Metrohm NH3-selective electrodes - Manual (Page 8)

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2 ￭￭￭￭￭￭￭￭
NH
3
-selective electrodes
2 Mode of operation
Ammonia (NH
3
) reacts in water with the formation of ammonium ions
(NH
4
+ ):
(1)
The equilibrium of this reaction, i.e. the relative proportion of ammonia
and ammonium ions, is determined by the pH value of the solution. In
acidic solutions, ammonia reacts almost completely to ammonium ions. At
a pH value of 9.2, the ratio of ammonia to ammonium ions is approxi-
mately 1 : 1. If the pH value of the solution is known, then the ratio of
ammonia to ammonium ions can also be calculated directly. The following
applies:
(2)
The value of the equilibrium constant K is also dependent on the tempera-
ture.
Table 1 Temperature dependency of the ammonia – ammonium ions
equilibrium
Temperature in °C – log(K)
5 4.13
25 4.78
35 5.08
The addition of an excessive amount of a strong base, e.g. concentrated
caustic soda (NaOH), causes the ammonium ions to be transformed into
ammonia in their entirety. Determination of the ammonium concentration
is thus indirectly possible by means of the determination of ammonia. In
the case of the NH
3
-selective electrode, a gas-permeable, hydrophobic
membrane separates the measuring solution from the measuring electro-
lyte. Ammonia now diffuses through the membrane for as long as the
partial pressure on the two sides is identical in size. The partial pressure of
ammonia is always proportional to its concentration. The measuring elec-
trolyte is comprised of a sufficiently concentrated ammonium chloride
solution, which means that the ammonium concentration can be assumed
to be constant.
The potential U of the measuring electrode is proportional to the hydrox-
ide concentration of the measuring electrolyte: