A mixture of concentrated sulphuric and nitric acids, used to introduce a nitro group (–NO2) into an organic compound. Its action depends on the presence of the nitronium ion, NO2+. It is mainly used to introduce groups into the molecules of aromatic compounds (the nitro group can subsequently be converted into or replaced by others) and to make commercial nitro compounds, such as the explosives cellulose trinitrate (nitrocellulose), glyceryl trinitrate (nitroglycerine),trinitrotoluene (TNT), and picric acid (trinitrophenol).