesirable microorganisms in the fermentation industry and as an antioxidant and inhibitor of enzyme-catalyzed oxidative discoloration and nonenzymatic browning, during preparation, storage and distribution of many foods. It is shipped as a liquid under pressure. Its vapor density is 2.26 times that of air at atmospheric pressure and 0°C. The specific gravity of the liquid is about 1.436 at 0°/4°C.

Chemical Properties

Sulfur dioxide occurs as a colorless gas at room temperature and pressure, with a strong, suffocating, pungent odor. It is noncombustible and is a strong reducing agent.

Chemical Properties

Sulfur dioxide,S02, also known as sulfurous acid anhydride,is a toxic, irritating, colorless gas. It is soluble in water, alcohol, and ether, and boils at -10 °C. Sulfur dioxide is used as a chemical intermediate, in artificial ice, in paper pulping,in ore refining,and as a solvent.

Chemical Properties

Sulfur dioxide is a colorless gas. It is stable, and non-corrosive when dry to common materials except zinc. Sulfur dioxide is corrosive when wet and incompatible with strong reducing or oxidizing agents, moisture, zinc, and its alloys. Sulfur dioxide has a large number of industrial applications. For instance, sulfur dioxide is used in the manufacture of sodium sulfi te, sulfuric acid, sulfuryl chloride, thionyl chloride, organic sulfonate,extensively as a bleaching agent, particularly in the bleaching of beet sugar, fl our, straw, textiles, and wood pulp. Sulfur dioxide has industrial utility in the tanning of leather, in brewing and preserving. Sulfur dioxide is a colorless gas with a characteristic and strong suffocating odor. Sulfur dioxide gas is released primarily from the combustion of fossil fuels (75%–85% of the industrial sources), the smelting of sulfi de ores, volcanic emissions, and several other natural sources. It is a US EPA priority air pollutant, but has many industrial and agricultural uses. It is sometimes added as a warning marker and fi re retardant to liquid grain fumigants.

Chemical Properties

Sulfur dioxide is a noncombustible colorless gas at ambient temperatures with a characteristic, strong, suffocating odor. The Odor Threshold is 1.1ppm. Shipped as a liquefied compressed gas.


Reported found in orange juice, grapefruit juice, onion, boiled and cooked beef,starfruit and weinbrand brandy.


Sulfur dioxide is used as a bleaching andfumigating agent; as a disinfectant, for treat ing wood pulp for manufacturing paper, inmetal refining, for preserving food and vegetables, and as a reducing agent. It is a majorair pollutant and is produced when soft coal,oils, or other sulfur-containing substances areburned. Automobile exhaust gases also contribute to air pollution. Sulfur dioxide in theatmosphere reacts with moisture to form sulfurous acid, or is oxidized to sulfur trioxide,which forms sulfuric acid, causing acid rain.


Sulfur Dioxide is a preservative, being a gas that dissolves in water to yield sulfurous acid. sulfite salts, such as sodium and potassium sulfite, sodium and potassium bisulfite, and sodium and potassium metabisulfite, yield free sulfurous acid at low ph. sulfur dioxide pre- vents the discoloration of foods by combining with the sugars and enzymes. it also inhibits bacterial growth. it is used in beverages, cherries, wines, and fruits.


Preserving fruits, vegetables, etc.; disinfectant in breweries and food factories; bleaching textile fibers, straw, wicker ware, gelatin, glue, beet sugars. Solvent and reagent in organic synthesis.

Production Methods

Sulfur dioxide can be made by burning sulfur, or by roasting sulfide ores such as pyrites, sphalerite, and cinnabar.


Sulfur dioxide usually is prepared industrially by the burning in air or oxygen of sulfur or such compounds of sulfur as iron pyrite or copper pyrite. Large quantities of sulfur dioxide are formed in the combustion of sulfur-containing fuels. In the laboratory, the gas may be prepared by reducing sulfuric acid (H2SO4) to sulfurous acid (H2SO3), which decomposes into water and sulfur dioxide, or by treating sulfites (salts of sulfurous acid) with strong acids, such as hydrochloric acid, again forming sulfurous acid.

Aroma threshold values

Recognition: 50 to 100 ppm

Air & Water Reactions

Dissolves in water to form sulfurous acid, a corrosive liquid. Moist Sulfur dioxide is very corrosive due to the slow formation of sulfuric acid [Handling Chemicals Safely 1980 p. 876].

