What is the Kjeldahl Method for Nitrogen Determination?
Historical Background
Nitrogen determination has a long-standing history in analytical chemistry.
The Kjeldahl method was first introduced in 1883 by Johan Kjeldahl during a meeting of the Danish Chemical Society.
At the time, Johan Kjeldahl was the laboratory manager at Carlsberg and was tasked with scientifically studying beer production processes. While investigating proteins during malt production, he developed a method to determine nitrogen content that was faster and more accurate than existing techniques.
Portrait of Danish Chemist Johan Kjeldahl (1849-1900) by Otto Haslund (1842-1917) Original color painting located in the boardroom of the Carlsberg Laboratory, Copenhagen. Photo: Wikimedia Commons.
Total Kjeldahl Nitrogen: A Versatile and Reliable Method
Kjeldahl analysis is highly versatile and suitable for a wide range of matrices, including:
- Food & feed: grains, meat, fish, milk, dairy products, fruits, vegetables
- Beverages
- Environmental samples: soil, fertilizers, water, wastewater, sludge
- Chemical & pharmaceutical industries: paper, textiles, rubber, plastics, polymers
This versatility makes Total Kjeldahl Nitrogen (TKN) a reference method across multiple regulated sectors.
TKN in Environmental Analysis
Total Kjeldahl Nitrogen (TKN) represents the sum of organic nitrogen, ammonia (NH₃), and ammonium (NH₄⁺) in soil, water, and wastewater samples.To calculate Total Nitrogen (TN), nitrate-N and nitrite-N concentrations must be determined separately and added to the TKN value.
Today, TKN is a mandatory parameter for regulatory reporting in many water treatment facilities.
TKN and Protein Determination
In food analysis, TKN is commonly used as a proxy for protein content.
Protein concentration is calculated using a nitrogen-to-protein conversion factor (N-factor), which depends on the protein source.
Typical conversion factors include:
- 6.38 for dairy products
- 6.25 for meat, eggs, and corn
- 5.70 for wheat flour
- 5.46 for peanuts
Despite variability, these factors fall within a relatively narrow and standardized range.
Kjeldahl Nitrogen Analysis: Working Principle
1. Digestion
During digestion, complex organic nitrogen compounds are broken down into simpler molecules, ultimately forming ammonium sulfate.
This process is accelerated by high temperature, acid, salts, and catalysts (selenium, copper, titanium).
Vapors released during digestion are aspirated via a suction cap using a recirculating water vacuum pump and neutralized in an SMS scrubber, ensuring operational efficiency and equipment protection.
Digestion is the most time-consuming step of the Kjeldahl method.
2. Distillation
Ammonium sulfate is converted into ammonia gas, which is distilled and captured in an acidic solution, forming ammonium ions.
Standard Kjeldahl distillation does not detect nitrates or nitrites. To include these forms, a pre-reduction step using Devarda alloy is required.
VELP Kjeldahl distillation units offer automation levels ranging from basic distillation to fully automated systems with automatic reagent addition, residue aspiration, and integrated titration.
3. Titration
Titration quantifies the amount of ammonia collected, allowing calculation of Kjeldahl nitrogen or protein content (%).
Titration options include:
- Manual titration (UDK 129, UDK 139)
- Automatic potentiometric titration (UDK 149)
- Automatic colorimetric titration (UDK 159, UDK 169 – AOAC recommended)
Standards and Compliance
The Kjeldahl method is approved by major international organizations, including AOAC, ISO, EPA, AACC, and AOCS.
All VELP Scientifica Kjeldahl instruments comply fully with these standards.
Comprehensive Kjeldahl Solutions for BeLux Laboratories
A complete range of Kjeldahl digestion, distillation, and titration systems is available in Belgium and Luxembourg, supporting laboratories in food, feed, environmental, chemical, and pharmaceutical sectors.
Both semi-automatic and fully automated solutions are designed for high accuracy, regulatory compliance, and reliable TKN and protein analysis in demanding analytical environments.
Automatic Digestion Units
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12 |
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20 |
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42 |
Accessories for DK and DKL series
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KS 1000 Scrubber is designed to neutralize toxic fumes produced during mineralization. |
Automatic Distillation and Titration
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Titration |
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Colorimetric + Autosampler |
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Colorimetric |
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Potentiometric |
Semi-Automatic Distillation
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Our products
The DK 18/26 Series Kjeldahl digestion unit with aluminium heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ±0.5°C at all selectable temperatures.
The DK 18/26 Series guarantees accuracy and repeatability. The DK 18/26 performs batch digestion of up to 18 samples with 100 ml test tubes - Ø 26 mm, reducing the energy consumption (-13% compared to the previous series). Low energy consumption provides lower costs of analysis and limits CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The DK 20 Series Kjeldahl digestion unit with aluminium heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ±0.5°C at all selectable temperatures.
The DK 20 Series guarantees accuracy and repeatability. The DK 20 series perform batch digestion of up to 20 samples with 250 ml test tubes - Ø 42 m, reducing the energy consumption (-13% compared to the previous series). Low energy consumption provides lower costs of analysis and limits CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The DK 42/26 Series digestion unit with aluminum heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ±0.5°C at all selectable temperatures. DK 42/26 allows digestions with 42 positions for 100 ml test tubes, Ø 26 mm.
