Sulfur dioxide

Physical 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.

Uses

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.

Production

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.

Toxicity

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

Description

Sulfur dioxide is a compound formed by the combination of the elements sulfur and oxygen. On a weight basis, the proportion of the elements is about I part sulfur to I part oxygen, or more exactly, 50.05 percent to 49.95 percent, respectively. At standard conditions of temperature and pressure, sulfur dioxide is a colorless gas with a characteristic pungent odor. It may be cooled and compressed to a colorless liquid, which, at one atmosphere pressure, boils at 14°F (10.0°C) and freezes at -104.6°F (-75.9°C). Sulfur dioxide liquid is heavier than water, and has a specific gravity of 1.436 at 32°F (0°C). As a gas, it is more than twice as heavy as air; its relative density is 2.2638 at atmospheric pressure and 32°F (0°C).
Sulfur dioxide is not flammable or explosive in either the gaseous or liquid state. It is a relatively stable chemical. Temperatures above 3632°F (2000°C) are required to bring about detectable decomposition of sulfur dioxide. Dry sulfur dioxide (less than 100 ppm water) is not corrosive to ordinary metals. However, in the presence of even small amounts of water, sulfur dioxide becomes corrosive to most metals, with exceptions including lead, Type 316 stainless steel, and certain alloys. Glass and certain plastics are also resistant to moist sulfur dioxide.

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,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.

sulfur dioxide lewis structure

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.

Occurrence

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

Uses

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.

Uses

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.
Intermediate in the manufacture of sulfuric acid and sulfite pulp; casting of nonferrous metal; used in the food industry as a biocide and a preservative.

Uses

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.

Definition

ChEBI: Sulfur dioxide is a sulfur oxide. It has a role as a food bleaching agent, a refrigerant and an Escherichia coli metabolite.

Preparation

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.

Production Methods

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

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).

Hazard

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

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.

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.

Materials Uses

Service conditions must be defined to properly specifY materials of construction for handling sulfur dioxide. It is customary, however, to use carbon steel for dry sulfur dioxide at ambient temperatures. Reference should be made to the latest edition of the ASME Code, Section VIlI, Division I for vessels, and to the latest edition of ANSI/ASME 831.3, Chemical Plant and Petroleum Refinery Piping for piping specifications.
Moist sulfur dioxide is corrosive to carbon steel; therefore, other materials of construction have to be considered in this case. A source of data on the corrosivity of sulfur dioxide to various materials is the Corrosion Data Survey, published by the National Association of Corrosion Engineers; suppliers of liquid sulfur dioxide are another source.

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.

Safety

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.

Physiological effects

Exposure to sulfur dioxide gas in low concentrations produces an irritating effect on the mucous membranes of the eyes, nose, throat, and lungs due to the fonnation of sulfurous acid as the gas comes in contact with the moisture on these surfaces. The effects of sulfur dioxide according to exposure pathway are as follows:
Inhalation
Acute exposure through inhalation may result in dryness and irritation of the nose and throat,choking, sneezing, coughing, and bronchospasm. Severe overexposure may cause death through a systemic acidosis, from pulmonary edema, or from respiratory arrest. Prolonged or repeated exposure may cause impaired lung function, bronchitis, hacking cough, nasal irritation and discharge, increased fatigue, alteration in the senses of taste and smell, and longer duration of common colds. In extreme cases, dental caries, loss of fillings, gum disorders, and the rapid and painless destruction of teeth may result from repeated overexposure.
Skin contact
Liquid sulfur dioxide can cause frostbite and skin burns, and it converts to sulfurous acid in moist environments, which may cause skin irritation.
Eye contact
Corneal bums, opacification of the cornea, and blindness may result if liquid sulfur dioxide is splashed in the eyes. Sulfur dioxide can penetrate the intact cornea and cause iritis.
Ingestion
Severe bums to the mouth, throat, and gastrointestinal system may occur.
Exposure limits
ACGIH recommends a Threshold Limit Value-Time-Weighted Average (TLV-TWA) of 2 ppm (5.2 mg/m3) for sulfur dioxide. The TLV-TWA is the time-weighted average concentration for a normal 8-hour workday and a 40-hour workweek, to which nearly all workers may be repeatedly exposed, day after day, without adverse effect. The ACGIH also recommends a Threshold Limit Value-Short Term Exposure Limit (TLV-STEL) of 5 ppm (13 mg/m3) for sulfur dioxide. The TLV-STEL is the 15-minute TWA exposure that should not be exceeded at any time during a workday even if the 8-hour TWA is within the TLV-TWA. Exposures above the TLV-TWA up to the STEL should not be longer than 15 minutes and should not occur more than 4 times per day. There should be at least 60 minutes between successive exposures in this range. OSHA lists an 8-hour Time-Weighted Average-Permissible Exposure Limit (TWA-PEL) of 5 ppm (13 mg/m3) for sulfur dioxide. TWAPEL is the exposure limit that shall not be exceeded by the 8-hour TWAin any 8-hour work shift of a 40-hour workweek.

