Analytical Chemistry Trinidad & 

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Ion Selective Electrodes - Determination of Fluoride in toothpaste by ISE

  • Determination of Potentiometric response of the Orion Fluoride
  • Ion-Selective electrode under varying solution conditions.
  • Application of the electrode to analysis of toothpaste sample solutions.

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.

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 used.


Gran's Plots

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 respectively
Cx = sample solution concentration
C = standard solution concentration
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:

  1. To generate a calibration curve of fluoride ion activity / fluoride ion concentration vs potentiometric response of the fluoride ion-selective electrode.

  2. 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 vortex.

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.


  1. For each solution, calculate the fluoride ion activity (TISAB composition to be provided) using the Debye-Huckel equation.
  2. 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 meter.

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.

  1. 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]).

  2. Direct readout: The meter used will provide direct readout values for your samples.
    Record the value for each sample solution.

  3. 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.
    Calculate the 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 sodium fluoride/g.

Treatment of Results and Discussion

  1. Tabulate the results for the three methods of determination.

  2. 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.

  3. 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

    Anal-Chem Resources
    Chem. Dept. UWI. St. Augustine Campus