EPA Method 351.2

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EPA Method 351.2:
Total Kjeldahl nitrogen in water by semiautomated colorimetry. Official Name: Method 351.2: Determination of Total Kjeldahl nitrogen by semi-automated colorimetry

The sample is heated in the presence of sulfuric acid, H2SO4 for two and one half hours.The residue is cooled, diluted to 25 mL and analyzed for ammonia.This digested sample may also be used for phosphorus determination.Total Kjeldahl nitrogen is the sum of free-ammonia and organic nitrogen compounds which are converted to ammonium sulfate (NH4)2SO4, under the conditions of digestion described.Organic Kjeldahl nitrogen is the difference obtained by subtracting the free ammonia value from the total Kjeldahl nitrogen value.Reduced volume versions of this method that use the same reagents and molar ratios are acceptable provided they meet the quality control and performance requirements stated in the method.

This method covers the determination of total Kjeldahl nitrogen in drinking, ground, and surface waters, domestic and industrial wastes.The procedure converts nitrogen components of biological origin such as amino acids, proteins and peptides to ammonia, but may not convert the nitrogenous compounds of some industrial wastes such as amines, nitro compounds, hydrazones, oximes, semicarbazones and some refractory tertiary amines.

Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)

High nitrate concentrations (10X or more than the TKN level) result in low TKN values.If interference is suspected, samples should be diluted and reanalyzed.Method interferences may be caused by contaminants in the reagent water, reagents, glassware, and other sample processing apparatus that bias analyte response.

QC Requirements:
Demonstration of laboratory capability, and periodic analysis of laboratory reagent blanks, fortified blanks, and other laboratory solutions as a continuing check on performance.

Maximum Holding Time:
28 days




Precision and accuracy values were calculated using interlaboratory data from performance evaluation studies.

MDL is established using reagent water fortified at a concentration of 2 to 3 times the estimated instrument detection limit and is determined from 7 replicate aliquots of the fortified reagent water and processed through the entire analytical method.

Revision Number:
Revision 2.0, August 1993

Test Description:
Nitrogen, Kjeldahl,

Report Number:

Instrument used for this test: