AMMONIA User Manual
AMMONIA_User 9/09
5
Some typical applications using ammonia sensors are:
- municipal water plant chloramination monitoring
- wastewater of gas scrubbers
- wastewater monitoring
- ammonia leaks in cooling towers
- the food sector, e. g. for wine and beer
- laboratories.
The measurement can be made directly in the liquid medium. This means that it is not necessary to carry out time-
consuming and complicated sample preparation. Colored or cloudy samples can also be measured. The pH value
must be greater than 7.3 or the chemical equilibrium of the ammonium ions shifts towards the ammonia state.
If the sample contains substances that could damage the sensor membrane, such as oils, fats or tensides, then the
ammonia can still be measured, using the technique known as “head space”. In this case, damage to the sensor is
ruled out, because there is no direct contact between the sensor and the sample. Measurement is performed in a
gas-tight space above the substance being measured. This gas-tight space above the substance being measured is in
equilibrium with the liquid as far as ammonia is concerned.
The simple sample preparation makes determination of the ammonia concentration a fast and economical operation
that can be applied with high precision over a wide range of concentration.
Principles of analyzer operation
The ammonia sensor consists of a glass pH electrode and a reference electrode which are surrounded by a common
electrolyte that is separated from the liquid being measured by a hydrophobic, gas-permeable membrane.
A thin layer of electrolyte is between the hydrophobic membrane and the glass pH electrode. Its pH value increases
when NH
3
gas makes its way through the membrane. The chemical processes in the electrolyte layer are described
by the following reaction equation.
NH3 + H2O
à
NH4
+
+ OH
-
(1)
This means that NH
3
functions as a base and thus increases the pH in the thin electrolyte layer in front of the glass
pH electrode. Since there is only a very small volume of liquid between the hydrophobic PTFE membrane and the
glass electrode, the electrode reacts very sensitively to the smallest amount of ammonia.
A strongly simplified derivation leads to the following relationship between the sensor signal and the concentration
of ammonia in the substance being measured: The reaction (Equation 1) is described by the law of mass action:
K = [NH4
+
] [OH
-
] / [NH3]
(2)
K = equilibrium constant
[x] = concentration component x