Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a colorless gas with a pungent smell that is responsible for the odor of burnt matches. It is released naturally by volcanic activity and is produced as a by-product of metals refining and the burning of Sour gas-bearing fossil fuels.[
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Sulfur dioxide is somewhat toxic to humans, although only when inhaled in relatively large quantities for a period of several minutes or more. It was known to medieval alchemy as "volatile spirit of sulfur".
Structure and bonding
SO2 is a bent molecule with C2v symmetry point group.
A valence bond theory approach considering just s and p orbitals would describe the bonding in terms of resonance between two resonance structures.
The sulfur–oxygen bond has a bond order of 1.5. There is support for this simple approach that does not invoke d orbital participation.
Occurrence
Sulfur dioxide is found on Earth and exists in very small concentrations in the atmosphere at about 15 ppb.
On other planets, sulfur dioxide can be found in various concentrations, the most significant being the atmosphere of Venus, where it is the third-most abundant atmospheric gas at 150 ppm. There, it reacts with water to form clouds of sulfurous acid ( + ⇌ + ), and is a key component of the planet's global atmospheric sulfur cycle. It has been implicated as a key agent in the warming of early Mars, with estimates of concentrations in the lower atmosphere as high as 100 ppm,
As an ice, it is thought to exist in abundance on the Galilean moons—as subliming ice or frost on the trailing hemisphere of Io,[ Europa's Hidden Ice Chemistry – NASA Jet Propulsion Laboratory. Jpl.nasa.gov (October 4, 2010). Retrieved on September 24, 2013.]
Production
Sulfur dioxide is primarily produced for sulfuric acid manufacture (see contact process, but other processes predated that at least since 16th century
Combustion routes
Sulfur dioxide is the product of the burning of sulfur or of burning materials that contain sulfur:
- + 8 → 8 , ΔH = −297 kJ/mol
To aid combustion, liquified sulfur ( is sprayed through an atomizing nozzle to generate fine drops of sulfur with a large surface area. The reaction is exothermic, and the combustion produces temperatures of . The significant amount of heat produced is recovered by steam generation that can subsequently be converted to electricity.[
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The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly. For example:
- 2 + 3 → 2 + 2
The roasting of sulfide ores such as pyrite, sphalerite, and cinnabar (mercury sulfide) also releases SO2:[Shriver, Atkins. Inorganic Chemistry, Fifth Edition. W. H. Freeman and Company; New York, 2010; p. 414.]
- 4 + 11 → 2 + 8
- 2 + 3 → 2 + 2
- 4 FeS + 7 → 2 + 4
A combination of these reactions is responsible for the largest source of sulfur dioxide, volcanic eruptions. These events can release millions of tons of SO2.
Reduction of higher oxides
Sulfur dioxide can also be a byproduct in the manufacture of calcium silicate cement; Calcium sulfate is heated with coke and sand in this process:
- 2 + 2 + C → 2 + 2 +
Until the 1970s commercial quantities of sulfuric acid and cement were produced by this process in Whitehaven, England. Upon being mixed with shale or marl, and roasted, the sulfate liberated sulfur dioxide gas, used in sulfuric acid production, the reaction also produced calcium silicate, a precursor in cement production.[ WHITEHAVEN COAST ARCHAEOLOGICAL SURVEY. lakestay.co.uk (2007)]
On a laboratory scale, the action of hot concentrated sulfuric acid on copper swarf produces sulfur dioxide.
- Cu + 2 →
Tin also reacts with concentrated sulfuric acid but it produces tin(II) sulfate which can later be pyrolyzed at 360 °C into tin dioxide and dry sulfur dioxide.
- Sn + →
- → +
From sulfites
The reverse reaction occurs upon acidification:
Reactions
Sulfites result by the action of aqueous base on sulfur dioxide:
Sulfur dioxide is a mild but useful reducing agent. It is oxidized by Halogen to give the sulfuryl halides, such as sulfuryl chloride:
Sulfur dioxide is the oxidising agent in the Claus process, which is conducted on a large scale in oil refineries. Here, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:
The sequential oxidation of sulfur dioxide followed by its hydration is used in the production of sulfuric acid.
- + + →
Sulfur dioxide dissolves in water to give "sulfurous acid", which cannot be isolated and is instead an acidic solution of bisulfite, and possibly sulfite, ions.
- Ka = 1.54; p Ka = 1.81
Laboratory reactions
Sulfur dioxide is one of the few common acidic yet reducing gases. It turns moist litmus pink (being acidic), then white (due to its bleaching effect). It may be identified by bubbling it through a dichromate solution, turning the solution from orange to green (Cr3+ (aq)). It can also reduce ferric ions to ferrous.
