Sulfur dioxide

August 28, 2023
I’m a dangerous but widely used gas.
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Sulfur dioxide (SO2) is a pungent, toxic gas that is the primary product of burning elemental sulfur. It exists widely in nature, mostly from volcanic activity and burning fossil fuels. It is found elsewhere in the solar system, as a gas in the atmospheres of Venus and Jupiter’s moon Io and as an ice on the other Galilean moons.

The major use of SO2 is in the manufacture of sulfuric acid (H2SO4), the most-produced chemical worldwide. Elemental sulfur and oxygen react to form SO2, which is catalytically oxidized with additional oxygen to make sulfur trioxide (SO3). The SO3 is mixed with existing H2SO4 to produce oleum (fuming sulfuric acid), which is added to water in a strongly exothermic process to make concentrated H2SO4. This is known as the contact process; it dates to an 1831 patent by British inventor Peregrine Phillips.

In chemical laboratories, it has multiple functions, including as a reducing agent, as a reagent in sulfonylation reactions, and as a low-temperature solvent. SO2 is also used to preserve dried fruits such as raisins and prunes and to prevent spoilage in wine.

The hazard information table shows that SO2 is pretty nasty stuff; but, in addition to its value as a chemical, it has another positive side: Volcanoes that emit the gas can have a beneficial effect on climate change. When SO2 spews into the stratosphere, it reacts photochemically with oxygen to form H2SO4 aerosols, which in turn reflect solar radiation and cool the atmosphere. But, as might be expected, even this has a downside because SO2 and H2SO4 contribute to acid rain.

Sulfur dioxide hazard information*

Hazard class**GHS code and hazard statement
Gases under pressure, liquefied gasH280—Contains gas under pressure; may explode if heatedChemical Safety Warning
Skin corrosion/irritation, category 1BH314—Causes severe skin burns and eye damageChemical Safety Warning
Serious eye damage/eye irritation, category 1H318—Causes serious eye damageChemical Safety Warning
Acute toxicity, inhalation, category 3H331—Toxic if inhaledChemical Safety Warning

*Most commonly cited hazard classes are listed. Other safety data sheets mention additional hazards.
**Globally Harmonized System (GHS) of Classification and Labeling of Chemicals. Explanation of pictograms.

Molecule in the News

Formaldehyde1, the simplest aldehyde and an important industrial and laboratory chemical, was the Molecule of the Week for May 29, 2006. Its many commercial uses include the production of resins used as adhesives in building materials, carpeting, and insulation materials.

But formaldehyde has a dark side. Small amounts of it can emanate from residences, work environments, and manufacturing plants, causing medical problems ranging from eye irritation to respiratory difficulties to nervous system disorders to various cancers. Many of its uses have been restricted or banned.

Although the National Toxicology Program designated formaldehyde as a human carcinogen in 2011, the US Environmental Protection Agency has yet to label it as such. In 2022, EPA commissioned the US National Academy of Sciences to review a draft risk assessment that formaldehyde is a human carcinogen; and this month, NAS reported that it concurred with EPA’s findings. Almost immediately, the American Chemistry Council, a trade association, challenged the assessment, saying that EPA’s and NAS’s scientific methods were flawed. Much is at stake because almost 1 million workers make formaldehyde-containing products that generate more than $500 billion in US sales.

1. CAS Reg. No. 50-00-0.

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Sulfur dioxide fast facts

CAS Reg. No.7446-09-5
SciFinder nomenclatureSulfur dioxide
Empirical formulaO2S
Molar mass64.06 g/mol
AppearanceColorless gas
Boiling point–10 °C
Water solubility110 g/L @ 20 °Ca

a. From solubility chart in a 2022 article by Junfu Yuan* and Meizhou Xu at Jiangsu Maritime Institute and Bureau (Nanjing, China).

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