Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 15)

Instruction Manual Fluoride Electrode
- 12 -
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
Fluoride concentration down to 1.0X10
-6
M (0.02 ppm) fluoride can be measured in neutral
solutions. Since sample contamination can be a factor in low level fluoride measurements, care
must be taken in making determinations below 1.0X10
-5
M. The upper limit of detection is a
saturated fluoride solution.
pH Effects
Hydrogen complexes a portion of fluoride in solution, forming the un-dissociated acid HF and the
ion HF
2
-1
in acid solutions with a pH below 5. The proportion of free fluoride ion in acid solutions
is shown in Figure 4.