Instruction Manual
Cadmium Electrode
13
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 cadmium nitrate solutions is 0.1M. In the presence of other
ions, the upper limit of detection is above 1.0x10
-2
M cadmium, 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 infuse 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.
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
cadmium. Neutral solutions containing free cadmium ions can be measured down to 1.0X10
-7
M
(0.01 ppm). Extreme care must be taken with measurements below 1.0X10
-5
M (1.1 ppm) to avoid
contamination of samples.
pH Effects
Figure 4 shows how the electrode responds to cadmium ion in solution at various pH levels.
Hydrogen ion interferes with low cadmium ion measurements. The minimum pH at which cadmium
ion concentrations can be measured without interference is given by the solid line
"limit of
detection"
above the shaded area in Figure 4.
At a high pH, free cadmium ion precipitates with hydroxide ion, thereby reducing the cadmium ion
concentration. The maximum pH at which cadmium concentration can be measured without
