Do you know the Kjeldahl method?
The Kjeldahl method is a chemical analysis method used for the determination of nitrogen content in different types of samples. It was developed in the 9th century by Johan Kjeldahl, a Danish chemist. It is an official method and is described in many regulations due to its multiple applications in many branches of industry (chemical, agricultural, environmental, etc.) and scientific research, but its main use is to determine the protein content in food. Since proteins are composed of amino acids that contain nitrogens, by determining the amount of nitrogen in the sample we can accurately determine the amount of protein in the food. Another main use is in water analysis.
How does the Kjeldahl method work?
The Kjeldahl method can be divided into three parts: digestion, distillation, and titration.
The digestion: in this step the organic nitrogen is converted into ammonium (NH4+). To carry out this reaction, the sample is placed in a digestion tube with the addition of sulphuric acid and a catalyst. The tube is then heated to approximately 400 °C. The heat and the sulphuric acid decompose the sample and the organic nitrogen is released. On contact with the acid, the nitrogen is reduced to ammonium sulphate.
Distillation: in this phase, the NH3 is distilled and separated from the remains of the digested sample. In order to separate the ammonia, we first need the deprotonated ammonium ion. To achieve this, sodium hydroxide is added to the digested mixture. Once it has passed through the distillation system, the ammonia is deposited in a container with a receiver solution containing boric acid and an indicator (methyl red, for example) which converts it back into ammonium.
Titration: at this stage, we use an acid-base reaction to determine the amount of nitrogen contained in our initial sample. To carry out the titration, we add a known quantity of hydrochloric acid (HCl) to the product of the previous distillation. We can identify when the sample is completely neutralised by the change in colour of the indicator. The amount of hydrochloric acid to be used depends on the amount of salt previously formed between boron and ammonium. Therefore, by means of conversion factors we can calculate the amount of initial nitrogen.
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