EPA-ORD / EPA-OST: 351.2:  Total Kjeldahl nitrogen in water by semiautomated colorimetry

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Official Method Name
Method 351.2: Determination of Total Kjeldahl nitrogen by semi-automated colorimetry
Current Revision
Revision 2.0, August 1993
Method Subcategory
Method Source
  Methods for the Chemical Analysis of Water and Wastes (MCAWW) (EPA/600/4-79/020)
Brief Method Summary
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.
Scope and Application
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.
Applicable Concentration Range
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.
Quality Control Requirements
Demonstration of laboratory capability, and periodic analysis of laboratory reagent blanks, fortified blanks, and other laboratory solutions as a continuing check on performance.
Sample Handling
Samples should be collected in plastic or glass bottles. All bottles must be thoroughly cleansed and rinsed with reagent water.
Samples must be preserved with H2SO4 to a pH < 2 and cooled to a 4oC at the time of collection.
Samples should be analyzed as soon as possible after collection. If storage is required, preserved samples should be maintained at 4oC.
Maximum Holding Time
28 days
Relative Cost
Less than $50
Sample Preparation Methods