EPA Method 351.4

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EPA Method 351.4:
TKN by ISE. Official Name: Nitrogen, Kjeldahl Total (Potentiometric, Ion Selective Electrode)

A sample is digested according to one of three procedures (macro-Kjeldahl, micro-Kjeldahl, or Block Digestion) to convert total Kjeldahl nitrogen (TKN) into ammonia. Following digestion and cooling, distilled water is added to the digestion flask and the pH adjusted to between 3 and 4.4 by the addition of NaOH. The sample is cooled and transferred to a 100 mL beaker. NaOH-NaI-EDTA is added to the beaker, and the ammonia concentration is measured using an ion selective electrode (EDTA prevents precipitation of hydroxides, thereby preventing deposition on the electrode membrane).

This method is applicable to the measurement of total Kjeldahl nitrogen (TKN) in drinking and surface water; domestic and industrial waste.

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

(A) Metals: Interference from metals is eliminated by the addition of NaI.(B) Nitrate: High nitrate concentrations (> 10 times the TKN level) result in low TKN values because of reaction between nitrate and ammonia. The reaction can be prevented by the use of an anion exchange resin (chloride form) to remove the nitrate prior to TKN analysis.

QC Requirements:
None given.

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



0.03 to 25 mg-TKN/L.

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


Revision Number:
Issued 1978

Test Description:
Nitrogen,Ammonia (as N)(Colorimetric,Automated Phenate)/Nitrogen,Kjeldahl,Total (Potentiometric,ISE)