Thermo Fisher Scientific Ion Selective Electrodes Nitrate - Manual (Page 12)

Instruction Manual Nitrate Ion Electrode
10
Limits of Detection
The upper limit of detection in pure sodium nitrate solutions is 1M. In the presence of other ions, the
upper limit of detection is above 10
-1
M nitrate, but two factors influence this upper limit. Both the
possibility of a liquid junction potential developing at the reference electrode and the salt extraction
effect influence this upper limit. Some salts may infuse into the electrode membrane at high salt
concentrations, causing deviation from the theoretical response. Either dilute samples between 1M and
10
-1
M or calibrate the electrode at 4 or 5 intermediate points.
The lower limit of detection is influenced by the slight water solubility of the ion exchanger used in the
sensing portion of the electrode. Refer to Figure 1 for a comparison of the theoretical response to the
actual response at low levels of nitrate. Nitrate measurements below 10
-5
M NO
3
-1
(0.6 ppm as NO
3
-1
)
should employ low level procedures.
pH Effects
The operating range of the nitrate electrode is from pH 2.5 to pH 11.
Electrode Life
A nitrate 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
decreases to the point calibration is difficult and electrode replacement is required.
Electrode Storage
The nitrate ion electrode may be stored for short periods of time in 10
-2
M nitrate solution. For longer
storage (longer than two weeks), rinse and dry the nitrate membrane and cover the tip with any
protective cap shipped with the electrodes. 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.
ELECTRODE THEORY
Electrode Operation
The nitrate ion electrode consists of an electrode body containing a liquid internal filling solution in
contact with a gelled organophilic membrane containing a nitrate ion exchanger. When the membrane
is in contact with a solution containing free nitrate ions, an electrode potential develops across the
membrane. This electrode potential is measured against a constant reference potential, using a standard
pH/mV meter or an ion meter. The level of nitrate ions, corresponding to the measured potential, is
described by the Nernst equation: