Thermo Fisher Scientific Ion Selective Electrodes Ammonia - Manual (Page 15)

Limits of Detection
The upper limit of detection in pure ammonia solutions is 1M. Ammonia is rapidly lost to the air
above a concentration of 1M. Dilution may be used if ammonia concentrations are above 1M. Also
dilute samples between 1M and 10-
1
M or calibrate the electrode at 4 or 5 intermediate points.
The lower limit of detection is around 1X10-
6
M. Refer to Figure 1 for a comparison of the
theoretical response to the actual response at low levels of ammonia. Ammonia measurements
below 10-
5
M NH3 should employ low level procedures.
pH Effects
The ammonia electrode can be used over the pH range 11 to 14. It is necessary to adjust the sample
pH to above 11 using the recommended ISA to convert all ammonium species in solution to
ammonia.
Electrode Life
The ammonia ion electrode will last six months in normal laboratory use. On-line measurements
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 membrane replacement is
required.
Electrode Storage
If erratic results are obtained from accidentally leaving the electrode in air, the space between the
sensing element and the inside of the membrane may be dry. To remedy this situation and allow
new filling solution to flow into the space, withdraw the glass electrode from the membrane by
pulling the cable slightly.
For low level measurements, immerse the tip of the electrode in pH 4 buffer between
measurements. For normal range measurements, keep the electrode tip immersed in a 0.001M or 10
ppm standard with added NaOH between measurements.
If storing the ammonia electrode overnight or over the weekend, immerse the tip in the 0.1M
standard without added NaOH. Do not store overnight in pH 4 buffer.
For longer periods of time, completely disassemble the electrode, rinse the inner body, the outer
body, and the cap with distilled water. After drying, reassemble the electrode without filling
solution.
ELECTRODE THEORY
Electrode Operation
A gas-permeable membrane is used to separate the electrode's internal solution from the sample
solution in the Eutech Ammonia Gas-Sensing Electrode. The sample diffuses dissolved ammonia
through the membrane until the partial pressure of ammonia is the same on both sides of the
membrane. The partial pressure of ammonia is proportional to the ammonia concentration.