KjeldahlAnalytik

Kjeldahl analysis, or KjeldahlAnalytik, is a classical chemical method for determining the total nitrogen content in organic compounds and many inorganic samples. Developed by Johan Kjeldahl in 1883, it remains widely used in chemistry, food science, agriculture, and environmental analysis.

The method combines a digestion, distillation, and titration procedure. In digestion, the sample is heated with concentrated sulfuric acid, often with a catalyst (such as copper sulfate) and a dehydrating salt (potassium sulfate). Organic nitrogen is oxidized and converted to ammonium sulfate. After digestion, the mixture is made alkaline with sodium hydroxide; the released ammonia is distilled from the solution and captured in a boric acid or another trapping solution. The trapped ammonia is then titrated with a standardized acid to determine its quantity. From this, the amount of nitrogen in the sample is calculated, and the result is often expressed as percent nitrogen or as percent protein using a conversion factor.

Applications cover a wide range of materials, including foods and feeds for protein content, fertilizers, soil and plant tissues, water, and environmental samples. In food and feed analysis, the nitrogen content is commonly converted to crude protein by multiplying by a factor, typically 6.25, though the factor may vary by protein source.

Advantages of the Kjeldahl method include its broad applicability and reliability for total nitrogen. Limitations include the use of corrosive reagents, relatively long analysis time, and the inability to differentiate between different forms of nitrogen (organic vs inorganic) without additional procedures. Modern alternatives, such as combustion-based methods, compete in speed and safety, but Kjeldahl analysis remains a standard reference method in official methods and routine laboratories.