Pyrosulfate

Pyrosulfate is a chemical term that can refer to the pyrosulfate dianion S2O7^2- or to the conjugate acid pyrosulfuric acid, H2S2O7. It is formed by the condensation of two sulfate units with the loss of a water molecule, a process that underlies its name and its role in sulfuric acid chemistry. In solid form or in concentrated sulfuric acid solutions, pyrosulfate exists in equilibrium with bisulfate and sulfate species.

The structure of the pyrosulfate anion consists of two SO4 tetrahedra linked by a shared oxygen atom, giving an overall dianionic species. In aqueous solution, S2O7^2- rapidly hydrolyzes to two bisulfate ions: S2O7^2- + H2O → 2 HSO4^-.

Pyrosulfates are strong dehydrating agents and are used as reagents in organic and inorganic chemistry. They can promote esterification and sulfation reactions under strongly acidic conditions and may serve as sources of sulfate groups in certain syntheses. They are also encountered in industrial processes involving sulfuric acid, oleum, and related systems, where they can act as fluxes or dehydrating components. Due to their corrosive nature and the exothermic reaction with water, handling requires appropriate safety precautions.

Salts of pyrosulfate are common and include potassium pyrosulfate (K2S2O7) and ammonium pyrosulfate ((NH4)2S2O7). These salts are used in fluxing applications for glass and ceramics and as reagents in sulfuric-acid–based processes. Pyrosulfate chemistry underpins several interconversions among sulfate species and remains relevant in both laboratory and industrial contexts.