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.
Nitrite interferes (same reaction), but can be eliminated by boiling with amidosulphuric acid (REF 918 973). Oxidising substances such as chlorine can lower the result or completely inhibit the reaction depending on their concentration.

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.