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Fluoride Ion Selective Electrode User Guide
Fluoride in Acid Solutions
In solutions with a pH below 5, hydrogen ions complex a
portion of the fl uoride ions, forming HF or HF
2
¯, which cannot
be detected by the fl uoride electrode. To free the complexed
fl uoride, the pH of the solution must be adjusted to the weakly
acidic to weakly basic region before making measurements.
A strong base, such as sodium hydroxide, should not be
used for pH adjustment, since the total ionic strength of the
adjusted samples and standards will vary according to the
original solution pH and the amount of sodium hydroxide
added. Variations in total ionic strength affect the accuracy
of concentration measurements. Dilution of samples and
standards with a large excess of sodium acetate, on the other
hand, will buffer the pH to above 5 and help adjust the total ionic
strength of samples and standards to the same level.
Procedure
1. Prepare a 15% sodium acetate solution. Dissolve reagent-
grade sodium acetate (CH
3
COONa) in distilled water.
Prepare a large enough quantity of 15% sodium acetate
solution to dilute all samples and standards.
2. Prepare a background solution that contains all sample
components except fl uoride. Use this solution to prepare
the standards.
3. Prepare standards in the concentration range of the
unknown samples by adding fl uoride to the background
solution. Dilute each standard 10:1 with the sodium acetate
solution (9 parts sodium acetate and 1 part standard).
Prepare fresh standards every two weeks if the standard
contains less than 10 ppm fl uoride. If an ISE (concentration)
meter is used, prepare at least two standards. If a meter
with a mV mode is used, prepare at least three standards.
4. Calibrate the electrode using the instructions in the
Checking Electrode Operation (Slope)
section.
5. Measure the unknown samples: Dilute each unknown
sample 10:1 with sodium acetate before performing taking
measurements (9 parts sodium acetate and 1 part unknown
sample).
Note:
In many cases, standards do not need to be prepared
using background solutions. If a standard prepared from the
background solution gives the same reading (after dilution with
sodium acetate) as a standard prepared from pure sodium
fl uoride, then the background solution is unnecessary.
