Kjeldahlmethode

Kjeldahlmethode, or the Kjeldahl method, is a widely used chemical technique for determining the amount of nitrogen in organic substances. Developed by Johan Kjeldahl in the late 19th century, it is applied in food analysis, agriculture, and pharmaceuticals to estimate protein content and other nitrogenous compounds. The method measures total Kjeldahl nitrogen, which includes most organic nitrogen and ammonium, but does not isolate nitrogen present as nitrates without modification.

The principle involves three main steps. First, the sample is digested with concentrated sulfuric acid in the presence of a catalyst and sometimes aid reagents, transforming organic nitrogen into ammonium sulfate. In the second step, the digest is neutralized with alkali, converting ammonium ions to ammonia. In the third step, the released ammonia is distilled from the mixture and captured in a boric acid solution or another receiving medium, where it can be quantified by titration or another suitable detection method.

A typical calculation relates the amount of ammonia (and thus nitrogen) to the nitrogen content of the sample. The volume and normality of the titrant used to determine the ammonia are used to compute the nitrogen percentage, often expressed as N%. For protein determination, nitrogen content is commonly converted to crude protein by multiplying N% by a factor such as 6.25, though this factor varies with the type of protein.

The method is standardized by organizations such as AOAC and ISO and is available in automated and semi-automatic formats. It remains a robust and versatile technique but has limitations: it is time-consuming, utilizes strong acids and bases, and measures total nitrogen without differentiating between different nitrogen forms. It may not accurately quantify nitrogen in samples containing interfering substances without appropriate method adjustments.