Metrohm NH3-selective electrodes - Manual (Page 15)

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6 Performing measurements
NH
3
-selective electrodes
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6 Performing measurements
6.1 Measured quantities
Ammonia concentrations are usually specified in one of the following
units:
￭ mol/L
￭ ppm (NH
3
)
￭ ppm (N)
Table 2 Conversion factors for various concentration specifications
Conc. in mol/L N content in ppm NH
3
content in ppm
10 –4 1.4 1.7
10 –3 14 17
10 –2 140 170
10 –1 1400 1700
1 14000 17000
6.2 General notes
￭ Standard and sample solutions must always be measured at the same
stirring speed and at the same temperature. A difference in tempera-
ture of 1 °C results in a deviation of approx. 2%.
￭ If possible, use narrow, high measuring vessels (minimum ratio of sur-
face to volume).
￭ Immediately prior to the measurement, a sodium hydroxide solution
must be added to each measuring solution. The solutions should have
a pH value of 11...14 after this addition and the concentration of all
dissolved particles should not exceed 1 mol/L.
￭ Alkali samples must be measured immediately. The ammonia loss in a
stirred alkali sample of 100 mL is approx. 50% within six hours.
For purposes of storage, the samples must be acidulated with hydro-
chloric acid to approx. pH 6 (approx. 0.5 mL c(HCl) = 1 mol/L per liter
of sample) and stored in well-sealed vessels.
Do not add the sodium hydroxide solution until just before the mea-
surement.