30
30.07
40
31.07
50
32.06
If changes in temperature occur, the electrodes should be re-calibrated. The temperature range for
the Eutech Lead Ion Electrode is 0
o
-80oC, provided that temperature equilibrium has occurred. If
the temperature varies substantially from room temperature, equilibrium times up to one hour are
recommended.
Electrode Response
Plotting the electrode mV potential against the lead concentration on semi-logarithmic paper results
in a straight line with a slope of about 25 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 several seconds in highly concentrated solutions to several minutes near the detection
limit.
A drifting potential reading or a decrease in electrode slope may mean that the electrode membrane
needs polishing.
To polish the membrane:
1.
If using polishing paper, cut off a 1-2" piece and place it face up on the lab bench.
2.
Put a few drops of distilled or deionized water in the center of the paper.
3.
Holding the paper (cotton) steady with one hand, bring the membrane of the electrode
down perpendicular to the paper and, with a slight swirling motion, gently polish the tip of
the electrode against the surface of the polishing paper (cotton) for a few seconds.
4.
Rinse the electrode surface with distilled or deionized water and soak the electrode tip in
standard solution for about five minutes before use.
5.
If using jeweller's rouge, place a cotton ball on the table top and flatten it using the bottom
of a beaker.
6.
Put 1-2 drops of distilled or deionized water in the center of the cotton pad.
7.
Add a small amount of jeweller's rouge to the damp cotton.
8.
Continue with Steps 3 and 4 above.
Limits of Detection
The upper limit of detection in pure lead perchlorate solutions is 0.1M. In the presence of other
ions, the upper limit of detection is above 1.0x10
-2
M lead, 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 extract into the electrode membrane at
high salt concentrations, causing deviation from the theoretical response. Either dilute samples
between 0.1M and 1.0x10
-2
M or calibrate the electrode at 4 or 5 intermediate points.