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Kjeldahl analysis

Kjeldahl analysis: a versatile and efficient method for the determination of Nitrogen and Protein

In 1889, while studying protein during malt production, Johan Kjeldahl developed a method for determining nitrogen content that was faster and more accurate than any method available at the time.

Kjeldahl analysis is extremely versatile, as it is suitable for a very wide range of samples, ranging from food and feed, and environmental samples to the chemical and pharmaceutical industries, as well as paper, textiles, rubber, plastics and polymers.

1. TKN in Environmental Analysis

Total Kjeldahl nitrogen (TKN) is the sum of organic nitrogen, ammonia (NH3), and ammonium (NH4+) in the chemical analysis of soil, water and wastewater.
To calculate Total Nitrogen (TN), the concentrations of nitrate-N and nitrite-N are determined and added to the total Kjeldahl nitrogen. 
Today, Total Kjeldahl Nitrogen is a required parameter for regulatory reporting at many water treatment plants.

2. TKN and Proteins

Total Kjeldahl nitrogen is used as a surrogate for protein in food samples. The conversion from TKN to protein depends on the type of protein present in the sample and what fraction of the protein is composed of nitrogenous amino acids.
However, the range of conversion factors is relatively narrow. Example conversion factors, known as N factors, for foods, ranging from 6.38 for dairy and 6.25 for meat, eggs and corn 5.70 for wheat flour, and 5.46 for peanuts.

3. The secret of a correct TKN determination

In most cases, the key to a successful Kjeldahl analysis can be the sample preparation step (before the digestion phase).
This method might not be the fastest method to use but thanks to its high reliability will always give satisfactory results if performed correctly (and following Standards).

Kjeldahl Nitrogen Analysis Working Principle

1. Digestion

The decomposition of nitrogen in organic samples utilizing a concentrated acid solution. This is accomplished by boiling a homogeneous sample in concentrated sulfuric acid. The end result is an ammonium sulfate solution.

2. Distillation

Adding an excess base to the acid digestion mixture to convert NH4+ to NH3, followed by boiling and condensation of the ammonia NH3 gas in a receiving solution.

3. Titration

To quantify the amount of ammonia in the receiving solution. The amount of nitrogen in a sample can be calculated from the quantified amount of ammonia ions in the receiving solution.

Today, various scientific associations approve the Kjeldahl method, including the AOAC International (Association of Official Analytical Chemists), AACC (Association of American Cereal Chemists), AOCS (American Oil Chemists Society), EPA (Environmental Protection Agency), ISO (International Standards Organization), and many others. All VELP Scientifica equipment for Kjeldahl nitrogen determination work in accordance with the above-mentioned associations.

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