Disulfuric acid

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 &rarr;  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 H20.(SO3)x e.g. H2S4O13(melting point 4°C).