Sulfur dioxide can react with certain 1,3- in a cheletropic reaction to form cyclic . This reaction is exploited on an industrial scale for the synthesis of sulfolane, which is an important solvent in the petrochemical industry.
Sulfur dioxide can bind to metal ions as a ligand to form metal sulfur dioxide complexes, typically where the transition metal is in oxidation state 0 or +1. Many different bonding modes (geometries) are recognized, but in most cases, the ligand is monodentate, attached to the metal through sulfur, which can be either planar and pyramidal hapticity1.
Uses
The overarching, dominant use of sulfur dioxide is in the production of sulfuric acid.[
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Precursor to sulfuric acid
Sulfur dioxide is an intermediate in the production of 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 dioxide to sulfuric acid is called the contact process. Several million tons are produced annually for this purpose.
Food preservative
Sulfur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried fruits, owing to its antimicrobial properties and ability to prevent oxidation,[ Current EU approved additives and their E Numbers, The Food Standards Agency website.] when used in this way in Europe. As a preservative, it maintains the colorful appearance of the fruit and prevents Decomposition. Historically, molasses was "sulfured" as a preservative and also to lighten its color. Treatment of dried fruit was usually done outdoors, by igniting sublimed sulfur and burning in an enclosed space with the fruits.
Winemaking
Sulfur dioxide was first used in winemaking by the Romans, when they discovered that burning sulfur candles inside empty wine vessels keeps them fresh and free from vinegar smell.
It is still an important compound in winemaking, and is measured in parts per million ( ppm) in wine. It is present even in so-called unsulfurated wine at concentrations of up to 10 mg/L.[ Sulphites in wine, MoreThanOrganic.com.] It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation – a phenomenon that leads to the browning of the wine and a loss of cultivar specific flavors.[Jackson, R.S. (2008) Wine science: principles and applications, Amsterdam; Boston: Elsevier/Academic Press]
Sulfur dioxide exists in wine in free and bound forms, and the combinations are referred to as total SO2. Binding, for instance to the carbonyl group of acetaldehyde, varies with the wine in question. The free form exists in equilibrium between molecular SO2 (as a dissolved gas) and bisulfite ion, which is in turn in equilibrium with sulfite ion. These equilibria depend on the pH of the wine. Lower pH shifts the equilibrium towards molecular (gaseous) SO2, which is the active form, while at higher pH more SO2 is found in the inactive sulfite and bisulfite forms. The molecular SO2 is active as an antimicrobial and antioxidant, and this is also the form which may be perceived as a pungent odor at high levels. Wines with total SO2 concentrations below 10 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of total SO2 allowed in wine in the US is 350 ppm; in the EU it 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 free SO2 concentrations over 50 ppm, SO2 becomes evident in the smell and taste of wine.
SO2 is also a very important compound in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery due to the risk of cork taint,[ Chlorine Use in the Winery. Purdue University] a mixture of SO2, water, and citric acid is commonly used to clean and sanitize equipment. Ozone (O3) is now used extensively for sanitizing in wineries due to its efficacy, and because it does not affect the wine or most equipment.[ Use of ozone for winery and environmental sanitation , Practical Winery & Vineyard Journal.]
As a reducing agent
Sulfur dioxide is also a good Reducing agent. 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. In municipal wastewater treatment, sulfur dioxide is used to treat chlorinated wastewater prior to release. Sulfur dioxide reduces free and combined chlorine to chloride.George Tchobanoglous (1979). 007041677X, McGraw Hill. 007041677X
Sulfur dioxide is fairly soluble in water, and by both IR and Raman spectroscopy; the hypothetical sulfurous acid, H2SO3, is not present to any extent. However, such solutions do show spectra of the hydrogen sulfite ion, HSO3−, by reaction with water, and it is in fact the actual reducing agent present:
- SO2 + H2O ⇌ HSO3− + H+
As a fumigant
In the beginning of the 20th century sulfur dioxide was used in Buenos Aires as a fumigant to kill rats that carried the Yersinia pestis bacterium, which causes bubonic plague. The application was successful, and the application of this method was extended to other areas in South America. In Buenos Aires, where these apparatuses were known as Sulfurozador, but later also in Rio de Janeiro, New Orleans and San Francisco, the sulfur dioxide treatment machines were brought into the streets to enable extensive disinfection campaigns, with effective results.
Biochemical and biomedical roles
Sulfur dioxide or its conjugate base bisulfite is produced biologically as an intermediate in both sulfate-reducing organisms and in sulfur-oxidizing bacteria, as well. The role of sulfur dioxide in mammalian biology is not yet well understood.
