Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 13)

Since water vapor reacts with ammonia in the gas phase, placing the ammonia electrode in a closed
system containing water vapor will allow measurements of solutions above 10-
3
M (14 ppm) to be
taken.
Ammonia in samples containing non-aqueous solutions or surfactants can be measured by adjusting
the sample pH to 11 to 13 with 10M NaOH. After measuring the sample, transfer it to an
erlenmeyer flask large enough to contain approximately 2/3 volume of air after sample addition.
Place a magnetic stir bar in the flask, fit the flask with a one-hole rubber stopper (opening large
enough to insert the electrode snugly), and insert the electrode until it is just above the level of
liquid in the flask. The closed flask now forms an air-tight closed system with the gas phase
saturated with water vapor. The partial pressure of ammonia is in equilibrium with the solution.
For gas phase measurements, calibrate the electrode in a closed flask using standards. The response
time for the ammonia electrode will be longer in the gas phase than if it were actually immersed in a
surfactant-free, aqueous solution.
ELECTRODE CHARACTERISTICS
Reproducibility
Electrode measurements reproducible to ±2% can be obtained if the electrode is calibrated every
hour. Factors such as temperature fluctuation, drift, and noise limit reproducibility. Reproducibility
is independent of concentration within the operating range of the electrode.
Interference
Volatile amines interfere with the operation of the ammonia electrode. Most gases, since they are
converted to the ionic form in basic solutions, do not interfere with ammonia electrode
measurement. The level of ions in solution can change the solubility of ammonia, though ionic
species cannot cross the gas-permeable membrane and are not considered direct electrode
interference. The level of ions in sample solution and standards do not interfere, given that they are
equal. The same holds true for dissolved species. Low results can occur in direct measurements, due
to the presence of some metallic ions and their complexation effect on ammonia.
Effect of Dissolved Species
One common substance that is a potential electrode interference is water vapor. The concentration
of the internal filling solution under the membrane is changed when water, in the form of water
vapor, moves across the electrode membrane. These changes will be seen as electrode drift. If the
total level of dissolved species in solution, the osmotic strength, is below 1M and the sample and
electrode temperatures are the same, water vapor transport is not a problem.
Samples of low osmotic strength are automatically adjusted to the correct level through addition of
10M NaOH. If samples have osmotic strengths greater than 1M, they should be diluted before
measurement. However, this dilution should not reduce the ammonia level below 10-
5
M. If dilution
is not possible, for the reason mentioned, the sample can be measured by adjusting the osmotic
strength of the electrode filling solution. The total level of dissolved species in the electrode filling
solution may be adjusted by adding 0.425 grams of reagent-grade sodium nitrate (NaNO
3
) to 10 ml
of the electrode filling solution.