Potentiometric Determination of Fluoride
Purpose: determine the amount of fluoride ion in samples of toothpaste, mouthwash, and
tap water using standard addition and serial dilution (calibration) methods.
Background Reading:
Principles of Instrumental Analysis 6th ed
Standard Additions, pp 13-17
Potentiometry, Chapter 23
p 672 The Fluoride Electrode
p 686 Direct Potentiometric Measurements
p 687-690 Activity versus concentration
Calibration Curves for Concentration Measurements
Standard-Addition Method
Materials & Equipment:
Out in the lab:
Fluoride ISE
10-mL volumetric pipet
25-mL volumetric pipet & bulb
One 500-mL volumetric flasks
Thermometer
In kit to be checked out from the stockroom:
Two 250-mL volumetric flasks
Three 100-mL volumetric flasks
pH paper
250ml beaker
3 plastic beakers
Wire gauze
Already prepared or available:
6M NaOH, 1.0 M Acetic Acid, Sodium acetate
NaF, dried
4M KCl for reference electrode filling solution
10 ppm fluoride ion soaking solution
Background
One type of an ion selective electrode (ISE) with which you have experience already is
the pH meter, which selectively measures the amount of H3O+ in solution. There are
several other types of ion selective electrodes, including the one used in this experiment
which measures for fluoride ion.
Potentiometry allows for the determination of concentration in a variety of samples over
a large concentration range. There are many advantages to using an ISE as means of
analysis, including its selectivity, efficiency, ease of sample preparation, and lack of
interference and reactivity with the sample itself. For a fluoride ISE one must be careful
to avoid measuring a false response to hydroxide ion by buffering the solution to be
slightly acidic, and this becomes increasingly important as fluoride concentrations
decrease.
Fluoride ion is a necessary evil, good at a low concentrations but detrimental at higher
ones. At 1 ppm ( 1ppm = 1 mg/L) it prevents tooth decay, which is why it is added to
water and dental products, but in the 2- 13.7 ppm range, staining, cracking, mottling, and
pitting of the teeth can result. At higher concentrations, it is a known cause of bone
cancer.
Water is generally fluoridated to 1 ppm but in some areas, such as the southwestern
United States, where fluoride exceeds 4 ppm, it is reduced to acceptable levels. You can
go online to www.epa.gov and follow the link for the Office of Water to obtain water
quality report for a particular area.
This experiment requires the use of an ion selective electrode (ISE), to detect fluoride.
Refer to p. 672 of your text for a detailed description of the fluoride electrode.
Briefly, the electrode contains a crystal of lanthanum fluoride as the membrane, through
which fluoride ions are mobile. The crystal is doped with europium fluoride to increase
conductivity. When immersed in a solution containing fluoride ions, a potential develops
across the membrane measured against a constant reference potential with a standard
pH/mV meter. The Nernst equation describes the measured potential as a function of the
activity of fluoride ions in solution.
Where
E = Eo - EJ
+
(RT/ZF) ln a
E = measured electrode potential
Eo = reference potential (a constant)
EJ = junction potential
R – 8.3145 J/molK
T = temperature in K
Z = charge (-1 for fluoride)
F = faraday’s constant = 96,485 C/mol
a = activity of fluoride ions in solution
The activity, a, represents the effective concentration while the total fluoride ion
concentration may include some bound ions as well. The electrode responds only to free
ions so it is important to avoid the formation of complexes that are meant to be measured.
In this case, complexation would lower the activity and therefore the electrode response.
The addition of a high concentration of non-interfering ions in the buffer help maintain a
constant total ionic strength and prevents a fluctuation in the activity coefficient of the
ion being measured.
pH and ionic strength
Potential is a function of activity rather than concentration, but if the ionic strength is
kept constant, then the activity will then be proportional to the concentration.
The ionic strength is defined as
I= ½ Σ cizi2
i
where c represents the ion concentration and z its charge.
The activity of an ion is also dependent on concentration as shown in the equation
ai= γi ci
where γ is the activity coefficient. Making substitutions into the Nernst equation
E = Eo - EJ
+
(RT/ZF) ln a
The equation becomes
E = Eo - EJ + (RT/ZF) ln γF + (2.303 RT/ZF) log CF
Note that the terms in the argument of the log are separated and ln is converted to log in
the last term. Also, if we consider that
pF = -log CF
The equation becomes
E = Eo - EJ + (RT/ZF) ln γF - (2.303 RT/ZF) pF
Collection of terms yields
E = (59.16) pF + constant
The significance of this equation is that it shows when one changes the concentration of
the fluoride sample by a factor of ten, the potential changes by 59 mV. This is a good
way to check if the electrode is functioning properly and that you are observing
“Nernstian” behavior.
