Electro-Chemical Devices (ECD) HYDRA Ammonium - Manual (Page 9)

HYDRA NH
4
+ -N Page 1
1.0 GENERAL DESCRIPTION
1.0.0 Description
The HYDRA Ammonium Analyzer measures the concentration of dissolved
ammonium as nitrogen (NH
4
+
-N) in water. The sensor uses three electrodes to
determine the NH
4
+
-N concentration, an Ammonium Ion Electrode, a Potassium Ion
Electrode and a pH electrode. It is designed for use in all kinds of water. Typical
applications include monitoring environmental waters, lakes, streams and wells as
well as wastewater treatment in aeration basins and effluent.
1.0.1 Potassium Ion Compensation
The Ammonium Ion Electrode provides the primary measurement. Any potassium
ion in the sample, due to its similar size and charge to the ammonium ion, causes a
positive interference in the measurement. A Potassium Ion Electrode measures the
amount of potassium ion present in the sample and HYDRA C22 Analyzer subtracts
the appropriate amount of signal from the Ammonium Measurement.
1.0.2 pH Compensation
The Ammonium Ion Electrode only measures the ammonium ion (NH
4
+ ) not
ammonia (NH
3
). Ammonium ion and ammonia coexist in a pH dependent ratio in
solution. At pH 9.2 the ratio is 1:1, at pH 7 nearly all of the ammonia is in the
ammonium state and at pH 11 it is nearly all ammonia which is invisible to the
sensor. The more acidic values favor the NH
4
+ and the more basic values favor
ammonia gas, NH
3
. The pH Electrode measures the pH and the HYDRA C22 Analyzer
calculates the total NH
4
+ -N concentration based on the pH vs. concentration profile.
1.0.3 Temperature Compensation
Temperature is also measured and used to compensate each of the three electrode measurements.
While the pH electrode response is well defined with respect to temperature the ion electrodes tend to
be less well behaved. For the best results
calibrate the sensors near the process
temperature.
1.0.4 Cleaning and Maintenance
The HYDRA Ammonium Analyzer is configured to
periodically actuate a cleaning cycle using the
integral spray cleaner in the sensor. The period
and duration of the cleaning cycle is user
configurable. The 4-20 mA output is held at
either the last value or a preset value during the
cleaning. This minimizes the formation biofilms
0
20
40
60
80
100
6 8 10 12
%
pH
NH
3
/NH
4
+
% concentration vs pH
% NH4+
% NH3