Kjeldahl nitrogen Kjeldahl nitrogen is the nitrogen content measured by Kjeldahl method. It includes ammonia nitrogen and organic nitrogen compounds which can be converted into ammonium salt under this condition. This kind of organic nitrogen compounds mainly include protein, amino acid, peptide, peptone, nucleic acid, urea and organic nitrogen compounds with negative trivalent nitrogen, but do not include azides, nitro compounds and so on. Because most of the organic nitrogen compounds in water are the former, the content of organic nitrogen can be expressed by the difference between Kjeldahl nitrogen and ammonia nitrogen. There are many organic nitrogen compounds in domestic sewage and industrial wastewater such as food, biological products and tannery, and the main components are proteins and their decomposition products (peptides and amino acids). The key point of Kjeldahl nitrogen determination is to take an appropriate amount of water sample into Kjeldahl flask, add concentrated sulfuric acid and catalyst (K2SO4), heat and digest, convert organic nitrogen into ammonia nitrogen, then distill ammonia in alkaline medium, absorb it with boric acid solution, and determine the content of ammonia nitrogen by Spectrophotometry or titration, that is, Kjeldahl nitrogen in water sample. When organic nitrogen is to be determined directly, the water sample can be pre distilled to remove ammonia nitrogen, and then determined by Kjeldahl method. Kjeldahl nitrogen is a very significant index in evaluating the eutrophication of lakes and reservoirs.