Metrohm NH3-selective electrodes - Manual (Page 19)

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6 Performing measurements
NH
3
-selective electrodes
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6.6 Influence of dissolved particles on the measurement
Water vapor is a potential interference factor. It can enter through the
membrane and change the concentration of ammonium chloride in the
measuring electrolyte. This leads to a potential drift. The introduction of
water should not be a problem, however, if the following conditions are
met:
￭ The total concentration of the dissolved particles is approx. 0.1 mol/L
(osmotic pressure).
￭ The electrode and the sample solution are maintained at the same
temperature.
The addition of the sodium hydroxide solution to samples with small ionic
strength results automatically in the correct concentration of particles in
solution. Samples with a total ion concentration > 1 mol/L should be
diluted prior to the measurement, although the ammonia concentration
should not be too close to the detection limit.
Samples with high total ion concentration (i.e. with high osmotic pressure)
and with low ammonia concentration can be measured after the osmotic
pressure of the measuring electrolyte has been increased. This is increased
through the addition of sodium nitrate (2.125 g NaNO
3
per 50 mL mea-
suring electrolyte).