EPA Method 351.3

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EPA Method 351.3:
Total Kjeldahl Nitrogen by Potentiometry. Official Name: Nitrogen, Kjeldahl Total (Colorimetric; Titrimetric; Potentiometric)

The sample is heated in the presence of sulfuric acid, potassium sulfate, and mercuric sulfate, and evaporated until sulfite fumes are obtained and the solution becomes colorless or pale yellow. The residue is cooled, diluted, and is treated and made alkaline with a hydroxide-thiosulfate solution. The ammonia is distilled and determined after distillation by potentiometry or other procedure.

This method determines Kjeldahl nitrogen in drinking, surface, and saline waters; domestic and industrial wastes. The method may not capture all nitrogenous compounds (e.g., some amines, nitro compounds, hydrozones); consult the method for more information.

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

Nitrate: High nitrate levels (> 10 times the TKN level) result in low TKN values. The reaction between nitrate and ammonia can be prevented by using an anion exchange resin (chloride form) to remove the nitrate prior to the TKN analysis.

QC Requirements:

Maximum Holding Time:
28 days (MCAWW, Table 1).



0.05 - 1,400 mg/L

Precision and accuracy values were calculated using interlaboratory data from EPA-managed Water Pollution (WP) performance evaluation studies.


Revision Number:
Editorial Revision 1974, 1978

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Examples:  8260

Examples:  Dioxin
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