36
Fluoride Ion Selective Electrode User Guide
Figure 6 Electrode Response in Alkaline Solutions
Complexation
Fluoride ions complex with aluminum, silicon, iron (+3), and
other polyvalent cations as well as hydrogen. The extent of
complexation depends on the concentration of complexing
agent, the total fl uoride concentration and pH of the solution,
and the total ionic strength of the solution.
TISAB II and III contain a reagent, CDTA, that preferentially
complexes aluminum or iron in the sample. In a 1 ppm fl uoride
sample, TISAB II or III complexes about 5 ppm aluminum or iron.
Higher levels of aluminum or iron can be complexed by using
TISAB IV.
solution pH
7
100
75
50
25
0
-25
8
9
10
11
electrode
potential
(mV)
10
-
3
M F
-
10
-
4
M F
-
10
-
5
M F
-
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