Disulfuric acid is a sulfur oxoacid. It is a major constituent of fuming sulfuric acid, oleum, and this is how most chemists encounter it. It is also a minor constituent of liquid anhydrous sulfuric acid due to the equilibria:

H₂SO₄ ⇌ H₂O + SO₃

SO₃ + H₂SO₄ ⇌ H₂S₂O₇

The acid is prepared by reacting excess SO₃ with sulfuric acid:

H₂SO₄ + SO₃ → H₂S₂O₇

Disulfuric acid is a strong acid and protonates sulfuric acid in the (anhydrous) sulfuric acid solvent system. There are salts of disulfuric acid, commonly called pyrosulfates, e.g. potassium pyrosulfate.

There are other related acids with the general formula H₂O.(SO₃)_x e.g. H₂S₄O₁₃(melting point 4°C).

See also

• Oleum
• Sulfuric acid
• sulfur oxoacids

Footnotes