EPA Method 351.1

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EPA Method 351.1:
TKN by Automated Colorimetry. Official Name: Nitrogen, Kjeldahl Total (Colorimetric, Automated Phenate)

This is an automated method. A sample is automatically digested with a sulfuric acid solution containing potassium sulfate and mercuric sulate as a catalyst to convert organic nitrogen to ammonium sulfate. The solution is then automatically neutralized with sodium hydroxide solution and treated with alkaline phenol and sodium hypochlorite solutions. The blue colored dye (indophenol) formed from reaction with ammonium is intensified by the addition of sodium nitroprusside. The absorbance is measured at 630 nm and is proportional to the total Kjeldahl nitrogen concentration.

The procedure determines Kjeldahl nitrogen in surface and saline waters. Approximately 20 samples per hour can be analyzed.

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

Iron and Chromium: Iron and chromium ions tend to catalyze, and copper ions tend to inhibit, the indophenol color reaction.

QC Requirements:

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



0.05 to 2.0 mg-N/L

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


Revision Number:
Issued 1971; Editorial Revision 1974 and 1978

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Examples:  8260

Examples:  Dioxin
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