Disulfuric acid

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Disulfuric acid
Dischwefelsäure - Disulfuric acid.svg
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[1]
Chemical formula H2S2O7
Molar mass 178.14 g/mol
Appearance colourless solid, very hygroscopic
Density 1.9 g/cm3, solid (20 ºC)
Melting point

35 °C

Boiling point

decomp.

Solubility in water reacts violently
Solubility in ethanol reacts
Hazards[2]
Material safety data sheet (MSDS) ICSC 1447
EU index number 016-019-00-2
GHS pictograms Skin Corr. 1ASTOT SE 3
GHS signal word DANGER
GHS hazard statements H314, H335
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 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:[3]

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.[3]

Ka (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–Oµ–S = 124º, S–Oµ = 164.5 pm and S–Ot = 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.[3]

References

  1. CRC Handbook of Chemistry and Physics, 62nd ed.; Weast, Robert C., Ed.; CRC Press: Boca Raton, FL, 1981; p B-154. ISBN 0-8493-0462-8.
  2. Index no. 016-019-00-2 of Annex VI, Part 3, to Regulation (EC) No 1272/2008 of the European Parliament and of the Council of 16 December 2008 on classification, labelling and packaging of substances and mixtures, amending and repealing Directives 67/548/EEC and 1999/45/EC, and amending Regulation (EC) No 1907/2006. OJEU L353, 31.12.2008, pp 1–1355 at p 400.
  3. 3.0 3.1 3.2 Greenwood, Norman N.; Earnshaw, A. Chemistry of the Elements; Pergamon: Oxford, 1984; pp 843, 845. ISBN 0-08-022057-6.
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