Sulfur dioxide Sulfur dioxide is the chemical compound with
the formula SO 2. At standard atmosphere it is a toxic gas
with a pungent, irritating and rotten smell. The triple point is 197.69 K and 1.67Kpa.
It is released naturally by volcanic activity. The specific person who discovered sulfur
dioxide is not known, but it was first 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. Structure and bonding
SO2 is a bent molecule with C2v symmetry point group. A valence bond theory approach which
just considered 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.
In terms of electron-counting formalism, the sulfur atom has an oxidation state of +4 and
a formal charge of +1. Occurrence
It is found on Earth and exists in very small concentrations and in the atmosphere at approximately
1 ppbv. On other planets, it can be found in various
concentrations, the most significant being the atmosphere of Venus, it is the third most
significant atmospheric gas, at 150 ppm and condenses to form clouds and is a key component
of chemical reactions in the planet's atmosphere and contributes to global warming. 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 100ppm though it currently only exists in trace amounts.
On both Venus and Mars, its primary source, like on Earth, is believed to be volcanic.
It is also believed to exist in trace amounts in the atmosphere of Jupiter and atmosphere
of Saturn. As an ice, it is thought to exist in abundance
on the Galilean moons – as sublimating ice or frost on the trailing hemisphere of Io,
a natural satellite of Jupiter and in the crust and mantle of Europa, Ganymede and Callisto
possibly also in liquid form and readily reacting with water.
Production Sulfur dioxide is primarily produced for sulfuric
acid manufacture (see contact process). In the United States in 1979, 23.6 million tonnes
of sulfur dioxide was used in this way, compared with 150 thousand tonnes used for other purposes.
Most sulfur dioxide is produced by the combustion of elemental sulfur. Some sulfur dioxide is
also produced by roasting pyrite and other sulfide ores in air.
Combustion routes Sulfur dioxide is the product of the burning
of sulfur or of burning materials that contain sulfur:
To aid combustion, liquefied sulfur (140–150 °C) 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 1000–1600 °C. The significant amount of
heat produced is recovered by steam generation that can subsequently be converted to electricity.
The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly. For example:
The roasting of sulfide ores such as pyrite, sphalerite, and cinnabar (mercury sulfide)
also releases SO2: A combination of these reactions is responsible
for the largest source of sulfur dioxide, volcanic eruptions. These events can release
millions of tonnes of SO2. Reduction of higher oxides
Sulfur dioxide can also be a by-product in the manufacture of calcium silicate cement:
CaSO4 is heated with coke and sand in this process:
Up 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.
On a laboratory scale, the action of hot sulfuric acid on copper turnings produces sulfur dioxide.
From sulfite Sulfite results from the reaction of aqueous
base and sulfur dioxide. The reverse reaction involves acidification of sodium metabisulfite:
Reactions Industrial reactions
Treatment of basic solutions with sulfur dioxide affords sulfite salts:
Featuring sulfur in the +4 oxidation state, sulfur dioxide is a reducing agent. It is
oxidized by halogens 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.
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-dienes
in a cheletropic reaction to give organosulfur compounds.
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 η1.
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 it is sometimes called E220 when used
in this way. As a preservative, it maintains the colorful appearance of the fruit and prevents
rotting. It is also added to sulfured molasses. In winemaking
The specific person who discovered sulfur dioxide is not known, but it was first 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.
Sulfur dioxide is an important compound in winemaking, and is designated as parts per
million in wine, E number: E220. It is present even in so-called unsulfurated wine at concentrations
of up to 10 mg/L. 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. It is the molecular SO2 which is active as an antimicrobial and
antioxidant, and this is also the form which may be perceived as a pungent odour 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 nose 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, 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 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. Sulfur dioxide is fairly soluble in water,
and by both IR and Raman spectroscopy, it is known that 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: Biochemical and biomedical roles
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. Sulfur
dioxide blocks nerve signals from the pulmonary stretch receptors (PSRs) and abolishes the
Hering–Breuer inflation reflex. 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 CFCs, 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 that has been 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 affords
the corresponding aryl sulfonyl chloride, for example:
As an air pollutant Sulfur dioxide is a noticeable component in
the atmosphere, especially following volcanic eruptions. According to the United States
Environmental Protection Agency (as presented by the 2002 World Almanac or in chart form),
the following amount of sulfur dioxide was released in the U.S. per year:
Sulfur dioxide is a major air pollutant and has significant impacts upon human health.
In addition the concentration of sulfur dioxide in the atmosphere can influence the habitat
suitability for plant communities as well as animal life. Sulfur dioxide emissions are
a precursor to acid rain and atmospheric particulates. Due largely to the US EPA’s Acid Rain Program,
the U.S. has witnessed a 33% decrease in emissions between 1983 and 2002. This improvement resulted
in part from flue-gas desulfurization, a technology that enables SO2 to be chemically bound in
power plants burning sulfur-containing coal or oil. In particular, calcium oxide (lime)
reacts with sulfur dioxide to form calcium sulfite:
Aerobic oxidation of the CaSO3 gives CaSO4, anhydrite. Most gypsum sold in Europe comes
from flue-gas desulfurization. Sulfur can be removed from coal during the
burning process by using limestone as a bed material in Fluidized bed combustion.
Sulfur can also be removed from fuels prior to burning the fuel, preventing the formation
of SO2 because there is no sulfur in the fuel from which SO2 can be formed. 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 or Sulferox. Fuel additives, such as calcium additives
and magnesium oxide, are being used in gasoline and diesel engines in order to lower the emission
of sulfur dioxide gases into the atmosphere. As of 2006, China was the world's largest
sulfur dioxide polluter, with 2005 emissions estimated to be 25,490,000 short tons (23.1 Mt).
This amount represents a 27% increase since 2000, and is roughly comparable with U.S.
emissions in 1980. Safety
Inhalation Inhaling sulfur dioxide is associated with
increased respiratory symptoms and disease, difficulty in breathing, and premature death.
In 2008, the American Conference of Governmental Industrial Hygienists reduced the short-term
exposure limit to 5 parts per million (ppm). The OSHA PEL is currently set at 5 ppm (13 mg/m3)
time weighted average. NIOSH has set the IDLH at 100 ppm. In 2010, the EPA "revised the
primary SO2 NAAQS by establishing a new 1-hour standard at a level of 75 parts per billion
(ppb). EPA revoked the two existing primary standards because they would not provide additional
public health protection given a 1-hour standard at 75 ppb."
A 2011 systematic review concluded that exposure to sulfur dioxide is associated with preterm
birth. 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. Symptoms of sensitivity to sulfiting agents, including sulfur dioxide, manifest
as potentially life-threatening trouble breathing within minutes of ingestion.