Disulfuric acid

Disulfuric acid (alternative spelling disulphuric acid) or pyrosulfuric acid (alternative spelling pyrosulphuric acid), also named oleum, is a sulfur oxoacid.[2] It is a major constituent of fuming sulfuric acid, oleum, and this is how most chemists encounter it. As confirmed by X-ray crystallography, the molecule consists of a pair of SO2(OH) groups joined by an oxide.[3]

Disulfuric acid
IUPAC name
Disulfuric acid [1]
Other names
Pyrosulfuric acid, Oleum
3D model (JSmol)
ECHA InfoCard 100.029.069
EC Number
  • 231-976-8
MeSH Pyrosulfuric+acid
  • InChI=1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Y
  • InChI=1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6)
  • OS(=O)(=O)OS(O)(=O)=O
  • OS(=O)(=O)OS(=O)(=O)O
  • O=S(=O)(O)OS(=O)(=O)O
Molar mass 178.13 g·mol−1
Appearance colorless
Melting point 36 °C (97 °F; 309 K)
Conjugate base Disulfate
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
N verify (what is YN ?)
Infobox references


It is also a minor constituent of liquid anhydrous sulfuric acid due to the equilibria:


The acid is prepared by reacting excess sulfur trioxide (SO3) with sulfuric acid:

Disulfuric acid can be seen as the sulfuric acid analog of an acid anhydride. The mutual electron-withdrawing effects of each sulfuric acid unit on its neighbour causes a marked increase in acidity. Disulfuric acid is strong enough to protonate "normal" 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 though none can be isolated.

See also


  1. International Union of Pure and Applied Chemistry (2005). Nomenclature of Inorganic Chemistry (IUPAC Recommendations 2005). Cambridge (UK): RSCIUPAC. ISBN 0-85404-438-8. p. 130. Electronic version.
  2. Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 978-0-08-037941-8.
  3. Hönle, Wolfgang (1991). "Crystal structure of H2S2O7 at 298 K". Zeitschrift für Kristallographie - Crystalline Materials. 196 (1–4). doi:10.1524/zkri.1991.196.14.279. S2CID 101311329.
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