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R9 Laboratory SOP 592
ANALYSIS OF TOTAL KJELDAHL NITROGEN IN WATER
Summary
This method is for the determination of total Kjeldahl nitrogen (TKN) in drinking and surface waters, domestic and industrial wastes. This SOP is based on EPA Method 351.2, Revision 2.0. The colorimetric method is based on reactions that are specific for the ammonia ion. The digestion converts organic forms of nitrogen to ammonium sulfate. Nitrate and some amines, nitro compounds, oximes, semicarbazones and some refractory tertiary amines are not converted to ammonium sulfate during digestion.
The applicable range is 0.3 to 10 mg/L. The range can be extended for high level samples through dilution either before or after the digestion procedure. The quantitation limit for total Kjeldahl nitrogen is 0.3 mg/L.
During the digestion procedure, the sample is heated in the presence of sulfuric acid, K2SO4, and HgSO4 for two and a half hours. This converts free ammonia, ammonium-nitrogen, and nitrogen components of biological origin such as amino acids, proteins and peptides to ammonium sulfate (NH4)2SO4. The residue is cooled, diluted with water and analyzed for ammonia. (This digested sample may also be used for total phosphorus determination). In-line neutralization of the digested sample with concentrated buffer on the chemistry manifold of the Lachat QuikChem AE converts the ammonium cation to ammonia, and also prevents undue influence of the sulfuric acid matrix on the pH-sensitive color reaction which follows. The ammonia that is produced is heated at 60°C with salicylate and hypochlorite to produce a dark blue-green color, which is proportional to the ammonia concentration. The absorbance is read at 660nm.
Total Kjeldahl nitrogen is the sum of free-ammonia and organic nitrogen compounds which are converted to ammonium sulfate during the digestion process. Organic Kjeldahl nitrogen is the difference obtained by subtracting the free-ammonia value from the total Kjeldahl nitrogen value.