CAS Number 7446-09-5
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Linear Formula SO2
·
Molecular Weight 64.06
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Beilstein Registry
Number 3535237
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EC Number 231-195-2
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MDL number MFCD00011450
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PubChem Substance ID 329769309
Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO
2. At standard atmosphere, it is a toxic gas with a pungent, irritating smell. The triple point is 197.69 K and 1.67 kPa. It is released naturally by volcanic activity.
2. At standard atmosphere, it is a toxic gas with a pungent, irritating smell. The triple point is 197.69 K and 1.67 kPa. It is released naturally by volcanic activity.
Detailed description
Sulfur
dioxide was used by the Romans in winemaking when they discovered that burning sulfur candles inside
empty wine vessels kept them fresh and free from vinegar smell.
Uses
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 billion kilograms are produced annually
for this purpose.
As a preservative
Sulfur dioxide is sometimes used as a preservative for dried apricots, dried figs, and other dried
fruits, owing to its antimicrobial properties, and is
called E220[18] when used in this
way in Europe. As a preservative, it maintains the colorful appearance of the
fruit and prevents rotting. It is also added to sulfured molasses.
In winemaking
Sulfur dioxide was used by the Romans in winemaking,
when they discovered that burning sulfur candles inside empty wine vessels
keeps them fresh and free from vinegar smell.[19]
Sulfur dioxide is still an important compound in winemaking, and is measured in parts per million in wine. It is present
even in so-called unsulfurated wine at concentrations of up to 10 mg/L.[20] It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation. Its
antimicrobial action also helps to minimize volatile acidity. Sulfur dioxide is
responsible for the words "contains sulfites" found on wine labels.
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 the risk
of cork
taint,[21] a mixture of SO2, water,
and citric acid is commonly used to clean and sanitize equipment. Compounds of
ozone (O3) are now used extensively as cleaning products in wineries
due to their efficiency, and because these compounds do not affect the
wine or equipment.
As a reducing agent[edit]
Sulfur dioxide is also a good reductant. 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.[22]
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+
Biochemical and biomedical roles[edit]
Sulfur dioxide is toxic in large amounts. It 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.[23] Sulfur dioxide
blocks nerve signals from the pulmonary stretch receptors and abolishes the Hering–Breuer
inflation reflex.
It was shown that endogenous sulfur dioxide plays a
role in diminishing an experimental lung damage caused by oleic
acid. Endogenous sulfur dioxide lowered lipid
peroxidation, free radical formation, oxidative stress and inflammation during
an experimental lung damage. Conversely, a successful lung damage caused a
significant lowering of endogenous sulfur dioxide production, and an increase
in lipid peroxidation, free radical formation, oxidative stress and
inflammation. Moreover, blockade of an enzyme that produces
endogenous SO2 significantly
increased the amount of lung tissue damage in the experiment. Conversely,
adding acetylcysteine or glutathione to the rat diet
increased the amount of endogenous SO2 produced and decreased the lung
damage, the free radical formation, oxidative stress, inflammation and
apoptosis.[24]
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, stenocardia.[25]
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.[26]
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.[27] Smooth muscle cell
proliferation is one of important mechanisms of hypertensive remodeling of
blood vessels and their stenosis, so it is an
important pathogenetic mechanism in arterial hypertension and atherosclerosis.
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 calcium
channels 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.[28]
Endogenous sulfur dioxide also diminishes myocardial
damage, caused by isoproterenol adrenergic hyperstimulation,
and strengthens the myocardial antioxidant defense reserve.[29]
As a refrigerant
Being easily condensed and possessing a high heat of evaporation, sulfur dioxide is a candidate material for refrigerants. Prior
to the development of chlorofluorocarbons, sulfur dioxide was used as a refrigerant in home refrigerators.
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 diazonium salts with sulfur dioxide and cuprous
chloride yields the corresponding aryl sulfonyl chloride, for example:[30]
Proposed use in climate engineering
Injections of sulfur
dioxide in the stratosphere has been proposed in climate engineering. The cooling effect would be similar to what has been observed
after the large explosive volcano eruption of Mount Pinatubo in 1991. However
this form of geoengineering would have uncertain regional consequences on
rainfall patterns, for example in monsoon regions.
Packaging details
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