Mixed acid

In the chemical industry, mixed acid means a specific mixture of sulfuric acid and nitric acid. Mixed acid is a powerful nitrating agent used to make nitro compounds used as fertiliser and explosives.

 Mechanism 
Nitric acid and sulfuric acid are both Brønsted acids. Sulfuric acid is the stronger acid, so the two can react, causing protonation of nitric acid. Elimination of water leaves a nitronium ion. This reaches a chemical equilibrium, which can be written in a simplified form as

Nitronium is what reacts with other molecules to make nitro compounds. It has a positive electrical charge on a nitrogen atom, which makes it a strong electrophile. When nitronium reacts with other molecules, Le Chatelier's principle drives the equilibrium to make more nitronium.

 Uses 
Nitration of aromatic compounds to nitrobenzene and its derivatives is the main use of mixed acid. Reduction of nitrobenzenes makes anilines, another important type of chemical.