Method 351.2

Total Kjeldahl Nitrogen(Semi-Automated Colorimetry)

This method is for the determination of total Kjeldahl nitrogen (TKN) in drinking and surface waters, domestic and industrial wastes. The sample is heated in the presence of sulfuric acid, potassium sulfate and mercuric sulfate for 2.5 hours. The residue is cooled, diluted to 25 mL and placed on an auto-analyzer for ammonia determination. This digested sample may also be used for phosphorus determination.  The applicable concentration range of the method is 0.1 - 20 mg/L.

Total Kjeldahl nitrogen is defined as the sum of free-ammonia and organic nitrogen compounds which are converted to ammonium sulfate. Organic Kjeldahl nitrogen is defined as the difference obtained by subtracting the free-ammonia value from the total Kjeldahl nitrogen value. 

The procedure converts nitrogen components of biological origin such as amino acids, proteins and peptides to ammonia, but may not convert the nitrogeneous compounds of some industrial wastes such as amines, nitro compounds, hydrazones, oximes, semicarbazones and some refractory tertiary amines. The applicable range of this method is 0.1 to 20 mg/L TKN. The range may be extended with sample dilution.

High nitrate concentrations (10x or more than the TKN level) results in low TKN values.  If an inteference is suspected, the sample should be diluted and re-analyzed. 

(EPA: Office of Water)


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Method Data

Hold Times, Preservatives, Preps, Collection, Analytical & Documentation
Holding Time:   28 days from sample collection to analysis.
Preservatives:   Preserve with concentrated sulfuric acid to a pH <2 and store at 4°C.
Required Preps:   500 mL HDPE or amber glass bottle
Collection Method:   Grab sampling
Analytical Methodology:   Spectrophotometry
Documentation:   351.2

Analyte List*

Analyte Formula CAS Number Detection Limit

* The analytes and detection limits listed for each method represent the typical detection limits and analytes reported for that particular method. Keep in mind that analyte lists may vary from laboratory to laboratory. Detection limits may also vary from lab to lab and are dependent upon the sample size, matrix, and any interferences that may be present in the sample.