Ammonia / Ammonium

Ammonium is a crucial component of the nitrogen cycle. Ammonia is the deprotonated form of ammonium. In drinking water treatment, ammonia monitors are usually situated at the front end of the process and are used for intake protection to the works. On a wastewater treatment works, ammonia is often monitored at both the inlet and the outlet to the works.

Low level online ammonia monitors work on the basis of colorimetry as it is more sensitive to low concentrations. For higher concentrations of ammonia, an ion selective electrode is more commonly used as it has a larger working range.

If colorimetry is the measurement principle, the sample is buffered to the correct pH for the chemistry and both ammonia and ammonium are measured. With an ion selective electrode approach, only the ammonium is measured but by simultaneously measuring pH and temperature, the ammonia concentration can be calculated. Typically, both approaches report the results in terms of mg/l nitrogen to overcome the issue of the form of the ammonia/ammonium in the original sample.

For the colorimetric method, Sodium dichloroisocyanurate is hydrolysed to form hypochlorite which reacts with ammonia to form monochloramine. This is then reacted with salicylate in the presence of nitroprusside to form indophenol blue. This is proportional to the initial concentration of ammonia and can be measured at 660nm