Delloyd's Lab-Tech Chemistry
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Ion Selective Electrodes - Determination of Fluoride by ISE
Determination of Potentiometric response of the Orion
The fluoride ion-selective electrode is a
homogeneous, solid state electrode and has as its sensing element, a
lanthanum fluoride crystal doped with europium fluoride (1,2). The
crystal is sealed flush at one end of the electrode body and the
internal filling solution in contact with the internal surface of the
element contain fluoride and chloride ions. A silver-silver chloride
element functions as the internal reference electrode.
Ion-Selective electrode under varying solution
Application of the electrode to analysis of toothpaste
The electrode shows extremely high sensitivity towards fluoride over
most other ions and its potentiometric response is essentially Nerstian
over several decades of concentration. The electrode however only
detects uncomplexed fluoride ions and the complexed forms must first be
decomposed before measurements of fluoride ion activity can be made
(3,4). Alternatively, known addition or subtraction techniques may be
A little used but highly accurate method of potentiometric
determination is by Gran's plots (5,6) which consists of a series of
standard additions and corresponding potentiometric measurements. The
relevant equation for fluoride ion determination is given as:
(Vx + V)X 10-e(F)/2.3RT = 10 -(Eo +
Ej)F/2.3RTxλ(CxVx + CV) where
Vx = sample solution volume
V = volume of standard added
Eo & Ej
= equilibrium potentials, normal potential and liquid potential
Cx = sample solution concentration
C = standard solution
λ = activity coefficient of specific ion
and the sign preceding E(F/2.3RT) indicates the direction of electrode slope.
Thus if sample solution strength is maintained at a high and almost
constant level, λ and Ej do not change appreciably. By plotting
(Vx + V) 10-e(F/2.3RT) vs V and extrapolating horizontally
to the axis, a Ve value is obtained where Cx Vx = -CVe, from which Cx
may be obtained.
This procedure has several advantages over the normal potentiometric
methods of selective ion determination (7) but will not be used in this
experiment. The sample to be investigated is a commercial brand ot
stannous or sodium fluoride-containing toothpaste.
Aims of the experiment:
- To generate a calibration curve of fluoride ion activity / fluoride
ion concentration vs potentiometric response of the fluoride
- Analysis of fluoride ion content of toothpaste samples using
- Standard calibration curve constructed from mV readings
- Direct readout using the ion-selective electrode and meter
calibrated with standard fluoride solutions.
- Standard addition technique using mV readings before and after
addition of known fluoride concentrations to sample solutions.
1. Activity /Response Measurements
From the 1M sodium fluoride solution provided, pipette 10 ml into a 100
ml volumetric flask and make up to the mark with distilled water, which will be 10-1 M. From
this solution, similarly dilute 10 ml into a second 100 ml volumetric
flask to produce a 10-2 M solution. Hence prepare by further
serial dilutions 10-3, 10 -4 and
10-5 M solutions.
Transfer about 100 ml of each solution into 250 ml beakers, begining
with the 10-5 M solution. Place the beaker on a magnetic
stirrer, put a small stirring magnet in the solution, and increase the
stirring rate to ensure rapid solution mixing without creating a
Connect the ion-selective combination electrode to the mV Ion
meter. Place the electrode into the solution such that about 1 cm of the
electrode tip is immersed, and allow the system to equilibrate before
taking the mV reading. Make a note of the settling time for each
solution to equilibrate from point of immersion to time when the
readings are taken.
After each measurement, remove the electrode from the test solution,
rinse with distilled water into a waste vessel, and dry lightly with a
clean tissue. Repeat measurements with the other standard solutions in
order of increasing concentration.
NB: When the electrode is not in use, it should be immersed in
distilled water and not allowed to dry out.
- For each solution, calculate the fluoride ion activity (TISAB
composition to be provided) using the Debye-Huckel equation.
- Plot mV vs log10 activity mV vs log10[conc]
on the same sheet of graph paper and obtain the slope of each
calibrated curve in the lower ranges of response.
2. Analysis of fluoride ion content of toothpaste:
Use the 10-1, 10-2, 10-3, and
10-4 fluoride solutions incorporating TISAB to calibrate the
Weigh accurately in triplicate about 2g of toothpaste sample dropped
directly inti 250 ml volumetric flasks. Add to each flask 12.5 ml TISAB
and make up to the mark with distilled water BEFORE SHAKING to
completely disperse the sample.
- Pipette 100 mls aliquots of each toothpaste suspension into 250 ml
beakers and measure their respective responses with the fluoride
electrode. Use these readings to determine the concentration of each
solution from the calibration curve (mV vs [conc]).
- Direct readout: The meter used will provide direct readout
values for your samples.
Record the value for each sample
- Standard Addition: Immediately after taking the mV reading
of each solution, pipette in 0.5 ml of an appropriate F- standard
(calculate value) and note the new reading.
potential (ΔE) change and hence the fluoride concentration of each
solution using the standard addition equation.
Express the mean
fluoride concentration of the toothpaste sample as µg stannous or
Treatment of Results and Discussion
- Tabulate the results for the three methods of determination.
- Comment on the calibration curves obtained with reference to linear ranges, and settling times obtained.
Compare the mean values obtained by each method in terms of precision, and difference from each other at the 95% confidence level.
- List what you consider to be advantages and disadvantages of each method of determination.
Frant, M.S. Ross J.W. Science 15 (1955) 1533
Lignane, J.J. Anal. Chem. 40 (1968) 935
Edmond, C.R. Anal. Chem. 41 (1969) 1327
Frant, M.S. Ross J.W. Anal. Chem. 40 (1968) 1169
Rosetti, F.J., Rosetti,H. J. Chem. Ed. 42 (1965) 375
Liberti. A., Mascini.M. Anal. Chem. 41 (1969) 676
Schick, A.L. J.A.O.A.C. 56 (1973) 798
Chem. Dept. UWI. St. Augustine Campus