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29 Fluoride Ion Selective Electrode User Guide
Titrations
The electrode makes a highly sensitive endpoint detector for
titrations of a fl uoride-containing sample using lanthanum
nitrate as the titrant. Titrations are more time-consuming than
direct electrode measurements, but results are more accurate
and reproducible. With careful technique, titrations can be
performed that are accurate to ± 0.2% of the total fl uoride
concentration of the sample. The sample should be at least
10
-3
M total fl uoride in concentration for a good endpoint break.
Titrations for fl uoride give low results in the presence of 1%
or more (based on total fl uoride) aluminum, iron, or trivalent
chromium.
The fl uoride electrode can also be used to detect titration
endpoints of samples containing species that react with
fl uoride, such as aluminum, lithium, lanthanum, and thorium.
Contact Technical Support or visit www.thermoscientifi c.com/
water for details.
The following procedure is for the titration of a fl uoride
containing sample with lanthanum nitrate.
1. Prepare a 0.1 M lanthanum nitrate solution by dissolving
43.3 g of reagent-grade La(NO
3
)
3
-6H
2
0 in a 1 liter volumetric
fl ask that contains approximately 700 mL of distilled water.
Once the solids are dissolved, fi ll the fl ask to the mark with
distilled water.
2. Standardize the lanthanum nitrate solution by titrating
against a 0.1 M fl uoride standard. Pipette exactly 25 mL of
fl uoride standard into a 250 mL plastic beaker and add
50 mL of distilled water. Place the electrode in the sample.
Stir the solution thoroughly throughout the titration.
3. Using a 10.0 mL burette, add increments of lanthanum
nitrate and plot the electrode potential against mL of
lanthanum nitrate added. The endpoint is the point of
the greatest slope. See
Figure 3 . Alternately, use a fi rst
derivative calculation or an on-line titration calculator to
determine the end point. Record the endpoint, V
t
o. Rinse
the electrode and blot dry.
4. Titrate the unknown samples. Pipette exactly 25 mL of
sample into a 250 mL beaker and add 50 mL of distilled
water. Place the electrode in the sample. Stir the solution
thoroughly throughout the titration.
5. Using a 10 mL burette, add increments of lanthanum nitrate
and plot the electrode potential against mL of lanthanum
nitrate added. Determine the endpoint, V
t
x.