Thermo Fisher Scientific Ion Selective Electrodes Fluoride - Manual (Page 14)

Instruction Manual Fluoride Electrode
- 11 -
Complexation
Hydrogen ion, as well as some other multivalent cations, aluminum, silicon, iron
+3
, will form
complexes with fluoride. The total ionic strength of the solution, the pH of the solution, the total
fluoride concentration, and the concentration of the complexing agent all contribute to the degree of
complexation. TISAB 1 and TISAB 2 complex about 5 ppm aluminum or iron in a 1 ppm fluoride
solution.
Temperature Influences
Samples and standards should be within
±1 o
C of each other, since electrode potentials are
influenced by changes in temperature. Because of solubility equilibrium on which the electrode
depends, the absolute potential of the reference electrode changes slowly with temperature. The
slope of the electrode, as indicated by the factor "S" in the Nernst equation, also varies with
temperature. Table 3 gives values for the "S" factor in the Nernstian equation for the fluoride ion.
TABLE 3: Temperature vs. Values for the Electrode Slope
Temp (
o
C) "S"
0 54.20
10 56.18
20 58.18
25 59.16
30 60.15
40 62.13
50 64.11
The temperature range for the Eutech Fluoride Ion Electrodes is 0
o
-80
o
C, provided that temperature
equilibrium has occurred. Only intermittent use is recommended at temperatures from 80o-100oC.
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 fluoride concentration on semi-logarithmic paper
results in a straight line with a slope of about 57 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 one minute or less in highly concentrated solutions to several minutes near the detection
limit. (Refer to Figure 3.)