Instruction Manual
Iodide Electrode
11
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 sodium iodide solutions is 1M. In the presence of other ions,
the upper limit of detection is above 1.0x10
-1
M iodide, 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 1M and 1.0x10
-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 electrode pellet.
Refer to Figure 1 for a comparison of the theoretical response to the actual response at low levels of
iodide.
pH Effects
The electrode can be used over the pH range 0-14.
Electrode Life
An iodide ion electrode will last for 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.