Peroxydisulfuric acid is an inorganic compound with a chemical formula (HO₃SO)₂. Also called Marshall's acid after Professor Hugh Marshall, who discovered it in 1891.^[1]

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Peroxydisulfuric acid

Names
IUPAC names μ-peroxido-bis(hydroxidodioxidosulfur) peroxydisulfuric acid
Other names Persulfuric acid, Peroxodisulfuric acid
Identifiers
CAS Number	13445-49-3 Y
3D model (JSmol)	Interactive image
ChEBI	CHEBI:29268 Y
ChemSpider	22822 Y
PubChem CID	24413
UNII	2RQ1860626 Y
CompTox Dashboard (EPA)	DTXSID00894926
InChI InChI=1S/H2O8S2/c1-9(2,3)7-8-10(4,5)6/h(H,1,2,3)(H,4,5,6) Y Key: JRKICGRDRMAZLK-UHFFFAOYSA-N Y InChI=1/H2O8S2/c1-9(2,3)7-8-10(4,5)6/h(H,1,2,3)(H,4,5,6)/f/h1,4H InChI=1/H2O8S2/c1-9(2,3)7-8-10(4,5)6/h(H,1,2,3)(H,4,5,6) Key: JRKICGRDRMAZLK-UHFFFAOYAS
SMILES O=S(=O)(OOS(=O)(=O)O)O
Properties
Chemical formula	H2O8S2
Molar mass	194.13 g·mol−1
Appearance	Colourless solid
Melting point	65 °C (149 °F; 338 K) (decomposes)
Solubility in water	soluble
Conjugate base	Peroxydisulfate
Related compounds
Other cations	Potassium persulfate Sodium persulfate Ammonium persulfate
Related compounds	Peroxymonosulfuric acid Pyrosulfuric acid
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). N verify (what is YN ?) Infobox references

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This oxoacid features sulfur in its +6 oxidation state and a peroxide group. Sulfur adopts the usual tetrahedral geometry.^[2]

The acid is prepared by the reaction of chlorosulfuric acid with hydrogen peroxide:^[3]

2 ClSO₃H + H₂O₂ → H₂S₂O₈ + 2 HCl

Another method is the electrolysis of moderately concentrated sulfuric acid (60-70%) with platinum electrodes at high current density and voltage:

H₂SO₄ + H₂O → H₃O⁺ + HSO₄⁻ (dissociation of sulfuric acid)

2 HSO₄⁻ → H₂S₂O₈ + 2 e⁻ (E₀ = +2.4V) (bisulfate oxidation)

2 H₂SO₄ → H₂S₂O₈ + H₂ (overall reaction)

3 H₂O → O₃ + 6 H⁺ (ozone produced as a side product)

Peroxydisulfuric acid is a precursor to several salts including sodium peroxydisulfate, potassium peroxydisulfate, and ammonium peroxydisulfate. These salts are used to initiate the polymerization of acrylonitrile, styrene, and related monomers. This application exploits the tendency of the peroxydisulfate anion to undergo homolysis to produce radicals. They are also used as cleaning of circuit boards.^[3]

• Peroxymonosulfuric acid (Caro's acid)
• Piranha solution