Disulfuric 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.