The Kjeldahl Method: Revolutionizing Nitrogen Determination.
Nitrogen determination has a long history in analytical chemistry.
Johan Kjeldahl first introduced the Kjeldahl method in 1883 at a meeting of the Danish Chemical Society.
Johan 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.
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 is versatile and effective
The 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).
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.
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 range from 6.38 for dairy and 6.25 for meat, eggs and corn to 5.70 for wheat flour, and 5.46 for peanuts.
What is Kjeldahl Nitrogen Analysis Working Principle?
Digestion
The goal of the digestion is to break down the bonds that hold the polypeptides together and convert them into simpler molecules (such as water, carbon dioxide and ammonium sulphate). These reactions can be speeded up by the temperature used during Kjeldahl digestion (the higher the temperature used, the faster the digestion can be obtained) and by the presence of acid, salt and catalysts (selenium, copper, titanium). Vapors that escape from the tubes are aspirated through the suction cap by a JP recirculating water vacuum pump and eliminated in an SMS scrubber. This configuration optimizes the efficiency of the operation. Avoid using Kjeldahl digestion units without an exhaust system: this will dramatically shorten its life and might cause expensive damage. This is the most time-consuming step of the analysis.
Distillation
The ammonium sulphate present in the digested sample is converted into ammonia gas, heated and distilled. The ammonia gas is led into an acid trapping solution where it dissolves and becomes a trapped ammonium ion once again. Using the Kjeldahl distillation, nitrites and nitrates are not detected. In order to quantify these elements, a reduction of the sample is necessary (using Devarda alloy) before the Kjeldahl digestion stage.
VELP Kjeldahl distillation units offer different levels of automation, from entry-level distillation to distillation with automatic reagents addition and residues aspiration, up to fully automated system with integrated colorimetric titration.
Titration
Titration is used to determine the amount of ammonia distilled off from the digested solution and hence calculate the Kjeldahl nitrogen or protein amount, as %.
2 NH4 H2BO3 + H2SO4 → (NH4)2SO4 + 2H3BO3
Titration can be performed:
• Manual after usage of UDK 129 or UDK 139
• Automatic with external potentiometric titrator, UDK 149
• Automatic colorimetric, UDK 159 or UDK 169 (AOAC recommended)
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.
Comprehensive Range of Kjeldahl Analysis Instruments for Laboratories.
Explore our extensive selection of Kjeldahl analysis instruments designed to meet the needs of laboratories in Belgium and Luxembourg. Our range includes both automatic and semi-automatic digestion units, as well as advanced distillation and titration units. Each instrument is engineered for precision and efficiency, ensuring accurate nitrogen determination in various applications. Discover the ideal solution for your laboratory’s requirements.
Automatic Digestion Units
DKL 8 Automatic Digestion Unit
Automatic Kjeldahl Digestion Unit with 8 positions aluminium heating block.
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DKL 12 Automatic Digestion Unit
Automatic Kjeldahl Digestion Unit with 12 positions aluminium heating block.
DKL 20 Automatic Digestion Unit
Automatic Kjeldahl Digestion Unit with 20 positions aluminium heating block.
DKL 42/26 Automatic Digestion Unit
Automatic Kjeldahl Digestion Unit with 42 positions aluminium heating block.
Semi-Automatic Digestion Units
DK 6/48 Digestion Unit
Kjeldahl Digestion Units with a 6-position aluminium heating block for test tubes of 48mm diameter.
DK 42/26 Digestion Unit
Kjeldahl Digestion Units with 42-position aluminum heating block for micro-kjeldahl applications.
KS 1000 Scrubber
Designed to neutralize toxic fumes produced during mineralization, in combination with DK(L) series.
Automatic Distillation and Titration Units
Semi-Automatic Distillation
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
