Metrohm NH3-selective electrodes - Manual (Page 9)

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2 Mode of operation
NH
3
-selective electrodes
￭￭￭￭￭￭￭￭ 3
(3)
U
N
is the Nernst potential (slope) of the electrode. Because of the fact that
the hydroxide concentration is proportional to the ammonia concentration
(equation 4), the electrode obeys the Nernst equation for the ammonia
concentration as well:
(4)
(5)
Using equation 5, either the ammonia concentration or the ammonium
concentration can be determined from the measured data. U‘
0
is deter-
mined essentially by means of the internal reference electrode. This
responds to the existing chloride concentration contained in the non-refill-
able reference electrolyte (gel).