Reactivity Profile

Sulfur dioxide is acidic. Reacts exothermically with bases such as amines, amides, metal oxides, and hydroxides. Frequently used as a reducing agent although Sulfur dioxide is not a powerful one. Acts as a reducing bleach to decolorize many materials. Can act as an oxidizing agent. Supports combustion of powdered aluminum [Mellor 5:209-212 1946-47]. Reacts explosively with fluorine [Mellor 2:1 1946-47]. Supports burning of manganese [Mellor 12:187 1946-47]. Readily liquefied by compression. Contact between the liquid and water may result in vigorous or violent boiling and extremely rapid vaporization. If the water is hot an explosion may occur. Pressures may build to dangerous levels if the liquid contacts water in a closed container [Handling Chemicals Safely 1980]. Supports incandescent combustion of monocesium acetylide, monopotassium acetylide, cesium oxide, iron(II) oxide, tin oxide, and lead oxide [Mellor]. Ethylene oxide and SO2 can react violently in pyridine solution with pressurization if ethylene oxide is in excess (Nolan, 1983, Case History 51).


Toxic by inhalation, strong irritant to eyes and mucous membranes, especially under pressure. Dangerous air contaminant and constituent of smog. Not permitted in meats and other sources of vitamin B1. U.S. atmospheric standard 0.140 ppm. Pulmonary function inhibitor and lower respiratory tract irritant. Questionable carcinogen.

Health Hazard

Sulfur dioxide may cause death or permanent injury after very short exposure to small quantities. 1,000 ppm causes death in from 10 minutes to several hours by respiratory depression. It is an eye and respiratory tract irritant. Persons with asthma, subnormal pulmonary functions or cardiovascular disease are at a greater risk.

Health Hazard

Exposures to sulfur dioxide cause adverse health effects to users and occupational workers. The gaseous sulfur dioxide is particularly irritating to the mucous membranes of the upper respiratory tract. Chronic exposure to sulfur dioxide produces dryness of the throat, cough, rhinitis, conjunctivitis, corneal burns, and corneal opacity. Acute exposure to high concentrations of sulfur dioxide may also result in death due to asphyxia. By contrast, chronic exposures to sulfur dioxide lead to nasopharyngitis, fatigue, and disturbances of the pulmonary function. Animals exposed to chronic doses of sulfur dioxide have shown thickening of the mucous layer in the trachea and also hypertrophy of goblet cells and mucous glands resembling the pathology of chronic bronchitis. It has been found that penetration of sulfur dioxide into the lungs is greater during mouth breathing than during nose breathing. In fact, an increase in the fl ow rate of the gas would markedly increase the penetration. Human subjects exposed for very brief periods to sulfur dioxide also showed alterations in pulmonary mechanics. More information on the adverse effects of sulfur dioxide and the manner of its potentiation in association with other chemicals may be found in literature.

Health Hazard

Exposure to sulfur dioxide can causesevere irritation of the skin, eyes, mucousmembranes, and respiratory system. Theeffects are coughing, choking, or suffocation;bronchoconstriction; and skin burn. A 10-minute exposure to 1000 ppm can be fatalto humans
LC50 value, inhalation (mice): 3000 ppm/30min (NIOSH 1986).

Fire Hazard

Sulfur dioxide is a noncombustible substance (NFPA rating = 0).

Fire Hazard

Containers may explode in heat of fire or they may rupture and release irritating toxic Sulfur dioxide. Sulfur dioxide has explosive properties when Sulfur dioxide comes in contact with sodium hydride; potassium chlorate at elevated temperatures; ethanol; ether; zinc ethylsulfurinate at very cool temperatures (-15C); fluorine; chlorine trifluoride and chlorates. Sulfur dioxide will react with water or steam to produce toxic and corrosive fumes. When the liquid is heated Sulfur dioxide may release irritating, toxic Sulfur dioxide gas. Avoid ammonia, monocesium or monopotassium acetylide; dicesium monoxide; iron (II) oxide; tin oxide; lead (IV) oxide; chromium; manganese; molten sodium, powder aluminum and rubidium. Sulfur dioxide has explosive properties when Sulfur dioxide comes in contact with sodium hydride; potassium chlorate at elevated temperatures; ethanol; ether; zinc ethylsulfurinate at very cool temperatures (-15C); fluorine; chlorine trifluoride and chlorates. Sulfur dioxide will react with water or steam to produce toxic and corrosive fumes. Hazardous polymerization may not occur.

Flammability and Explosibility

Sulfur dioxide is a noncombustible substance (NFPA rating = 0).

Pharmaceutical Applications

Sulfur dioxide is used as an antioxidant for pharmaceutical injections. It is also used as a preservative and antioxidant in the food and cosmetics industries.

