Test kits for water analysis
Test kits for water analysis

Start this free course now. Just create an account and sign in. Enrol and complete the course for a free statement of participation or digital badge if available.

Free course

Test kits for water analysis

3.1 Ammonia

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:

NH3 (g) + H2O (l) = NH4+ (aq) + OH (aq)
Equation label: (4)

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.

NH4+ (aq) + 2[HgI4]2 (aq) + 4OH (aq) = HgO.Hg(NH2)I (s) + 7I (aq) + 3H2O (l)
Equation label: (5)

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.


Take your learning further

Making the decision to study can be a big step, which is why you'll want a trusted University. The Open University has 50 years’ experience delivering flexible learning and 170,000 students are studying with us right now. Take a look at all Open University courses.

If you are new to University-level study, we offer two introductory routes to our qualifications. You could either choose to start with an Access module, or a module which allows you to count your previous learning towards an Open University qualification. Read our guide on Where to take your learning next for more information.

Not ready for formal University study? Then browse over 1000 free courses on OpenLearn and sign up to our newsletter to hear about new free courses as they are released.

Every year, thousands of students decide to study with The Open University. With over 120 qualifications, we’ve got the right course for you.

Request an Open University prospectus371