Kjeldahlmetoder

Kjeldahlmetoder refers to a widely used analytical chemistry technique for determining the nitrogen content in a sample. Invented by Danish chemist Johan Kjeldahl in 1883, the method is primarily employed to assess the protein content of food and animal feed, as proteins are the major source of organic nitrogen in these matrices. The Kjeldahl method involves a three-step process: digestion, neutralization, and titration.

During the digestion stage, the sample is heated in concentrated sulfuric acid, often in the presence of a catalyst like copper sulfate or selenium. This process converts the organic nitrogen in the sample into ammonium sulfate. Following digestion, the solution is cooled and then neutralized with a strong base, typically sodium hydroxide. This liberates ammonia gas from the ammonium sulfate.

The liberated ammonia is then distilled, usually into a boric acid solution, which traps the ammonia. Finally, the amount of trapped ammonia is determined by titration with a standardized acid, such as hydrochloric acid. The volume of acid required to neutralize the boric acid-ammonia mixture is directly proportional to the amount of nitrogen in the original sample. By multiplying this nitrogen content by a conversion factor (typically 6.25 for most foods), the crude protein content can be estimated. While the Kjeldahl method is considered a reference method due to its accuracy and reliability, it is time-consuming and requires hazardous chemicals.