It is considered that endogenous sulfur dioxide plays a significant physiological role in regulating cardiac and blood vessel function, and aberrant or deficient sulfur dioxide metabolism can contribute to several different cardiovascular diseases, such as arterial hypertension, atherosclerosis, pulmonary arterial hypertension, and stenocardia.
It was shown that in children with pulmonary arterial hypertension due to congenital heart diseases the level of homocysteine is higher and the level of endogenous sulfur dioxide is lower than in normal control children. Moreover, these biochemical parameters strongly correlated to the severity of pulmonary arterial hypertension. Authors considered homocysteine to be one of useful biochemical markers of disease severity and sulfur dioxide metabolism to be one of potential therapeutic targets in those patients.
Endogenous sulfur dioxide also has been shown to lower the proliferation rate of endothelial smooth muscle cells in blood vessels, via lowering the MAPK activity and activating adenylyl cyclase and protein kinase A.
Endogenous sulfur dioxide in low concentrations causes endothelium-dependent vasodilation. In higher concentrations it causes endothelium-independent vasodilation and has a negative inotropic effect on cardiac output function, thus effectively lowering blood pressure and myocardial oxygen consumption. The vasodilating and bronchodilating effects of sulfur dioxide are mediated via ATP-dependent and L-type ("dihydropyridine") calcium channels. Endogenous sulfur dioxide is also a potent antiinflammatory, antioxidant and cytoprotective agent. It lowers blood pressure and slows hypertensive remodeling of blood vessels, especially thickening of their intima. It also regulates lipid metabolism.
Endogenous sulfur dioxide also diminishes myocardial damage, caused by isoproterenol adrenergic hyperstimulation, and strengthens the myocardial antioxidant defense reserve.
As a reagent and solvent in the laboratory
Sulfur dioxide is a versatile inert solvent widely used for dissolving highly oxidizing salts. It is also used occasionally as a source of the sulfonyl group in organic synthesis. Treatment of aryl with sulfur dioxide and cuprous chloride yields the corresponding aryl sulfonyl chloride, for example:
As a result of its very low Lewis basicity, it is often used as a low-temperature solvent/diluent for superacids like magic acid (FSO3H/SbF5), allowing for highly reactive species like tert-butyl cation to be observed spectroscopically at low temperature (though tertiary carbocations do react with SO2 above about −30 °C, and even less reactive solvents like SO2ClF must be used at these higher temperatures).
As a refrigerant
Sulfur dioxide was one of the earliest adopted for mechanical refrigeration owing to its ease of liquefaction and high latent heat of vaporization. In 1784, Jean-François Clouet and Gaspard Monge first demonstrated that sulfur dioxide gas could be liquefied at low temperatures. In the mid-1870s, Raoul Pictet successfully employed sulfur dioxide in a prototype refrigeration system. Beginning in 1920, it saw widespread use in the "Rollator" rotary-compressor home refrigerators produced by Norge. Following the introduction of less toxic, non-flammable chlorofluorocarbon (CFC) refrigerants, the use of sulfur dioxide in refrigeration systems gradually declined.
As an indicator of volcanic activity
Sulfur dioxide content in naturally-released geothermal gasses is measured by the Icelandic Meteorological Office as an indicator of possible volcanic activity.
Safety
Ingestion
In the United States, the Center for Science in the Public Interest lists the two food preservatives, sulfur dioxide and sodium bisulfite, as being safe for human consumption except for certain asthmatic individuals who may be sensitive to them, especially in large amounts.
Inhalation
Incidental exposure to sulfur dioxide is routine, e.g. the smoke from matches, coal, and sulfur-containing fuels like bunker fuel. Relative to other chemicals, it is only mildly toxic and requires high concentrations to be actively hazardous.[ Sulfur Dioxide Basics U.S. Environmental Protection Agency] However, its ubiquity makes it a major air pollutant with significant impacts on human health.[ Sulfur Dioxide (SO2) Pollution. United States Environmental Protection Agency]
In 2008, the American Conference of Governmental Industrial Hygienists reduced the short-term exposure limit to 0.25 parts per million (ppm). In the US, the OSHA set the PEL at 5 ppm (13 mg/m3) time-weighted average. Also in the US, NIOSH set the IDLH at 100 ppm.