Carcinogenicity

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.

Environmental Fate

Concentrations of sulfur dioxide as low as 1–2 ppm have been reported to cause severe stress to green plants, and dissolved sulfur dioxide can be toxic to aquatic life. Sulfur, however, is the sixth most abundant element in living creatures, and is important in the structure and synthesis of proteins and kinetics of cellular components. For the most part, sulfur dioxide and other sulfites are rapidly metabolized by living organisms, which avoids cytotoxic effects. Sulfite metabolism pathways abound in plants and animals due to the ubiquity of sulfur and its compounds. For example, in certain plants, it has been shown that more than 80% of injected sulfite was metabolized to sulfate within 3 h.
Sulfur dioxide and other sulfites are generally highly soluble compounds that interact with the environment through a variety of processes. The primary functions of sulfites are those of reducing agents, which can remove dissolved oxygen from waterways; in the air, this ismanifested in the oxidation of sulfur dioxide to produce someinsoluble particulate sulfate salts as well as sulfuric acid. In waterways, the reduction of dissolved oxygen in turn generates a favorable environment for anaerobic bacteria, disrupting the local microbiota. Decreases in dissolved oxygen caused by the presence of sulfites – typically below 5 ppm dissolved oxygen – can negatively affect fish and other organisms present in polluted waterways. Another effect of sulfite contamination of waterways is the production of hydrogen sulfide gas, which is a by-product of sulfite-induced redox processes.

storage

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.

Shipping

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.

Toxicity evaluation

Conversion of sulfur dioxide to bisulfite in the airway may initiate bronchoconstriction, due to the ability of the more reactive bisulfite ion to disrupt disulfide bonds in tissue proteins, resulting in tissue damage and an inflammatory response. Sulfites and bisulfites can further inhibit DNA synthesis and cause human lymphocyte aberrations and can lead to crosslinking in proteins and nucleic acids in general, as well as generate free radicals during their oxidation to sulfates.
Bronchoconstriction and other related effects may be mediated by release of leukotrienes, prostaglandins, or other inflammatory factors. Some evidence suggests that free radicals and oxidative stress may play a role, and that metabolites of SO2 (especially sulfites) may be responsible for clastogenicity. Sulfur dioxide–induced bronchoconstriction, occurs when the gas acts on tracheobronchial receptors to induce a cholinergic reflex. Inhaled sulfur dioxide elicited a stronger reaction in sulfite oxidase–deficient rats than endogenously accumulated sulfites and S-sulfocysteine (a reaction product of sulfite with cysteine residues in proteins). Noncholinergic mechanisms for sulfur dioxide–induced bronchoconstriction have been demonstrated in humans as well. In one study, asthmatic subjects were administered indomethacin – a prostaglandin synthetase inhibitor – followed by challenge with sulfur dioxide gas; a reduction in airway responsiveness was observed.

Incompatibilities

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).

Precautions

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.

GRADES AVAILABLE

Sulfur dioxide is available in technical and food grades for use in both commercial and industrial applications.