Kjeldahlnitrogen

Kjeldahl nitrogen is a measure of the total nitrogen content in a sample, determined using the Kjeldahl method, developed by Johan Kjeldahl in 1883. It is widely used to estimate protein content in foods, feeds, and related products, as nitrogen is a key component of amino acids.

Principle and procedure: The sample is digested with concentrated sulfuric acid, often with a catalyst and inhibitors to speed digestion, converting organic nitrogen to ammonium sulfate. After digestion, the mixture is neutralized with sodium hydroxide, which liberates ammonia. The ammonia is distilled into a receiving solution, commonly containing boric acid, and then titrated with a standardized acid to determine the amount of ammonia present. The nitrogen content is calculated from the titration data and the sample mass.

Applications and interpretation: Kjeldahl nitrogen is used to estimate crude protein by applying a conversion factor (the most common factor is 6.25, though some foods and species require different values). It is also used to determine total nitrogen in fertilizers, pharmaceuticals, and environmental samples. The method yields total organic and inorganic nitrogen but does not distinguish among nitrogen forms or identify specific nitrogen-containing compounds.

Limitations and considerations: The method does not differentiate inorganic nitrogen from organic nitrogen and can be biased by the presence of non-protein nitrogen in some foods. Volatilization or incomplete digestion can affect accuracy. It is time-consuming, involves corrosive reagents and hazardous waste, and requires careful standardization and calibration. Alternatives, such as the Dumas combustion method, can provide faster results and different analytical advantages, depending on the sample type.

See also: Kjeldahl method, Kjeldahl nitrogen, protein determination, nitrogen analysis.