Peroxymonosulfuric acid, H
5, also known as persulfuric acid, peroxysulfuric acid, or Caro's acid. 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. It is one of the strongest oxidants known (E0 = +2.51 V) and is highly explosive.
|Systematic IUPAC name|
Peroxomonosulfuric acid
Persulfuric acid
3D model (JSmol)
CompTox Dashboard (EPA)
|Molar mass||114.078 g mol−1|
|Density||2.239 g cm−3|
|Melting point||45 °C|
|Tetrahedral at S|
|Occupational safety and health (OHS/OSH):|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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5 is sometimes confused with H
8, 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.
Published patents include more than one reaction for preparation of Caro's acid, usually as an intermediate for the production of potassium monopersulfate (PMPS), a bleaching and oxidizing agent. One patent for production of Caro's acid for this purpose gives the following reaction:
This is the reaction that produces the acid transiently in "piranha solution".
5 has been used for a variety of disinfectant and cleaning applications, e.g., swimming pool treatment and denture cleaning. Alkali metal salts of H
5 show promise for the delignification of wood. It is also used in laboratories as a last resort in removing organic materials since H
5 can fully oxidize any organic materials.
Ammonium, sodium, and potassium salts of H
5 are used in the plastics industry as radical initiators for polymerization. They are also used as etchants, oxidative desizing agents for textile fabrics, and for decolorizing and deodorizing oils.
Pure Caro's acid is highly explosive. Explosions have been reported at Brown University and Sun Oil. As with all strong oxidizing agents, peroxysulfuric acid should be kept away from organic compounds such as ethers and ketones because of its ability to peroxidize these compounds, creating highly unstable molecules such as acetone peroxide.