Ammonia-Nitrogen by Konelab - phenate colorimetric method
Reference Method: Baird, R.B., Eaton, A.D. and Rice, E.W. Standard Methods for the Examination of Water and Wastewater, 23rd Edition, American Public Health Association, Washington, DC, 2017. "Nitrogen (Ammonia)". Method 4500-NH3. Sections A, and G.
This method is applicable to the analysis of ammonia in drinking, ground and surface waters, domestic and industrial wastes, and biosolids (municipal sewage sludge). Extracts of soil samples are also analyzed by this method.
Ammonia is commonly encountered in domestic, wastewater, and industrial effluents. It is also found as a biochemical reduction product in ground waters. The presence of ammonia is indicative of domestic pollution, while its presence in ground water indicates microbiological processes. Ammonia is toxic to fish. It exerts a high oxygen demand as it converts to nitrite and nitrate. It interferes with water treatment processes by reacting with chlorine to form chloramines, which reduce the effectiveness of the treatment process. It also acts as a nutrient, promoting the growth of algae and other aquatic plants. Ammonia is seldom found in concentrations high enough to be harmful to man. In terms of water treatment processes ammonia is indicative of the effectiveness of the treatment.
Ammonia is determined colorimetrically using the Berthelot reaction (indophenol blue reaction; see attachment in section 18). In the first stage of the reaction, the sample's acid preservative is neutralized to a target pH of 6.5 to 7.0; the ammonia then undergoes a chlorination reaction with hypochlorite to form a monochloramine. In the second stage the monochloramine is reacted with phenate, under alkaline conditions (approximately pH 10) and in the presence of nitroprusside, to form quinonechlorimine. In the third stage quinonechlorimine reacts further with phenate to form the blue indophenol complex. The amount of indophenol is proportional to the amount of ammonia present and is measured colorimetrically at 660 nm.
The Konelab is a random access, discrete colourimetric analyzer. A sampler arm obtains and dispenses into a cuvette, specific quantities of sample and the various reagents required to form a color. The samples are incubated for specific times to achieve optimum color development. This process is rapidly repeated for each sample into 12 cuvette banks. Light from a polychromatic halogen source passes through a filter wheel to select the correct wavelength. The beam then passes through a chopper. The chopped beam them goes via a quartz fibre to a beam divider which sends 90% of the beam through the sample cuvette to the sample detector and the remaining 10% to the reference detector. Both signals go to the measurement electronics where a baseline corrected absorbance is calculated. The process is illustrated in the schematic below. Page 3 of 3 Success Through Science ®
This process is conducted twice for each sample, once before the color forming reagents are added and the second after the color is formed. The software subtracts the background absorbance from the color formed absorbance to give the net absorbance for each sample. This value is used in calculating the sample concentrations from the slope and intercept of a calibration curve.