Instruction Manual
Ammonium Electrode
9
Electrode Response
Plotting the electrode mV potential against the ammonium concentration on semi-logarithmic paper
results in a straight line with a slope of about 56 mV per decade. Refer to Figure 1.
The time needed to reach 99% of the stable electrode potential reading, the electrode response time,
varies from one minute or less for ammonium concentration above 1.0X10-
5
M to several minutes
near the detection limit. Refer to Figure 2.
Electrode Storage
The ammonium ion electrode may be stored in 1.0X10-
2
M ammonium standard for short periods of
time. For storage over 3 weeks, rinse and dry the ammonium membrane electrode and cover the tip
with any protective cap shipped with the electrode(s). The reference portion of the combination
electrode (or the outer chamber of the reference electrode) should be drained of filling solution, if
refillable, and the rubber insert placed over the filling hole.
Detection Limit
The upper limit of detection is 1M in pure ammonium chloride solutions. The upper limit of
detection is above 1.0X10-
1
M when other ions are present, but the possibility of a liquid junction
potential developing at the reference electrode and the "salt extraction effect" are two limiting
factors. Some salts may be extracted into the electrode membrane at high salt concentrations
causing deviation from theoretical response. Calibrate the electrode at four or five intermediate
points, or dilute the sample, to measure samples between 1.0X10-
1
M and 1M.
The slight water solubility of the ion exchanger in the sensing module, which causes deviation from
theoretical response, determines the lower limit of detection. The theoretical response at low levels
of ammonium chloride compared to actual response is shown in Figure 1. A low level measurement
is recommended if ammonium measurements are made below 1.0X10-
5
M (0.18 ppm as
ammonium).
