Synthesis of Caro’s acid

Preparation of Caro’s acid (Peroxymonosulfuric acid)

Synthesis of peroxymonosulfuric acid

Synthesis of peroxymonosulfuric acid

A slight excess of 100% (or at least nearly anhydrous) hydrogen peroxide  is slowly added to pure, thoroughly cooled chloro­sulfonic acid. Vigorous evolution of hydrogen chloride occurs. When all the hydrogen peroxide has been added and the gas evolution has subsided, the reaction mixture is gradually warmed and the dissolved or still evolving hydrogen chloride is removed in aspirator vacuum. The mixture cannot be allowed to stand too long prior to hydrogen chloride removal because the peroxymonosulfuric acid tends to oxidize the hydrogen chloride to chlorine and to oxides of chlorine. The hydrogen chloride-free liquid can then be crystallized in a well-sealed flask. If, after standing for some time at room temperature, it is still not solid, the flask is placed in a cooling bath. After 12 hours the crystals are separated from the mother liquor either by rapid suction filtration on a glass frit. The effectiveness of this separation determines the purity of the product, which is normally 94-97%. The impurities present are H2S2O8 and traces of CI. The substance can be further purified by partial remelting. The yield is 50-70%. A small additional fraction can be obtained from the mother liquor. The handling of large quantities of peroxymonosulfuric acid is somewhat dangerous. Local overheating, caused, for example, by addition of H2O, can cause explosive decomposition. Safety glasses should always be worn. Peroxymonosulfuric acid is colorless, beautiful crystals; hygroscopic; pure material stable for a few days, although with slight loss of active oxygen, the rate of decomposition is catalysed by the impurities formed; vigorous oxidizing agent. M.p. +45°C with slight decomposition.

Reference: G. Brauer, Handbook of Preparative Inorganic Chemistry, Vol 1, 388-389 p, Academic Press, New York, 1965.


hydroxy hydrogen sulfate



InChI Key


Canonical SMILES


Depositor-Supplied Synonyms

Caro’s acid, peroxysulfuric acid, Peroxomonosulfuric acid, Peroxomonosulphuric acid, hydroxy hydrogen sulfate, Caro acid, sulfuroperoxoic acid, AC1MBPM0, AC1Q7DKP, H2SO5, [SO2(OH)(OOH)], CHEBI:29286, (dioxidanido)hydroxidodioxidosulfur, FHHJDRFHHWUPDG-UHFFFAOYSA-N, IN003374, OR339852

Removed Synonyms

persulfuric acid, Peroxymonosulfuric acid, CID2754594, 7722-86-3

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