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Nitrating mixture is the name given to the mixture of concentrated nitric acid and sulfuric acid, in a 1:1 ratio, used in the nitration of organic substances, such as aromatic compounds. The sulfuric acid in this mixture is sufficiently strong to protonate nitric acid, producing the nitronium ion (NO2+), which is the active species in aromatic nitration. It is also sometimes referred to as mixed acid.
The mixed acid solution is used in the preparation of aromatic nitrates, such as nitrobenzene, TNP, TNT, etc.
Mixed acid reacts vigorously with organic substances, generating heat and nitrogen dioxide gas.
Nitrating mixture is prepared by cooling a volume of concentrated sulfuric acid, usually on an ice bath. An equal volume of concentrated nitric acid is slowly added, keeping the solution at constant temperature. A cheaper nitrating mixture may be made by adding potassium nitrate to sulfuric acid though it may not work for some reactions.
Mixed acid is highly corrosive and is a fire hazard. It is unstable, and should be generated in situ.