Disulfuric acid

Disulfuric acid
IUPAC name	Disulfuric acid
Other names	Pyrosulfuric acid
Identifiers
InChI	InChI=1/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6)
InChIKey	VFNGKCDDZUSWLR-UHFFFAOYAZ
Standard InChI	InChI=1S/H2O7S2/c1-8(2,3)7-9(4,5)6/h(H,1,2,3)(H,4,5,6)
Standard InChIKey	VFNGKCDDZUSWLR-UHFFFAOYSA-N
CAS number	[7783-05-3]
EC number	231-976-8
ChemSpider	56433
Properties
Chemical formula	H2S2O7
Molar mass	178.14 g/mol
Melting point	36 °C
Hazards
Flash point	non-flammable
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)

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 (approx. 3.6 mmol/kg H₂S₂O₇ and 4.4 mmol/kg HS₂O₇⁻ at 25 ºC) due ionic self-dehydration:

2H₂SO₄ ⇌ H₃O⁺ + HS₂O₇⁻

K (25 ºC) = 5.1 × 10⁻⁵

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

H₂SO₄ + SO₃ → H₂S₂O₇

Disulfuric acid is a strong acid and protonates sulfuric acid in the (anhydrous) sulfuric acid solvent system.

K_a (25 ºC, H₂SO₄) = [H₃SO₄⁺][HS₂O₇⁻]/[H₂S₂O₇] = 1.4 × 10⁻²

There are salts of disulfuric acid, commonly called disulfates or pyrosulfates, e.g. potassium disulfate, K₂S₂O₇. The disulfate ion, S₂O₇²⁻, is a pair of corner-sharing SO₄ tetrahedra, with S–O_µ–S = 124º, S–O_µ = 164.5 pm and S–O_t = 144 pm.

There are other related acids with the general formula H₂O·(SO₃)_x though none are isolable. The trisulfate ion, S₃O₁₀²⁻, and the pentasulfate ion, S₅O₁₆²⁻, have been crystallographically characterised: the central sulfur atoms have tetrahedral coordination, with corner-sharing SO₄ tetrahedra, but the terminal SO₃ groups are almost planar as in sulfur trioxide.

References

Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 843, 845. ISBN 0-08-022057-6.