PROCEDURAL INFORMATION
Buffer
Prepare 500.00 ml of pH 5.25 acetic acid/sodium acetate buffer. Before class calculate
the mass of sodium acetate and volume of 1.0 M acetic acid to make a buffer with a pH
of 5.25. The total acetate (HA and A-) must equal 0.5 M. Test the pH of your buffer with
a pH strip and add dropwise 6M NaOH if necessary. Addition of base may also help in
dissolving the sodium acetate. STORE IN A PLASTIC BOTTLE.
Calibration Curve Method (Serial Dilution)
Volumetrically prepare 250.00 mL of 0.02 M sodium fluoride, which has been previously
oven-dried for one hour at 100 °C and stored in a dessicator.
Use a volumetric pipet to transfer 25.00 mL of your standard to a beaker and add
25.00 mL of buffer. Swirl solution gently before taking readings of you sample.
Before discarding the stock solution of 0.020M sodium fluoride, measure the potential of
the unbuffered solution to discover what effect the buffer has on the solution.
Use 25.00 mL of the excess 0.02 M sodium fluoride solution to make a 0.002M dilution
with DI water. As was done with the stock solution, add an equal amount of buffer to the
diluted solution to prepare the sample for a reading. Use the plastic beakers when taking
measurements. Continue to make serial dilutions until you have a total of five
measurements. Note that the electrode should be rinsed with DI water and blotted dry in
between measurements of each of the solutions you have made.
Unknown Fluoride Sample Preparation by Calibration Curve Method
The three samples to be analyzed for fluoride are mouthwash, tap water, and natural
water. For each, take 25.00 mL of each add 25.00 mL of buffer. Measure the potential of
each of these three samples and use the calibration curve generated from the data in the
previous section to determine the fluoride ion content.
Standard Addition Method
You will need a standard that is approximately 0.030 ppm F- or 1.6 x 10^ -6 M NaF.
Figure out the mass required to make 100 mL of 0.030 ppm F-. You'll see this isn't
something you can weigh out on the analytical balance (it goes to the 0.1mg). You will
have to weigh out a reasonable amount and dissolve to 1L or 100mL (your choice) and
then dilute an aliquot of that solution to 100ml to make a 0.0300 ppm standard solution.
There are several way to do this, so just make sure it makes sense to you and your partner
Now prepare four samples for analysis using four 100-mL volumetric flasks.
In each flask add 50 mL of buffer and 5 mL of tap water.
One sample will have no standard added to it and can simply be diluted to 100-mL with
water.
Before diluting the other three samples, add 10 mL of standard to one , 20 mL to another,
and 30 mL to the last one.
.
Make sure all solutions are mixed well before taking measurements.
Potentiometric Measurements – Trouble shooting
A difference of 1 °C will result in a 2 % error in measurement, so keep the temperature of
all samples constant.
Constant stirring is vital but avoid stirring vigorously enough to generate heat that will
affect the measurements. If the stirrer seems to produce heat, use a Styrofoam sheet or
wire gauze in between the stirrer and the beaker.
Rinse the electrodes with distilled water and blot dry in between .
Improper storage of the electrode can cause irreparable damage. Make sure at the end of
the day that the electrode is immersed in 10 ppm F- and that the electrode has filling
solution (4M KCl) up to the level of the filling hole.
STORAGE
When you are done for the day, the fluoride electrode must be stored in 10ppm
fluoride solution with an equal volume of buffer added. Make sure that the
electrode remains filled with the 4M KCl filling solution. The electrode should never
be stored in water as it will disrupt the behavior of the electrode itself.
NAME___________________
Partner___________________
Date___________________
Kit #________
Balance #________
Fluoride Ion Electrode #________
REPORT SHEET
Turn in yellow sheets with the raw data and calculations.
Provide graphs and regression analysis.
DATA & CALCULATIONS
Calibration Curve Data Collected
Molarity
p[F-]
mV
1._____________________________________
2._____________________________________
3._____________________________________
4._____________________________________
5._____________________________________
Stock without buffer
Mouthwash
Tap water
Natural water
_______________mV
_______________mV
_______________mV
_______________mV
Standard Addition for Tap Water Analysis
Water w/ no stock
_______________mV
Water w/ 10 mL of stock
_______________mV
Water w/ 20 mL of stock
_______________mV
Water w/ 30 mL of stock
_______________mV
Graph of EMF vs. log [F-]
Slope of linear region ____________
Standard error of slope____________
Results Using the Calibrations Curve - Show sample calculations below:
Concentration of F- in mouthwash in M and ppm (ppm= mg/L).
Concentration of F- in tapwater in M and ppm.
Concentration of F- in natural water in M and ppm.
Using the standard addition curve, what is the concentration of fluoride ion in your
water sample? How does this compare to results from the calibration curve?
Discuss the difference in potential between the stock solution of 0.020 M NaF with and
without buffer.
Suppose you have a contaminant, such as an organic solvent, in the sample. Which
method- the calibration curve method or standard addition method would be more
advantageous in terms of data analysis. Which method works better? Why do you think
this is more advantageous?