What is the Kjeldahl method of nitrogen test?
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 available in Belgium and Luxembourg, designed to meet the needs of laboratories in food, environmental, feed, chemical, and pharmaceutical industries.
Our range includes both automatic and semi-automatic digestion units, as well as advanced distillation and titration systems for precise nitrogen and protein determination.
Each instrument is engineered for regulatory compliance (AOAC, ISO, EPA) and offers high-performance solutions for TKN and total nitrogen analysis.
Discover the ideal Kjeldahl system for your lab’s analytical requirements in the BeLux market.
Automatic Digestion Units
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Model |
Positions |
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8 |
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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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Model |
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KS 1000 Scrubber is designed to neutralize toxic fumes produced during mineralization. |
Automatic Distillation and Titration
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Model |
Titration |
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Colorimetric + Autosampler |
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Colorimetric |
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Potentiometric |
Semi-Automatic Distillation
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Model |
Our products
The UDK 139 is a convenient and reliable solution for safe, semi-automated distillations. The perfect distiller for laboratories looking for precise and reproducible results of Kjeldahl nitrogen TKN, proteins, ammoniacal nitrogen, nitric nitrogen (Devarda), TVBN, sulfites, phenols, volatile acids, cyanides, and alcohol content.
The Kjeldahl distillation unit UDK 139 combines excellent value-for-money with high reliability and advanced performance.
- The semi-automatic process ensures efficient operations
- Programmable water and sodium hydroxide addition
- Selectable steam generation output level 10% – 100%
- Auto removal of residues from sample tube
- Clear and intuitive operations thanks to the Smart User Interface and digital display
- Maximum safety for the operator
- Unmatched flexibility with a wide accessory range
The UDK 149 is designed for maximum versatility and reproducible results when determining Kjeldahl nitrogen TKN, proteins, ammoniacal nitrogen, nitric nitrogen (Devarda), TVBN, sulfites, phenols, volatile acids, cyanides, and alcohol content.
UDK 149 can be used in combination with several models of external potentiometric titrators for higher sample throughput and direct output of the final result.
- Connection to various external titrators for automated processing and efficient operations
- Premium result accuracy and precision
- Programmable boric acid, water, sodium hydroxide addition
- Selectable steam generation output level 10% – 100%
- Auto removal of residues from sample tube
- Clear and intuitive operations thanks to the Smart User Interface and digital display
- Maximum safety for the operator
- Unmatched flexibility with a wide accessory range
Illustration bellow showcase the combination of UDK 149 Automatic Kjeldahl Nitrogen Protein Analyzer (Reference: VEL-F30200145) together with TITROLINE 5000 Automatic titrator (Reference: VEL-R30800194):
The UDK 159 is designed for high-throughput laboratories looking for precise and reproducible results of Kjeldahl nitrogen TKN, proteins, ammoniacal nitrogen, nitric nitrogen (Devarda), TVBN, sulphites, phenols, volatile acids, cyanides, and alcohol content.
- The fully automatic process ensures efficient operations, distillation and titration performed simultaneously
- Shortest time-to-results with online titration and automatic results calculation
- Premium result accuracy and precision thanks to the integrated colorimetric titrator with high precision burette
- Programmable boric acid, water, sodium hydroxide addition
- Selectable steam generation output level 10% – 100%
- Auto removal of residues from titrator & sample tube
- Clear and intuitive operations thanks to the Smart User Interface and digital display
- Maximum safety for the operator
- Unmatched flexibility with a wide accessory range
The UDK 169 is designed for high-throughput laboratories looking for precise and reproducible results of Kjeldahl nitrogen TKN, proteins, ammoniacal nitrogen, nitric nitrogen (Devarda), TVBN, sulfites, phenols, volatile acids, cyanides, and alcohol content.
UDK 169 can be used in combination with the AutoKjel autosampler, for a highly productive system capable of autonomously processing sample tubes, which are fed directly into the fully automatic distillation and titration unit.
- The fully automatic process ensures efficient operations, distillation and titration performed simultaneously
- Shortest time-to-results with online titration and automatic results calculation
- Premium result accuracy and precision thanks to the integrated colorimetric titrator with high precision burette
- Programmable boric acid, water, sodium hydroxide addition
- Selectable steam generation output level 10% – 100%
- Auto removal of residues from titrator & sample tube
- Clear and intuitive operations thanks to the Smart User Interface and digital display
- Maximum safety for the operator
- Unmatched flexibility with the autosampler and a wide accessory range
The TitroLine® 5000 is a very compact titrator for simple routine titrations.
Thanks to the standard methods the creating of methods is no longer required. Just load the method and press the start button. GLP compliant results can be documented on a connected printer or USB-memory stick. Connection to various external titrators for automated processing and efficient operations.
- Color display that can be easily viewed from a distance and at extreme angles.
- Export the results as PDF or CSV on USB sticks.
- Pre-installed standard methods
Following titration modes are possible:
- Titrations to pH, mV - endpoint (2 EP)
- Titrations to one equivalence point (pH und mV) with dynamic or linear titration addition
- Manual titrations with the controller
- Dosing tasks
- Number of methods: 5
Typical applications for the TitroLine® 5000 are:
- Salt content in foodstuffs (cheese, soy sauce, ketchup)
- pH-value, total acidity in wine and beverages
- Acid value in bread and bread dough
- Total nitrogen according to Kjeldahl
- Chloride in drinking water and other samples
- pH-value, Alkalinity (p + m-value)
- FTA/TA („FOS/TAC“)
TitroLine 500 Automatic titrator Includes:
- 10006572 TITROLINE 5000 230V
- 10000284 Rod Stirrer TZ1847
- 10000285 pH Combination Electrode N61
- 10000286 Plug Cable Combination K1A
- 10000287 Stand Rod (145) SA TZ 1510
- 10000288 PTFE Ground-in NS 14,5 TZ 1520
- 10000289 Titration Vessel (200) TZ1759
- A00000211 Connection Kit Schott RS232
- A00191223 Power supply x Schott mechanical stirrer
