Thermo Fisher Scientific Ion Selective Electrodes Lead - Manual (Page 17)

Instruction Manual Lead Electrode
15
The lower limit of detection is influenced by the slight water solubility of the electrode pellet. Refer
to Figure 1 for a comparison of the theoretical response to the actual response at low levels of lead
ion. Neutral solutions containing free lead ions can be measured down to 1.0X10
-6
M (0.2 ppm).
Extreme care must be taken with measurements below 1.0X10
-5
M (2.0 ppm) to avoid adsorption of
lead ions in the sample onto container walls.
pH Effects
Figure 2 shows the electrode response to lead ion in solution at various pH levels. Hydrogen ion
interferes with low lead ion measurements. The minimum pH at which lead ion concentrations can
be measured without interference is given by the shaded area to the left in Figure 2.
At a high pH, free lead ion precipitates with hydroxide ion, thereby reducing the lead ion
concentration. The maximum pH at which the lead concentrations can be measured without
interference from hydroxide is given by the shaded area to the right in Figure 2. Within this shaded
area, lead combines with hydroxide to form Pb(OH)
2
. Since only free lead concentration can be
measured with the lead ion electrodes, a false reading results.
Electrode Life
A lead 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 electrode replacement is required.
Electrode Storage
The lead electrode may be stored for short periods of time in 1.0x10
-2
M lead solution. For longer
storage (longer than two weeks), rinse and dry the sensing pellet and cover the membrane tip with
any protective cap shipped with the electrode. 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 lead ion electrodes are composed of sulfides of lead and silver bonded into an epoxy or glass
body. When an electrode potential develops across the membrane, the membrane is in contact with
a solution containing lead ions. This electrode potential is measured against a constant reference
potential, using a pH/mV meter or an ion meter. The level of lead ion, 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 (
∼25 mV/decade)
X = level of lead ions in solution