Analytical Method: Nitrogen (total) according to Kjeldahl (Others)


Instruments and Reagents:

Cat.No. 173016  Spectroquant® VIS Spectrophotometer Prove 100 or

Cat.No. 173017  Spectroquant® UV/VIS Spectrophotometer Prove 300 or

Cat.No. 173018  Spectroquant® UV/VIS Spectrophotometer Prove 600 or

Cat.No. 109748  Spectroquant® Photometer NOVA 30 or

Cat.No. 109751  Spectroquant® Photometer NOVA 60 or

Cat.No. 173632  Spectroquant® Colorimeter Move 100

Cat.No. 114946  Rectangular cells 10 mm and/or

Cat.No. 114947  Rectangular cells 20 mm and/or

Cat.No. 114944  Rectangular cells 50 mm 
Cat.No. 114752  Spectroquant® Ammonium Test
Cat.No. 100748  Sulfuric acid 98 % for the determination of nitrogen
Cat.No. 110958  Kjeldahl tablets for Wieninger method
Cat.No. 158793  Sodium hydroxide solution 50 % for analysis
Cat.No. 106798  4-Nitrophenol indicator 1g/l
Cat.No. 114946  Rectangular cell 10 mm
Cat.No. 107913  Boiling chips granules
Kjeldahl flasks and digestion apparatus
200-ml volumetric flask
Pipettes for pipetting volumes 0.6 and 8.0 ml


The method for the determination of total nitrogen according to Kjeldahl is defined in EN ISO 25663. Ammonium sulfate can be measured directly from the digestion solution by photometry.
Alternatively it is possible to determine the ammonium in the distillate by photometry in analogy with EPA 351.3 and Standard Methods 4500 Norg B. According to EPA the concentration of boric acid in the resultant sample must not exceed 0.2 %. When sulfuric acid is used as the distillation receiver as per Standard Methods, the sample solution must be neutralized prior to the photometric determination of ammonium.
By digestion e.g. according to EN 25663 with potassium sulfate-containing sulfuric acid, in the presence of a catalyst organic nitrogen compounds are converted into ammonium sulfate. Kjeldahl's nitrogen is determined photometrically according to the indophenol method.
Measuring range:
1 mg/l - 500 mg/l N at optical path lengths 10 mm and 690 nm
Sample preparation:
Digestion acc. to EN 25663.
Depending on the anticipated nitrogen content, pipette the corresponding sample volume into the Kjeldahl flask and add 10 ml of sulfuric acid 98 % and one Kjeldahl tablet. After digestion is complete, allow the solution to cool and neutralize with sodium hydroxide solution for analysis as described in the standard.
Quantitatively transfer the solution to a 200-ml volumetric flask and make up to the mark with water for analysis:
Prepared sample

A blank using water for analysis must be prepared in the same way as the sample.

Concentration of Kjeldahl's nitrogen [mg/l] Sample volume
Dilution factor
0.10 - 5.00 100 2
0.2 - 10.0 50 4
0.5 - 20.0 25 8
1 - 50 10 20
2 - 100 5 40
4 - 200 2.5 80
10 - 500 1 200


Perform the determination in the prepared sample using the Spectroquant® Ammonium Test according to the package insert.
The dilution (see Table) must be duly considered in calculating the analysis result.

Analysis result = measurement result x dilution factor


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