Nitrate is reduced to nitrite with an inorganic reducing agent. Nitrite is then diazotised with an aromatic amine and simultaneously coupled to form an azo dye.

Nitrates occur in most ground and surface waters in concentrations of up to 20 mg/l. In addition to mere geological influences nitrate concentrations can also increase from agricultural sources (fertilisers). The VISOCOLOR® test kits are intended for the determination of nitrate in surface and drinking water and in industrial waste waters which do not contain high concentrations of interfering ions.