Safety Profile

A poison gas. Experimental reproductive effects. Human mutation data reported. Human systemic effects by inhalation: pulmonary vascular resistance, respiratory depression, and other pulmonary changes. Questionable carcinogen with experimental tumorigenic and teratogenic data. It chiefly affects the upper respiratory tract and the bronchi. It may cause edema of the lungs or glottis, and can produce respiratory paralysis. A corrosive irritant to eyes, skin, and mucous membranes. This material is so irritating that it provides its own warning of toxic concentration. Levels of 400-500 ppm are immediately dangerous to life. Its toxicity is comparable to that of hydrogen chloride. However, less than fatal concentration can be borne for fair periods of time with no apparent permanent damage. It is a common air contaminant. A nonflammable gas. It reacts violently with acrolein, Al, CsHC2, Cs2O, chlorates, ClF3, Cr, FeO, F2, Mn, KHC2, KClO3, Rb2C2, Na, Na2C2, SNOx diaminolithiumacetylene carbide. Will react with water or steam to produce toxic and corrosive fumes. Incompatible with halogens or interhalogens, lithium nitrate, metal acetylides, metal oxides, metals, polymeric tubing, potassium chlorate, sodium hydride.


Sulfur dioxide is used in food and pharmaceutical products. However, in large amounts, sulfur dioxide gas is highly irritant to the eyes, skin, and mucous membranes. Inhalation can lead to severe irritation of the respiratory tract. Direct contact with the liquid form may cause frostbite. Sulfur dioxide and sulfites may also cause allergic reactions and asthma.

Potential Exposure

Sulfur dioxide is used in the manufacture of sodium sulfite, sulfuric acid; sulfuryl chloride; thionyl chloride; organic sulfonates; disinfectants, fumigants, glass, wine, ice, industrial and edible protein; and vapor pressure thermometers. It is also used in the bleaching of beet sugar, flour, fruit, gelatin, glue, grain, oil, straw, textiles, wicker ware; wood pulp; and wool; in the tanning of leather; in brewing and preserving; and in the refrigeration industry. Exposure may also occur in various other industrial processes as it is a by-product of ore smelting, coal and fuel oil combustion; paper manufacturing and petroleum refining.


Sulfur dioxide may act as a cancer promoter. The mortality of arsenic smelter workers was higher when they had also been exposed to sulfur dioxide. In addition, rats exposed to 3.5 or 10 ppm of sulfur dioxide developed squamous cell carcinomas from inhalation of benzo[a]pyrene, but neither compound alone produced carcinomas under the conditions of this experiment.


Sulfur dioxide is noncorrosive and stable when dry. It is usually stored under pressure in cylinders, and should be kept in a cool, dry, well-ventilated area, away from flammable materials.


UN1079 Sulfur dioxide, Hazard Class: 2.3; Labels: 2.3-Poisonous gas, 8-Corrosive material, Inhalation Hazard Zone C. Cylinders must be transported in a secure upright position, in a well-ventilated truck. Protect cylinder and labels from physical damage. The owner of the compressed gas cylinder is the only entity allowed by federal law (49CFR) to transport and refill them. It is a violation of transportation regulations to refill compressed gas cylinders without the express written permission of the owner

Purification Methods

Dry it by bubbling through conc H2SO4 and by passage over P2O5, then through a glass-wool plug. Freeze it with liquid air and pump it to a high vacuum to remove dissolved gases. It is easily liquefied by compression (2.5atmospheres at 15o), or by passing it through a glass spiral column in a freezing mixture of ice and salt. It is a colourless liquid with a density of 1.434 at 0o, which on rapid evaporation forms a snow white solid. It could be used as a solvent in certain reactions. HARMFUL SUFFOCATING VAPOURS.


Reacts with water to form sulfurous acid, a medium-strong acid. Reacts violently with ammonia, acrolein, acetylene; alkali metals; such as sodium, potassium, magnesium, and zinc; chlorine, ethylene oxide; amines, butadiene. Attacks many metals including aluminum, iron, steel, brass, copper, nickel; especially in presence of water or steam. Incompatible with halogens. Attacks plastics, rubber and coatings.


Sulfur dioxide reacts vigorously with strong alkalis and oxidizing agents. The moist gas corrodes most metals. Sulfur dioxide is incompatible with chlorates, fluorine, interhalogens, powdered metals, metal oxides, metal acetylides, sodium hydroxide, and diethyl zinc. It is also incompatible with thiamine and gelatin.

Waste Disposal

Return refillable compressed gas cylinders to supplier. Pass into soda ash solution, then add calcium hypochlorite; neutralize and flush to sewer with water (A-38).


Occupational workers should be careful at workplaces because exposure to sulfur dioxide occurs from breathing it in the air. It affects the lungs and at high levels may result in burning of the nose and throat, breathing diffi culties, and severe airway obstructions.

Regulatory Status

GRAS listed. Accepted for use as a food additive in Europe. Included in the FDA Inactive Ingredients Database (IV infusions; injection solutions). Included in the Canadian List of Acceptable Non-medicinal Ingredients.


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