Metrohm NH3-selective electrodes - Manual (Page 18)

6.5 Performing measurements with very low concentrations
￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭￭
12 ￭￭￭￭￭￭￭￭
NH
3
-selective electrodes
Buret volume 10 mL
Sample size 10 mL
ISA/TISAB 10 mL
Total volume 20 mL
Factor c
Standard solution
/c
Sample
20
This results in a sample concentration in the measuring solution of 2.5
mg/L. The optimum concentration of the standard solution is thus 2.5
mg/L · 20 = 50 mg/L. Please note that this is merely to be considered a
guideline for standard additions. Even if you deviate from this recommen-
dation, precise measurements will still be possible.
6.5 Performing measurements with very low concentra-
tions
NOTE
The electrode 6.0506.100 or the membrane module 6.1255.000 must
be used for measurements with very low concentrations.
The response time of the electrode is relatively long with low ammonia
concentrations. The response time decreases with increasing concentra-
tion. You can improve the response time of the electrode and the preci-
sion of the result for ammonia concentrations < 6 · 10
–5 mol/L with the
following measures:
￭ Condition the NH
3
-selective electrode prior to the measurement in an
ammonia-free pH 4 buffer solution.
￭ Dilute the measuring electrolyte 1 : 9 with distilled water.
￭ Work with sealed measuring vessel.
￭ Use as large a sample volume as possible so that the ratio of surface to
volume is as low as possible. This will minimize the absorption of
ammonia from the air.
Even when these measures are applied, the response time of the electrode
may still be as long as ten minutes, not only for the samples but also
when conditioning in pH 4 buffer.