Hach-Lange NH4D sc Ammonium Sensor - Manual (Page 9)

9
General Information
2.3 Theory of operation
The NH4D sc sensor utilizes ion-selective electrode technology to
measure ammonium ions (NH
4
+ ) in a waste water sample.
Well-known interferences of temperature and potassium are
compensated by using appropriate built-in sensors. The reference
electrode is a differential pH technology and does not have direct
contact with the process and therefore, it is particularly stable
against drift.
Ion-selective electrodes have a special membrane to which only a
specific type of ion can adhere. As a result an ion-specific potential
forms on the membrane surface. To be able to measure a potential
difference, a reference electrode is required, which is not affected
by the sample to be measured.
CARTRICAL TM technology reduces cross-sensitivities by not only
calibrating each electrode individually but also calibrating all 3
electrodes to each other.
pH value at the time of ammonium measurement:
In water, an equilibrium of ammonium (NH
4
+
) and ammonia (NH
3
)
is established. This equilibrium depends on the pH value.
NH
4
+
+ OH
–
<--> NH
3
+ H
2
O
Since ammonia in most instances has a pH value above 8,
ammonia is not directly measured by an ISE electrode.