Sulfur Dioxide (SO₂)

Sulfur dioxide (also sulphur dioxide) is the chemical compound with the formula SO₂. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide. Further oxidation of SO₂, usually in the presence of a catalyst such as NO₂, forms H₂SO₄, and thus acid rain. This is one of the causes for concern over the environmental impact of the use of these fuels as power sources.

Preparation

Sulfur dioxide can be prepared by burning sulfur:

8 + 8O₂ → 8SO₂

The combustion of hydrogen sulfide and organosulfur compounds proceeds similarly.

2H₂S (g) + 3O₂ (g) → 2H₂O (g) + 2SO₂ (g)

The roasting of sulfide ores such as pyrites, sphalerite (zinc blende), and cinnabar (mercury sulfide) also releases SO₂:

4FeS₂ (s) + 11O₂ (g) → 2Fe₂O₃ (s) + 8SO₂ (g)
2ZnS (s) + 3O₂ (g) → 2ZnO (s) + 2SO₂ (g)
HgS (s) + O₂ (g) → Hg (g) + SO₂ (g)

Sulfur dioxide is a by-product in the manufacture of calcium silicate cement: CaSO₄ is heated with coke and sand in this process:

2CaSO₄ (s) + 2SiO₂ (s) + C (s) → 2CaSiO₃ (s) + 2SO₂ (g) + CO₂ (g)

Action of hot sulfuric acid on copper turnings produces sulfur dioxide.

Cu (s) + 2H₂SO₄ (aq) → CuSO₄ (aq) + SO₂ (g) + 2H₂O (l)

Structure and bonding

SO₂ is a bent molecule with C₂v symmetry point group. In terms of electron-counting formalisms, the sulfur atom has an oxidation state of +4, a formal charge of 0, and is surrounded by 5 electron pairs and can be described as a hypervalent molecule. From the perspective of molecular orbital theory, most of these valence electrons are engaged in S–O bonding.

The S–O bonds are shorter in SO₂ (143.1 pm) than in sulfur monoxide, SO (148.1 pm), whereas the O–O bonds are longer in O₃ (127.8 pm) than in dioxygen, O₂ (120.7 pm). The mean bond energy is greater in SO₂ (548 kJ/mol) than in SO (524 kJ/mol), whereas it is less in O₃ (297 kJ/mol) than in O₂ (490 kJ/mol). These pieces of evidence lead chemists to conclude that the S–O bonds in sulfur dioxide have a bond order of at least 2, unlike the O–O bonds in ozone, which have a bond order of 1.5.

Reactions

Treatment of basic solutions with sulfur dioxide affords sulfite salts:

SO₂ + 2NaOH → Na₂SO₃ + H₂O

Featuring sulfur in the +4 oxidation state, sulfur dioxide is a reducing agent. It is oxidized by halogens such as chlorine to give the sulfuryl halides:

SO₂ + Cl₂ → SO₂Cl₂

However, on rare occasions, it can also act as an oxidising agent: in the Claus process, sulfur dioxide is reduced by hydrogen sulfide to give elemental sulfur:

SO₂ + 2H₂S → 3S + 2H₂O

Sulfur dioxide can bind to metal ions as a ligand, 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 petroleum, 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 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 appearance of the fruit and prevents rotting. Its presence gives the fruit a distinctive chemical taste.

In winemaking

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 unsulphurated wine at concentrations of up to 10 milligrams per litre. It serves as an antibiotic and antioxidant, protecting wine from spoilage by bacteria and oxidation. It also helps to keep volatile acidity at desirable levels. Sulfur dioxide is responsible for the words "contains sulfites" found on wine labels. Wines with SO₂ concentrations below 10 ppm do not require "contains sulfites" on the label by US and EU laws. The upper limit of SO₂ allowed in wine is 350 ppm in US, in the EU 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 over 50ppm, SO₂ becomes evident in the nose and taste of wine.

SO₂ is also a very important element in winery sanitation. Wineries and equipment must be kept clean, and because bleach cannot be used in a winery, 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.

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. Sulfur dioxide has no role in mammalian biology. Sulfur dioxide blocks nerve signals from the pulmonary stretch receptors (PSR's) 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 Refrigerant, 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.

Temperature dependence of aqueous solubility

The values are tabulated for 101.3 kPa partial pressure of SO₂. Solubility of gas in a liquid depends on the gas partial pressure according to Henry's law. The solubility is given for "pure water", i.e., water that contains only SO₂ in the amount at equilibrium with the gas phase. This "pure water" is going to be acidic. The solubility of SO₂ in neutral (or alkaline) water is generally going to be higher because of the pH-dependent speciation of SO₂ in the solution with the production of bisulfite and some sulfiteions.

Safety

Sulfur dioxide is an allergen to which some consumers are sensitive. SO₂ is associated with increased respiratory symptoms and disease, difficulty in breathing, and premature death.

Effects of Various SO₂ Levels