An Overview of the Kjeldahl Method Including State-of-the-Art Equipment and a New Traceable Standard for IQ and OQ

By: Michaela Buffler, Nicole Amann, AnalytiX Volume 8 Article 3

Michaela Buffler, Product Specialist Kjeldahl Solutions, Büchi Labortechnik AG, Meierseggstr. 40, CH-9230 Flawil, Switzerland buffler.m@buchi.com
Nicole Amann, Product Manager Analytical Standards

The Kjeldahl Method

For almost 130 years the determination of nitrogen using the method developed by Danish chemist Johan Gustav Christoffer Thorsager Kjeldahl (1849–1900) has been an internationally accepted standard. He introduced the method in 1883 at a meeting of the Danish Chemical Society as a means to determine nitrogen in barley and yeast. The method, named after its inventor, has since found widespread application in agricultural, environmental, chemical, biochemical and pharmaceutical markets. Today, the Kjeldahl method is applied to the determination of nitrogen, primarily as a means to calculate the protein content, in dairy products, meat, beer, cereals and many other samples.

Kjeldahl Process

When determining protein according to the Kjeldahl method, the sample is first treated (digested) with concentrated H2SO4, which leads to the formation of ammonium sulphate. Through alkalinisation with NaOH, the ammonia is displaced from the ammonium sulphate and over-distilled into a boric acid receiver via steam distillation. This solution is then titrated with HCl or H2SO4 to determine the nitrogen content (and hence protein) in the sample.

Today, the Kjeldahl method can largely be automated by half- or fully automated Kjeldahl Distillation Units.

The three primary steps in the Kjeldahl method:

  1. Digestion 
    Organic nitrogen is converted into NH4

  2. Distillation 
    NH3 is distilled into the receiver vessel 

  3. Titration 
    Ammonia is determined

New for Kjeldahl: KjelFlex K-360

Büchi Labortechnik AG, a market leader in analytical laboratory solutions, has recently developed the KjelFlex K-360, the latest innovation in a wide range of Kjeldahl digestion and distillation instruments.

Thanks to its unique modular design, the KjelFlex K-360 offers flexibility to meet changing laboratory needs. By means of four add-on modules, the user can adjust the K-360 according to each individual application. The K-360 can be delivered completely with all modules; the modules can also be purchased separately and installed at any time for easy upgrading.

The basic configuration provides automatic dosage of boric acid, NaOH and water as standard. If needed, the unit can be equipped with an additional acid-resistant pump for the dosage of stronger acids. The application range is extended by a number of non-Kjeldahl applications, such as phenol distillation and the determination of formaldehyde, sulphur dioxide and volatile acids.

By connecting an external titrator (Mettler, Metrohm, Schott, Radiometer, for example), the K-360 capabilities are extended to provide an automatic system for nitrogen and protein determination. With such a configuration, manual work is reduced to an absolute minimum. The titration starts automatically when the distillation is finished. After titration, the contents of the sample tube and receiving vessel are drawn off automatically.

Besides the unique modularity, the K-360 offers a variety of additional features, including storage capacity of up to 50 customer-specific methods or 500 results, password security, data printout using a USB interface, multilingual software and numerous safety functions.

Qualification of the KjelFlex K-360

The qualification of laboratory instruments is an important aspect when following GMP/GLP regulations. The Installation Qualification (IQ) and Operational Qualification (OQ) assure that the instrument is delivered and operates according to the manufacturer’s specifications. Büchi and Sigma-Aldrich have also collaborated to develop a complete service package that comprises Installation Qualification, Operational Qualification, reference substance for the validation and several certificates. Visit www.buchi.com for more information on the K-360.

NIST-Traceable Analytical Standard for KjelFlex K-360 from Sigma-Aldrich

Sigma-Aldrich has developed reference material for KjelFlex K-360: an ammonium phosphate monobasic analytical standard for nitrogen determination according to the Kjeldahl method.  Its nitrogen content is traceable to NIST SRM 194.

Please visit www.sigma-aldrich.com, and type in the word “Kjeldahl” under Keyword (full text) in the search engine to see our complete offering of products and literature on the Kjeldahl method.

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