The Digestion Phase

Kjeldahl digestion converts nitrogen 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.

Weigh your sample
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.

Add the sample, acid and catalyst in the test tube
Place the sample into the most suitable VELP glass test tube along with 12 - 20 ml of concentrated sulfuric acid, as specified in the method. The total amount of acid needed during 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).
Add KjTabs^TM catalyst tablets according to the Official Methods requirements. Copper catalysts are satisfactory but very slow to react. Selenium catalysts usually react very fast and are typically used for substances that are resistant to Kjeldahl digestion (e.g. fats and oils).

Set the program in the digester
Temperature and time ramps are critical for the correct digestion step and we strongly suggest following Offical Methods. In the majority of the methods, there are 2 or 3 temperature ramps to be set.
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 complete Kjeldahl digestion in each tube. It can reach 450 °C / 842 °F, ensuring a nitrogen recovery higher than 99% in the following stages).

Mineralization and Cooling
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 an 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 incomplete mineralization of the sample which cannot be processed further.

Raise the samples (automatically on DKL Kjeldahl digestion units) and cool them by natural dispersal. 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.