PEROXYACIDS

The acids involved include:
Oxydisulfuric acid is a chemical compound having the formula H2S2O5. In structural terms, it can be written as HO3S–SO2H. The salts are commonly called “metabisulfites”. Essentially, this acid is a “sulfate–sulfite acid”. It is related to the “dithionites” except for the addition of an extra oxygen atom to the molecule. The dithionite anion, S2O4^2- , is an anion formally derived from the hypothetical compound “dithionous acid”, H2S2O4.
Although the alkalis such as Na and K form metabisulfites, very few alkaline earths do. Such compounds are commonly used as food preservatives. Metabisulfite compounds have been demonstrated to cause asthma and other allergic reactions in sensitive individuals.
Peroxymonosulfuric acid, also known as persulfuric acid, peroxysulfuric acid, or as Caro’s acid, has the formula, H2SO5, a liquid at room temperature. In this acid, the S(VI) center adopts its characteristic tetrahedral geometry; the connectivity is indicated by the formula HO-O-S(O)2-OH. H2SO5 is sometimes confused with H2S2O8, known as “peroxydisulfuric acid”. The disulfuric acid, which appears to be more widely used as its alkali metal salts, has the structure HO-S(O)2-O-O-S(O)2-OH.
The laboratory scale preparation of Caro’s acid involve the combination of chlorosulfuric acid and hydrogen peroxide:
H2O2 + ClSO2OH ? H2SO5 + HCl
Peroxydisulfuric acid has the formula H2S2O8. It is also called Marshall’s acid. In structural terms, it can be written as HO3SOOSO3H. It is one of a group of sulfur oxoacids. Its salts, commonly known as persulfates, are industrially important but the acid itself is not. Alkali salts are commonly used but most alkaline earth peroxydisulfates remain unknown.