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Kjeldahl nitrogen

Kjeldahl analysis is definitely the most common method for performing nitrogen and protein determination for diversified applications.

Nitrogen determination has a long history in the area of analytical chemistry.
Johan Kjeldahl first introduced the Kjeldahl method in 1883 at a meeting of the Danish Chemical Society.
Kjeldahl, at that time Carlsberg laboratory manager, was assigned to scientifically observe the processes involved in beer production.
While studying proteins during malt production, he developed a method of determining nitrogen content that was faster and more accurate than any method available at the time.

Kjeldahl nitrogen

Johan Kjeldahl, working at Carlsberg Laboratory in the 1880s

Kjeldahl analysis is extremely versatile, as it can handle a very wide range of samples, from food & feed (grain, meat, fish, milk, dairy, fruit, vegetables), beverages, environmental (agriculture, oilseeds, soil, fertilizers, water, wastewater, sludge) to chemical and pharmaceutical industries (paper, textiles, rubber, plastic, polymer).

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 treatment plants.

Total Kjeldahl nitrogen is often 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 range from 6.38 for dairy and 6.25 for meat, eggs and maize (corn) to 5.70 for wheat flour, and 5.46 for peanuts.

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

The Kjeldahl analysis may be broken down into three main steps:

  • 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.
  • Distillation - adding excess base to the acid digestion mixture to convert NH4+ to NH3, followed by boiling and condensation of the NH3 gas in a receiving solution.
  • 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.

Kjeldahl nitrogen - Modern VELP Scientifica solutions for digestion

Kjeldahl nitrogen - Modern VELP Scientifica solutions for distillation and titration