Calcium Pyrosulfate

Description

Calcium pyrosulfate has the molecular formula of CaS₂O₇ and the molecular weight of 216.2078 g/mol. There are few references in the scientific literature concerning this salt. This salt is not expected to be stable in aqueous solution and is expected to decompose to the sulfate. This may explain why the data on the Ca salt is sparse to nonexistent. It can be prepared via a solid-state reaction with ammonium pyrosulfate:
CaCl2+(NH4)2S2O7+heat
? CaS2O7+2NH3(gas)+H2O(gas)
The solubility of this salt has not been reported. The existence of hydrates is unlikely. However, if one heats the persulfate, a pyrosulfate salt can be obtained:
2CaS2O8 ? 2CaS2O7+O2
Further heating then forms the sulfate plus oxygen. These reactions have not been studied critically and no decomposition temperatures can be quoted. Another method of preparation involves the sulfate. If calcium sulfate is mixed with fuming sulfuric acid (to minimize the water present), it dissolves, forming a syrup which upon heating to 150°C deposits the pyrosulfate crystals which are stable. Further heating removes the excess acid. These crystals are stable to about 720°C where they decompose to form the sulfate plus sulfur trioxide.