In acidic solution sulphanilic acid or sulphanilamide are diazotised with nitrite. The diazonium salt is then coupled with an aromatic amine, forming an intensely coloured azo dye.
Organic colloids, chlorine, humic acids and coloured heavy metal ions interfere.

In surface waters nitrite ions are generally present in low concentrations. Their presence in ground water is less common. In waste waters nitrite frequently occurs, even in fairly high concentrations.