Product

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Main Product
Sulfuric Acid
Segment
Chemicals
Main-Family
Inorganics
Sub-Family
Mineral Acids
Physical State

Liquid

Product
Oleum
Names
Fuming Sulfuric Acid; Disulfuric Acid; Pyrosulfuric Acid; Sulfo hydrogen sulfate; μ-Oxido-bis(hydroxidodioxidosulfur)

Description

Oleum is a chemical solution consisting of sulfur trioxide (SO₃) dissolved in concentrated sulfuric acid (H₂SO₄). Its composition is variable, depending on the percentage of free SO₃ present, and it is also known as "fuming sulfuric acid" due to its tendency to release SO₃ vapors in air.

Oleum is generally assessed according to the free SO3 content by mass. It can include strengths from a few percent up to 65% SO₃ and is sometimes specifically defined as disulfuric (pyrosulfuric) acid (H₂S₂O₇ or H₂SO₄·SO₃) when referring to 100% oleum, which has a density of a molecular weight of 178.13–178.14 g/mol and a density of 1.840 g/mL at 25°C.

Oleum is produced in the contact process, where sulfur is oxidized to sulfur trioxide which is subsequently dissolved in concentrated sulfuric acid. Sulfuric acid itself is regenerated by dilution of part of the oleum.

References

  1. Wikipedia (last edited on Oct 22, 2025). Disulfuric acid
  2. Wikipedia (last edited on Oct 21, 2025). Oleum
  3. Decachem (Oct 30, 2024). Oleum - Fuming Sulfuric Acid
  4. PubChem. Disulfuric acid (Compound CID 62682)
  5. PubChem. Fuming sulfuric acid (Compound CID 24681).
  6. WebQC (2024). Properties of H₂S₂O₇
  7. ChemicalBook (last updated on Dec 18, 2024). Pyrosulfuric acid
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Products Picture
Molecular representation of disulfuric acid (100% oleum)
  Product Communicator
Title Date
  UserPic Oleum is an intermediate product in the sulfuric acid contact process 11/9/2025
Identifiers
logo CAS Number
7783-05-3
logo EC Number
616-954-1
logo ECHA InfoCard
100.116.872
logo IUPAC Name
Sulfo hydrogen sulfate
logo PubChem ID
24681
Chemical Data
Chemical Formula

H2S2O7

Molecular Weight (g/mol)
178.13
Specific Gravity
1.84
Crude Data
API Gravity
-54.6
Country
Product Settings
Default
Status
A
Content provided by
Transaction Name Date
Modified by UserPic   Kokel, Nicolas 11/9/2025 9:22 AM
Added by UserPic   Kokel, Nicolas 11/9/2025 7:06 AM