Encyclopedia
Oleum
Fuming sulfuric acid (oleum) is sulfuric acid with an excess amount (up to 80%) of sulfur trioxide. Oleums can be in liquid or solid form, have an intense brown color; they react vigorously with water, burn skin and clothing, emit dangerous "smoke" (in particular, free sulfur trioxide). They are stored in glass, ceramic containers or in sheet iron vessels.
Oleum is often used in sulfonation reactions in organic chemistry (production of naphthalenesulfonic acid, hydroxyanthraquinone, thiindigo, derivatives of alizarin, etc.)
Source: "Explanations to the Harmonized System of the Russian Federation for Foreign Economic Activity (HS Russia)" (prepared by the Customs Authority of the Russian Federation) (volume 1, sections I - VI, groups 1 - 29)