Present naturally in surface and wastewaters, ammonia mainly results from the deamination of organic nitrogen containing compounds and hydrolysis of urea. Ammonia may also be present from water treatment processes that utilize chloramines for disinfection, where ammonia is added to the water to react with chlorine. Ammonia is less likely to appear in groundwater due to adsorption by soil particles.
Ammonia can be measured in two ways, using either the Nessler colorimetric method or an ammonia Ion-Selective Electrode (ISE). In the Nessler method, Nessler reagent (K2HgI4) is added to a sample under strong alkali conditions. The ammonia in the sample reacts with the Nessler reagent producing a yellow color, which is proportional to the concentration of ammonia. Portable and benchtop photometers allow the results to be displayed as ammonia, ammonia-nitrogen or ammonium, each being a mathematical transformation based on the respective formula weight.
The ammonia ISE is an alternative measuring method. The ammonia-selective electrode uses a hydrophobic gas-permeable membrane to separate the sample solution from an internal solution of ammonium chloride. Dissolved ammonia NH3 and NH4+ are converted to NH3 (gas) by raising pH to above 11 with a strong base. NH3 (gas) diffuses through the membrane and changes the internal solution pH that is then sensed by the internal pH electrode. Potentiometric measurements are made with a pH meter having an expanded millivolt scale or with a specific ion meter. Hanna portable and benchtop ISE meters have the ion charge and formula weight pre-programmed into the meter allowing measurements in a variety of concentration units including moles/liter, mg/l and ppm.
It is important to note that the pH of the sample is needed to understand the concentration of ammonia versus ammonium by using the Henderson-Hasselbach equation. Knowing the pH of the sample and ammonia concentration make it possible to determine the proportion that is ammonium. The pKa of ammonia is 9.25.