Ammonium Electrode
Instruction Manual
10
pH Effects
Hydrogen ion interferes with measurements of low levels of ammonium ion although the electrode
can be used over a wide pH range. Table 3 should be used to determine the minimum pH at which
low level ammonium measurements can be made without more than a 10% error due to hydrogen
ion interference.
Electrode Life
An ammonium ion electrode will last six months in normal laboratory use. On-line measurement
might shorten operational lifetime to several months. In time, the response time will increase and
the calibration slope will decrease to the point calibration is difficult and electrode replacement is
required.
ELECTRODE THEORY
Electrode Operation
An ammonium ion electrode consists of an electrode body containing an ion exchanger in a sensing
module. This sensing module contains a liquid internal filling solution in contact with a gelled
organophilic membrane containing an ammonium selective ion exchanger.
An electrode potential develops across the membrane when the membrane is in contact with an
ammonium solution. Measurement of this potential against a constant reference potential with a
digital pH/mV meter or with a specific ion meter depends on the level of free ammonium ion in
solution. The level of ammonium ions, corresponding to the measured potential, is described by the
Nernst equation:
E = Eo + S log X
where:
E = measured electrode potential
Eo= reference potential (a constant)
S = electrode slope (~ 56 mV/decade)
X = level of ammonium ions in solution
The activity, X, represents the effective concentration of the ions in solution. Total ammonium
concentration, Ct, includes free ammonium ions, Cf, plus bound or complexed ammonium ions, Cb.
Since the ammonium electrodes only respond to free ion, the free ion concentration is:
Cf = Ct - Cb
The activity is related to the free ion concentration, Cf, by the activity coefficient,
γ
,by:
X =
γ
Cf
Activity coefficients vary, depending on total ionic strength, I, defined as:
I = ½
Σ
CxZ
2
x
where:
Cx = concentration of ion X
Zx = charge of ion X
