Determination of Kjeldahl Nitrogen

Nitrogen is one of the five main elements found in organic materials such as protein. This fact was first recognized by the Danish chemist Kjeldahl as a method for determining the amount of protein in samples from a wide variety of organisms. Kjeldahl developed this method to determine the amount of nitrogen in mixtures of substances containing ammonium salts, nitrate or organic nitrogen compounds.

The main principle in this procedure is the oxidation of the organic compound using strong sulfuric acid. As the organic matter oxidizes, the carbon it contains turns into carbon dioxide and the hydrogen turns into water. Nitrogen from amine groups is converted to ammonium ion, which dissolves in the oxidizing medium.

The Kjeldahl method of nitrogen analysis is a globally applied standard for calculating protein content in a wide variety of materials, from human and animal feed, fertilizer, wastewater and fossil fuels.

The Kjeldahl method consists of three steps that must be performed in the following order:

• The sample is first digested in strong sulfuric acid in the presence of a catalyst that helps convert the amine nitrogen into ammonium ions.
• The ammonium ions are then converted to ammonia gas, heated and distilled. The ammonia gas is directed into a capture solution where it dissolves and becomes an ammonium ion once again.
• Finally, the amount of ammonia retained by titration with a standard solution is determined and a calculation is made.

The first step (sample digestion) is the most time consuming step in the analysis. The purpose of this step is to break the bonds that hold the polypeptides together and turn them into simpler chemicals such as water, carbon dioxide and ammonia. The purpose of the next step, distillation, is to separate ammonia (i.e. nitrogen) from the digestion mixture. In the third step (titration), ammonia neutralizes some of the hydrochloric acid it finds there as it dissolves in the acid-retaining solution, and the remaining acid is then back-titrated. In this way, the amount of ammonia distilled from the digestion solution can be calculated, thereby determining the amount of nitrogen in the protein.

The element nitrogen is the primary growth component found in hundreds of chemical compounds from fertilizers, foods, oils, water/wastewaters and pharmaceuticals to textiles. High concentrations of nitrogen within lakes or streams indirectly increase oxygen demands, often depleting oxygen levels to toxic conditions and killing fish and other aquatic organisms.

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