Ammonia occurs as a breakdown product of nitrogenous material and is harmful to fish and other forms of aquatic life. It occurs naturally in water due to the breakdown of plant and animal matter, but high concentrations can indicate contamination from raw sewage, fertilisers or industrial effluent.
Ammonia dissolves in water to produce an alkaline solution of ammonium hydroxide:
Two classic reactions have been utilised in test kits to detect the ammonium ion.
One method, nesslerisation, is the reaction of ammonium ions (NH4+) with the tetraiodomercurate anion in an alkaline solution (Nessler's reagent) to form a yellow-brown compound.
In fact, there has been some dispute over the actual formula of the coloured product, which has also been reported as HgO.Hg (NH3)2I2 and as NH2.Hg2I3. But, whichever is correct doesn't really matter to us; it's the colour change that is important. Of more practical relevance is the fact we are dealing with highly toxic mercury compounds in the field, so due care must be exercised during use and when disposing of spent materials.
Another approach was originally reported in 1859 by French chemist Marcellin Berthelot. Ammonium ions and phenol react in the presence of an oxidising agent, at high pH (11-12) to form a vivid blue colour. It has been shown that the intensity of this colour is proportional to the concentration of ammonium ions. The oxidising agent used is hypochlorite (more familiar as a component of household bleach) and the blue product is known as indophenol blue.