Disulfuric acid

Disulfuric acid
IUPAC name Disulfuric acid [1]
Other names Pyrosulfuric acid
Identifiers
CAS number	7783-05-3 Y
PubChem	62682
ChemSpider	56433 Y
EC number	231-976-8
MeSH	Pyrosulfuric+acid
ChEBI	CHEBI:29211 Y
Jmol-3D images	Image 1 Image 2 Image 3
SMILES OS(=O)(=O)OS(O)(=O)=O OS(=O)(=O)OS(=O)(=O)O O=S(=O)(O)OS(=O)(=O)O
InChI InChI=1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Y Key: VFNGKCDDZUSWLR-UHFFFAOYSA-N Y InChI=1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6) Key: VFNGKCDDZUSWLR-UHFFFAOYAZ
Properties
Molecular formula	H2O7S2
Molar mass	178.14 g mol−1
Melting point	36 °C; 97 °F; 309 K
Y (verify) (what is: Y/N?) Except where noted otherwise, data are given for materials in their standard state (at 25 °C (77 °F), 100 kPa)
Infobox references

Disulfuric acid (or pyrosulfuric acid) is an oxoacid of sulfur. 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 can be seen as the sulfuric acid analogue 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 are isolable.

See also

• Disulfurous acid
• Oleum
• Sulfuric acid
• Sulfur oxoacid

References

• Greenwood, Norman N.; Earnshaw, Alan (1997). Chemistry of the Elements (2nd ed.). Butterworth-Heinemann. ISBN 0080379419.