Kjeldahl nitrogenIt refers to the nitrogen content measured by Kjeldahl method. It includes ammonia nitrogen and organic nitrogen compounds that can be converted to ammonium salts under these conditions. This type of organic nitrogen compound mainly includes proteins, amino acids, peptides, peptones, nucleic acids, urea, and synthesized organic nitrogen compounds in the negative trivalent form of nitrogen, but does not include azides, nitro compounds, etc. Due to the fact that most organic nitrogen compounds present in water are the former, the difference between Kjeldahl nitrogen and ammonia nitrogen can be used to represent the organic nitrogen content.

Domestic sewage, as well as industrial wastewater from food, biological products, and leather manufacturing, often contain a significant amount of organic nitrogen compounds, mainly proteins and their decomposition products (peptides, amino acids).

The key points of the measurement are to take an appropriate amount of water sample into a Kjeldahl flask, add concentrated sulfuric acid and catalyst (K)₂SO₄）Heating digestion converts organic nitrogen into ammonia nitrogen, then distills ammonia in an alkaline medium, absorbs it with boric acid solution, and determines the ammonia nitrogen content in the water sample by spectrophotometry or titration. When directly measuring organic nitrogen, the water sample can be pre distilled to remove ammonia nitrogen, and then measured using the Kjeldahl method.
When evaluating eutrophication of lakes, reservoirs, and other water sources, It is a very meaningful indicator.