EXPLORING THE DIFFERENCES BETWEEN TOTAL KJELDAHL NITROGEN (TKN) AND TOTAL NITROGEN (TN)

Nitrogen is a fundamentally important element for all living organisms, entering, along with carbon, hydrogen, oxygen and sulfur, into the composition of proteins and, along with the aforementioned elements and phosphorus, into the constitution of nucleic acids. 

In order to better perform you analysis, it is helpful to understand the different forms of nitrogen and the parameters to be determined to get the best results.

The Kjeldahl analysis is a highly versatile method, as it can handle an extensive range of samples from food & feed, beverages, environmental, and agriculture to chemical and pharmaceutical industries.

Also known as Total Kjeldahl Nitrogen (TKN), it is used for the quantitative determination of organic nitrogen plus ammonia (NH3), and ammonium (NH4+) in the chemical analysis of a compound. Other forms of inorganic nitrogen such as nitrite-nitrogen and nitrate-nitrogen, are not included in this measurement.

The TKN analysis is typically used as a surrogate for protein in food, feed and beverage 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.

Even if TKN might not be the fastest method to use, since it requires numerous steps, thanks to its high reliability will always give satisfactory results if performed correctly and following the correct standard.

The Kjeldahl method doesn’t measure, or partially measures, some nitrogen compounds such as azides, azocompounds, hydrazones, hydrazine and hydroxylamine, nitrite, and nitrate.

By contrast, Total Nitrogen is the sum of the nitrate-nitrogen (NO3-N), nitrite-nitrogen (NO2-N), ammonia-nitrogen (NH3-N) and organic bounded nitrogen structures: it is determined in those samples where the presence of inorganic nitrogen is relevant.

Knowing the composition and nitrogen compounds is essential for water and wastewater treatment plants, where organic nitrogen is converted into inorganic nitrogen. To determine the total nitrogen in soil and in sludges from wastewater treatment plants for agricultural uses, the value of ammonia, nitrite, nitrate, urea and organic nitrogen has to be summed. 

In these cases, the Dumas combustion method is a strong alternative to the Kjeldahl analysis, as it offers more accurate results - usually a little bit higher. 

Kjeldahl’s method gives reliable results if the sought value is the sum of organic nitrogen (proteins, nucleic acids, urea, synthetic organic chemicals) and ammonia nitrogen while other nitrogen compounds are not present.

For the determination of total nitrogen in samples when the most relevant substances are ammonium, organic nitrogen, nitrate and nitrite, depending on the standard and the purpose of the determination, an elemental analyzer is needed. 

Therefore, depending on the type of sample, the purpose of the analysis and the different substances, the following values are analyzed:

• TKN (Total Kjeldahl Nitrogen) = Total organic nitrogen + ammonium
• Total organic nitrogen = TKN - ammonium
• Total inorganic nitrogen = (nitrate + nitrite) + ammonium
• TN (Total Nitrogen) = TKN + Nitrate + Nitrite