AMMONIA User Manual
AMMONIA_User 9/09
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With the standard addition method, a known amount of ammonia (in the form of ammonium chloride) is added to
the sample with the unknown concentration. From the increase in the signal after this augmentation, one can work
back to derive the unknown initial concentration, whereby the increased volume of the total augmented solution
must be taken into account in the calculation. With the standard addition method, the user can exclude matrix-
specific influences that are not accounted for when setting up a calibration curve (the first method). The remarks
made previously about the shift of the chemical equilibrium in the ammonia direction also apply to the standard
addition method.
Procedure for laboratory measurements
In order to ensure that the chemical equilibrium between ammonium ions and ammonia is shifted towards ammonia
as completely as possible, alkali must be added to the samples with the unknown concentration and the standard
solutions just before starting measurement. One can, for instance, proceed as follows: 100ml of standard solution
resp. 100ml of the sample have 1ml 10- mol caustic soda added until the pH value has reached at least 11 (samples
with a low pH may require the addition of a substantial amount of caustic soda to reach a pH of at least 11. If in
doubt, the pH must be measured).
Samples and standard solution should be stirred with a magnetic mixer and mixer paddle, so that the measurement
stabilizes quickly. But care must be taken that the samples and standard solutions are not warmed up by action of
the magnetic mixer. If necessary, a thermally insulating plate must be placed between the vessel and the mixer
plate. The ammonia sensor should be mounted on a stand in such a way that it is immersed in the liquid in the
sample vessel placed on the mixer, but does not touch the magnetic mixer paddle or the bottom of the vessel. Any
air bubbles on the membrane must be removed.