A toxic colourless dense liquid, C₆H₅NHNH₂, b.p. 240°C, which turns brown on exposure to air. It is a powerful reducing agent, made from diazonium salts of benzene. It is used to identify aldehydes and ketones, with which it forms condensation products called hydrazones. With glucose and similar sugars it forms osazones. For such tests, the nitro derivative 2,4-dinitrophenylhydrazine (DNP) is often preferred as this generally forms crystalline derivatives that can be identified by their melting points. Phenylhydrazine is also used to make dyes and derivatives of indole.