Kjeldahlmetoden

The Kjeldahl method is a standard analytical chemistry procedure for determining the nitrogen content of a sample. Developed by the Danish chemist Johan Kjeldahl in 1883, it is widely used in the food industry for protein analysis, as well as in environmental testing and pharmaceutical quality control. The method relies on the principle that nitrogen in organic compounds can be converted to ammonia through a series of chemical reactions.

The process begins with digestion, where the sample is heated with concentrated sulfuric acid. This breaks down the organic matrix and converts the nitrogen into ammonium sulfate. Copper or selenium are often added as catalysts to speed up this digestion process. Following digestion, the ammonium sulfate solution is made alkaline by adding a strong base, typically sodium hydroxide. This liberates ammonia gas, which is then distilled into a known volume of a weak acid, such as boric acid.

The ammonia reacts with the boric acid to form ammonium borate. The amount of ammonia absorbed is then determined by titrating the excess acid with a standard solution of a strong base, such as hydrochloric acid. By knowing the concentration of the titrant and the volume used, the amount of nitrogen in the original sample can be calculated. Since most nitrogen in organic materials is present in proteins, the Kjeldahl method is often used as an indirect measure of protein content, with a conversion factor typically applied.