The DK 42/26 Series guarantees accuracy and repeatability. The DK 42/26 performs batch digestion of up to 42 samples with 100 ml test tubes - Ø 26 mm, reducing the energy consumption (-13% compared to the previous series). Low energy consumption provides lower costs of analysis and limits CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The DK 6 Series Kjeldahl digestion unit with aluminium heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ±0.5°C at all selectable temperatures.
The DK 6 Series guarantees accuracy and repeatability. The DK 6 series perform batch digestion of up to 6 samples with 250 ml test tubes - Ø 42 m, reducing the energy consumption (-13% compared to the previous series). Low energy consumption provides lower costs of analysis and limits CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The DK 6/48 Series Kjeldahl digestion unit with aluminium heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ± 0.5°C at all selectable temperatures.
The DK 6/48 Series guarantees accuracy and repeatability. The DK 6/48 series perform batch digestion of up to 6 samples with 300 ml test tubes - Ø 48 mm, reducing the power consumption of 13% compared to the previous series. Low energy consumption permit to have low costs for every single analysis and limit CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The DK 8 Series digestion unit with aluminium heating block ensures high thermal homogeneity heating up to 450 °C. The temperature of the block is constantly controlled by a microprocessor and is extremely stable, homogeneous and precise: ±0.5°C at all selectable temperatures.
The DK 8 Series guarantees accuracy and repeatability. The DK 8 performs batch digestion of up to 8 samples with 250 ml test tubes - Ø 42 m, reducing the energy consumption (-13% compared to the previous series). Low energy consumption provides lower costs of analysis and limits CO2 release.
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes.
DKL digesters are fully automatic and immediately ready to use, as they consist of an aluminium heating block, a lift for automatic sample handling, suction hood, test tubes, test tube rack and drip tray. The DKL 12 automate digestion with12 positions for both 250 and 400 ml test tubes, Ø 42 mm.
The DKL series provides the operator with the ultimate technology that allows the laboratory technician to set up the analysis choosing from a library of 54 programs (30 pre-installed + 24 customizable), press START and walk away, boosting the lab productivity.
Ease of loading and unloading make the VELP DKL a pleasure to use when carrying out Kjeldahl analysis, and there is no need to handle the sample when the tubes are hot!
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
DKL digesters are fully automatic and immediately ready to use, as they consist of an aluminium heating block, a lift for automatic sample handling, suction hood, test tubes, test tube rack and drip tray. DKL 20 automates digestion with 20 positions for both 250 and 400 ml test tubes, Ø 42 mm.
The DKL series provides the operator with the ultimate technology that allows the laboratory technician to set up the analysis choosing from a library of 54 programs (30 pre-installed + 24 customizable), press START and walk away, boosting the lab productivity.
Ease of loading and unloading make the VELP DKL a pleasure to use when carrying out Kjeldahl analysis, and there is no need to handle the sample when the tubes are hot!
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
DKL digesters are fully automatic and immediately ready to use, as they consist of an aluminium heating block, a lift for automatic sample handling, suction hood, test tubes, test tube rack and drip tray. DKL 42/26 allows automated digestion with 42 positions for 100 ml test tubes, Ø 26 mm
The DKL series provides the operator with the ultimate technology that allows the laboratory technician to set up the analysis choosing from a library of 54 programs (30 pre-installed + 24 customizable), press START and walk away, boosting the lab productivity.
Ease of loading and unloading make the VELP DKL a pleasure to use when carrying out Kjeldahl analysis, and there is no need to handle the sample when the tubes are hot!
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
DKL digesters are fully automatic and immediately ready to use, as they consist of an aluminium heating block, a lift for automatic sample handling, suction hood, test tubes, test tube rack and drip tray.
The DKL series provides the operator with the ultimate technology that allows the laboratory technician to set up the analysis choosing from a library of 54 programs (30 pre-installed + 24 customizable), press START and walk away, boosting the lab productivity.
Ease of loading and unloading make the VELP DKL a pleasure to use when carrying out Kjeldahl analysis, and there is no need to handle the sample when the tubes are hot!
For applications where the digestion process produces fumes, acid gases or corrosive substances, the below specific accessory is strongly recommended:
- KS 1000 Scrubber neutralizes all the fumes
The KS 1000 Scrubber is engineered to safeguard the laboratory environment and the health of the operators by eliminating exposure to harmful acid fumes produced during the Kjeldahl digestion or other processes.
The KS 1000 Scrubber effectively neutralizes corrosive and toxic gases in 2 steps:
- Dilution: in the right tank, reaction gases flow into a water solution, where they are effectively dissolved. This step allows for trapping even part of the sulfur dioxide produced during digestion.
- Neutralization: in the left glass tank, reaction gases are neutralized in a sodium hydroxide solution. Thanks to a wide contact surface between gas and liquid, the KS 1000 Scrubber achieves maximum neutralization efficiency.
In specific applications, an optional activated carbon filter can be used to absorb odors produced during oxidative mineralization.
The UDK 129 is an entry-level and reliable solution for safe distillations. The most suitable distiller for laboratories looking for a simple but efficient instrument for Kjeldahl nitrogen TKN, proteins, ammoniacal nitrogen, nitric nitrogen (Devarda), TVBN, sulfites, phenols, volatile acids, cyanides, and alcohol content.
The Kjeldahl distillation unit UDK 129 includes features that ensure efficient and reliable distillation, far beyond the expectations of an ordinary entry-level unit.
- Programmable sodium hydroxide addition
- High-precision pump ensuring constant accurate dosing of the reagent
- Intuitive interface for easy analysis settings
- Maximum safety for the operator
- Unmatched flexibility with a wide accessory range
