Analytical instruments - VELP Scientifica
VELP Scientifica - Via Stazione, 16 - 20865 Usmate (MB) ITALY - Tel. +39 039 62 88 11 - Fax. +39 039 62 88 120
Analytical instruments - VELP Scientifica
Send us your request
Contact Us
08:30 – 17:30 (GTM +1)
VELP Inc. U.S.
8.30 AM – 5.30 PM (EST)

Kjeldahl digestion

Kjeldahl digestion converts nitrogen compounds (proteins, amines, organic compounds) into ammonia compounds.

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.

Kjeldahl digestion sample weighing

If the sample is solid, weigh out approximately 1 - 3 g of the sample in a VELP weighing boat (nitrogen-free) (CM0486000 or CM0486001) and record the weight (the particle size of the sample should be reduced to < 1 mm, for better results. The sample might need to be homogenized, before any operation).
If the sample is liquid, measure the volume with a pipette and place it in a beaker and stir it using one of VELP’s hot plate stirrers. If necessary, remove any CO2 (e.g. fizzy drinks) before measuring the volume.


Kjeldahl digestion acid and catalyst addition

Place the sample into a VELP glass test tube (where nitrogen content could be quite low, larger sample amounts need to be used) along with 12 - 20 ml of concentrated sulfuric acid, as specified in the method. The total amount of acid needed during a Kjeldahl digestion can vary from one sample type to another. Another factor to consider is the loss of acid that occurs due to the evaporation through the exhaust system used. The VELP exhaust system and heat shield control acid loss (around 1.2 ml acid per sample).
A problem that might occur during the Kjeldahl digestion is the drying out of the digested sample, a process called “the salting out effect”, due to the too high flow rate.

Add catalyst tablets (select the correct variety according to the protocol):
Copper catalyst (CM, CT0006650) is satisfactory but very slow to react; environmentally friendly. Selenium catalysts (W, CT0006613) usually react very fast and are typically used for substances that are resistant to Kjeldahl digestion (e.g. fats and oils). Other formulas are available (ST, CT0006609; TCT, CT0006621).

Kjeldahl digestion DKL programming

Select the program from the menu of the Kjeldahl digestion unit (on DKLs the most used applications are pre-installed and others are user-programmable). Just by pressing Menu, Programs, you can choose which Standard Program to select or create a new Customizable Program.
Lower the samples (automatically on DKL Kjeldahl digestion units) into the aluminum heating block (maintenance-free and highly durable) and heat the mixture to the temperature indicated in the Standard Method. (The DKL aluminum heating block ensures the best possible homogeneity across all tubes and a complete Kjeldahl digestion in each tube. It can reach 450 °C / 842 °F, ensuring a nitrogen recovery higher than 99% in the following stages).

Heat the mixture for the time indicated in the Standard Method in order to obtain a clear and colorless solution. During this phase the sulfuric acid reacts with the sample, converting all nitrogen in organic form into inorganic form that is stable and ready to be analyzed.

H2SO4 + Organic N →(NH4)2SO4 + CO2 +  H2O + other compounds

If any problem occurs during the Kjeldahl digestion and/or if the sample preparation was not correct, the customer can notice the presence of carbon residues (black-brown colored) in the digested mixture and on the walls of the tubes. These are symptoms of an incomplete mineralization of the sample which cannot be processed further.

Raise the samples (automatically on DKL Kjeldahl digestion units) and cool by natural radiation.
Separate the suction cap (press the up arrow on DKL Kjeldahl digestion units) - a drip tray needs to be introduced below the suction cap to collect any drops of acid that might fall from the suction cap glass bells.
Now the tube rack can be removed and the samples are ready to be moved to the distillation phase.

Typical process for Kjeldahl digestion

Typical process for Kjeldahl digestion

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 digestion related products