AMMONIA User Manual
AMMONIA_User 9/09
2
5
2.2 Sensor Calibration
General advice on performing ammonia sensor calibration
Possible calibration procedures for the potentiometric measurement of ammonia are: direct calibration with a series
of calibration solutions, or the addition method. For the latter method, a known quantity of a standard solution is
added to a sample of the liquid with the unknown concentration, and from the change in signal one can work back
to calculate the initial concentration.
During calibration and measurement, care must be taken that the sample and the calibration solution have the same
temperature, otherwise the transport of water vapor through the membrane can cause drifting of the electrode.
Furthermore, the temperature also affects the slope of the sensor characteristic (which is temperature-dependent in
accordance with the Nernst equation). Basically, the ammonia sensor can be used over the temperature range 0 to
50°C. Ammonium chloride is recommended for making up calibration solutions. A strong alkali is used to adjust the
pH of the calibration solutions to pH
>
11. The sample solutions are prepared in the same way.
For standard on-line operation the preparation of ppm standards for a two point calibration differing by at least a
factor of 10 is described in appendix E.
Sensor calibration
The sensor is, of course, also suitable for measurement in situations where the user needs to determine the
ammonia concentration as accurately as possible. In this case, more complex means are required for the calibration,
in order to account for the various influences on the measurement.
There are two ways of making a quantitative measurement of ammonia:
•
direct measurement, with a multi-point calibration
•
the standard addition method.
The first method requires the setting up of a calibration curve, using reference solutions with known ammonia
concentration levels. The concentration levels of the reference solutions should cover the expected concentration
range of the samples. The calibration solutions and sample solutions must be treated in the same way: in order to
achieve maximum measurement accuracy, the chemical equilibrium between the ammonium ions and the ammonia
must be shifted towards ammonia.
This is done by adding an alkali, which is used to achieve a pH value
>
11. The adjustment of the pH is carried out not
only for the sample solution (with the unknown concentration) but also for the calibration solutions. Furthermore,
the measurement of the samples and reference solutions must be performed at the same temperature, because the
results will otherwise be influenced by the temperature.
Calibration starts off with the reference solution that has the lowest concentration, and proceeds through
increasingly higher concentrations. Before changing over to the next sample vessel, the sensor must be flushed with
ammonia-free distilled water and dried off by dabbing with a soft paper tissue (without touching the membrane!).
The result of the calibration measurements is a calibration curve that can be evaluated with normal analytical
methods. In some laboratory instruments (ion meters), the instrument itself performs the evaluation, so that the
concentration can be displayed directly after calibration. For lengthy series of measurements, the calibration should
be repeated at intervals.