Metrohm NH3-selective electrodes - Manual (Page 16)

6.3 Performing direct measurements
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
10 ￭￭￭￭￭￭￭￭
NH
3
-selective electrodes
￭ The electrode must be conditioned in distilled water for at least 10
minutes between the measurements. If measurements are being per-
formed at very high concentrations, then the electrode should be con-
ditioned for 30 minutes.
￭ If a standard or sample solution is measured repeatedly, then it must
be stored in a sealed or covered vessel between the measurements.
This ensures that ammonia will not escape.
￭ Ammonia dissipates very rapidly out of solution when ammonia con-
centrations are > 1 mol/L. These kinds of samples must therefore be
diluted.
￭ You will find many additional useful notes on working with the NH
3
-
selective electrode in Application Bulletin 133.
6.3 Performing direct measurements
Direct measurement is a simple and rapid method for testing numerous
samples across a wide range of concentrations.
Observe the following notes when performing direct measurements:
￭ Select the concentrations of the standard solutions (e.g. NH
4
Cl solu-
tions) in such a way that the ammonia concentration of the sample
solution to be anticipated falls in the middle of the calibration range.
￭ Make sure that the temperatures of all standard and sample solutions
are identical.
￭ The diffusion of ammonia through the membrane is slowed down con-
siderably in the presence of ammonia concentrations < 6 · 10
–5 mol/L.
The response time of the electrode is prolonged accordingly. A special
measurement technique is required for this concentration range (see
Chapter 6.5, page 12).
￭ Ammonia dissipates very rapidly out of solution when ammonia con-
centrations are > 1 mol/L. These kinds of samples must therefore be
diluted (see Chapter 6.6, page 13).