EPA Method 350.3

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EPA Method 350.3:
Ammonia by Potentiometry. Official Name: Nitrogen, Ammonia (Potentiometric, Ion Selective Electrode)

Ammonia is determined potentiometrically using an ion-selective ammonia electrode and a pH meter having an expanded millivolt scale or a specific ion meter.The ammonia electrode uses a hydrophobic gas permeable membrane to separate the sample solution from an ammonia chloride internal solution. Ammonia in the sample diffuses through the membrane and alters the pH of the internal solution, which is sensed by a pH electrode. The constant level of chloride in the internal solution is sensed by a chloride selective ion electrode which acts as a reference electrode.

This method determines ammonia-nitrogen in drinking, surface, and saline waters; domestic and industrial wastes.

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

(A) Volatile Amines: Volatile amines act as a positive interference (measured value is greater than the true value).(B) Mercury: Mercury forms a strong complex with ammonia. DO NOT preserve samples with mercuric chloride.

QC Requirements:
None given.

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



0.03 to 1400 mg/L ammonia as nitrogen.

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


Revision Number:
Issued 1974