Environmental role
Air pollution
Major volcanic eruptions have an overwhelming effect on sulfate aerosol concentrations in the years when they occur: eruptions ranking 4 or greater on the Volcanic Explosivity Index inject and water vapor directly into the stratosphere, where they react to create sulfate aerosol plumes.[IPCC, 1990: Chapter 1: Greenhouse Gases and Aerosols R.T.. In: Climate Change: The IPCC Scientific Assessment J.T.Houghton,. Cambridge University Press, Cambridge, United Kingdom and New York, NY, USA, pp. 31–34,] In the eastern United States in the early 2000s, sulfate particles were estimated to account for 25% or more of all air pollution.[US EPA: Effects of Acid Rain – Forests ][ Effects of Acid Rain – Human Health . Epa.gov (June 2, 2006). Retrieved on February 9, 2013.] Further, this form of pollution is linked to preterm birth and low birth weight, with a study of 74,671 pregnant women in Beijing finding that every additional 100 μg/m3 of in the air reduced infants' weight by 7.3 g, making it and other forms of air pollution the largest attributable risk factor for low birth weight ever observed.
Control measures
Due largely to the US EPA's Acid Rain Program, the U.S. has had a 33% decrease in emissions between 1983 and 2002 (see table). This improvement resulted in part from flue-gas desulfurization, a technology that enables SO2 to be chemically bound in burning sulfur-containing coal or petroleum.
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In particular, calcium oxide reacts with sulfur dioxide to form calcium sulfite:
- CaO + SO2 → CaSO3
Aerobic oxidation of the CaSO3 gives CaSO4, anhydrite. Most gypsum sold in Europe comes from flue-gas desulfurization.
To control sulfur emissions, dozens of methods with relatively high efficiencies have been developed for fitting of coal-fired power plants.
Sulfur can also be removed from fuels before burning, preventing formation of SO2 when the fuel is burnt. The Claus process is used in refineries to produce sulfur as a byproduct. The Stretford process has also been used to remove sulfur from fuel. Redox processes using iron oxides can also be used, for example, Lo-Cat[ FAQ's About Sulfur Removal and Recovery using the LO-CAT® Hydrogen Sulfide Removal System. gtp-merichem.com] or Sulferox.[ Process screening analysis of alternative gas treating and sulfur removal for gasification. (December 2002) Report by SFA Pacific, Inc. prepared for U.S. Department of Energy (PDF) Retrieved on October 31, 2011.]
Fuel additives such as calcium additives and magnesium carboxylate may be used in marine engines to lower the emission of sulfur dioxide gases into the atmosphere.[May, Walter R. Marine Emissions Abatement . SFA International, Inc., p. 6.]
Effects on ozone layer
Sulfur dioxide aerosols in the stratosphere can contribute to ozone depletion in the presence of chlorofluorocarbons and other halogenated ozone-depleting substances.[ Injection of sulfur dioxide and large amounts of water vapor into the stratosphere following the 2022 eruption of Hunga Tonga-Hunga Haʻapai resulted in altered atmospheric circulation that promoted a decrease in ozone in the southern latitudes but an increase in the tropics.][
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Impact on climate change
Projected impacts
Solar geoengineering
As the real world had shown the importance of sulfate aerosol concentrations to the global climate, research into the subject accelerated. Formation of the aerosols and their effects on the atmosphere can be studied in the lab, with methods like ion-chromatography and mass spectrometry
Properties
Table of thermal and physical properties of saturated liquid sulfur dioxide:| Temperature (°C) | Density (kg/m^3) | Specific heat (kJ/kg K) | Kinematic viscosity (m^2/s) | Conductivity (W/m K) | Thermal diffusivity (m^2/s) | Prandtl Number | Bulk modulus (K^-1) |
| −50 | 1560.84 | 1.3595 | 4.84E-07 | 0.242 | 1.14E-07 | 4.24 | – |
| −40 | 1536.81 | 1.3607 | 4.24E-07 | 0.235 | 1.13E-07 | 3.74 | – |
| −30 | 1520.64 | 1.3616 | 3.71E-07 | 0.23 | 1.12E-07 | 3.31 | – |
| −20 | 1488.6 | 1.3624 | 3.24E-07 | 0.225 | 1.11E-07 | 2.93 | – |
| −10 | 1463.61 | 1.3628 | 2.88E-07 | 0.218 | 1.10E-07 | 2.62 | – |
| 0 | 1438.46 | 1.3636 | 2.57E-07 | 0.211 | 1.08E-07 | 2.38 | – |
| 10 | 1412.51 | 1.3645 | 2.32E-07 | 0.204 | 1.07E-07 | 2.18 | – |
| 20 | 1386.4 | 1.3653 | 2.10E-07 | 0.199 | 1.05E-07 | 2 | 1.94E-03 |
| 30 | 1359.33 | 1.3662 | 1.90E-07 | 0.192 | 1.04E-07 | 1.83 | – |
| 40 | 1329.22 | 1.3674 | 1.73E-07 | 0.185 | 1.02E-07 | 1.7 | – |
| 50 | 1299.1 | 1.3683 | 1.62E-07 | 0.177 | 9.99E-08 | 1.61 | – |
See also
External links
