Star Series
Method Note
M-1800
Fluoride in
Toothpaste
Key Words
Introduction
• Fluoride
• Toothpaste
• Fluoride
Ion Selective
Electrode (ISE)
Fluoride is incorporated in oral health products in several chemical forms, including sodium fluoride,
sodium monofluorophosphate, amine fluoride and stannous fluoride. This method provides rapid and
uncomplicated determinations for fluoride in the form of sodium fluoride in toothpaste. This procedure
eliminates interferences associated with the presence of color agents and solids. The direct measurement of
fluoride ions with the fluoride ion selective electrode is a well-established technique that is frequently used
as a standard method of analysis.
Recommended Equipment
Cat. No.
1. 4-Star Plus benchtop pH/ISE meter
1115000
2. Fluoride combination electrode
9609BNWP
3. Stirrer probe (or magnetic stir plate and bar)
096019
4. Swing arm electrode stand
090043
5. 500 mL volumetric flasks
6. 100 mL and 200 mL beakers
7. 50 mL graduated cylinders
Required Solutions
Cat. No.
1. 1 ppm fluoride standard with TISAB II
040906
2. 2 ppm fluoride standard with TISAB II
040907
3. 10 ppm fluoride standard with TISAB II
040908
4. Optimum Results™ A electrode filling solution
900061
5. TISAB II solution
940909
6. Deionized water
Calibration Standard Preparation
1. Using a graduated cylinder, transfer 25 mL of the 1 ppm (mg/L) fluoride standard into a beaker. The
1 ppm (0.0001 % w/v) standard will be used as “0.002 % w/v” for the calibration. The “0.002 %
w/v” value takes into account and corrects for the 1:20 dilution of the sample (25 grams dispersed in
500 mL of deionized water).
2. Using a graduated cylinder, transfer 25 mL of the 10 ppm (mg/L) fluoride standard into a beaker. The
10 ppm (0.0010 % w/v) standard will be used as “0.020 % w/v” for the calibration. The “0.020 %
w/v” value takes into account and corrects for the 1:20 dilution of the sample (25 grams dispersed in
500 mL of deionized water).
1 of 4
Fluoride in
Toothpaste
Sample Preparation
1. Mix 25 grams of toothpaste in about 150 mL of deionized water.
2. Quantitatively transfer the mixture to a 500 mL volumetric flask. Dilute to the mark with deionized water
and thoroughly mix the solution.
3. Using a graduated cylinder, transfer 25 mL of the prepared sample into a beaker. Using a graduated
cylinder add 25 mL of the TISAB II solution.
4. Repeat steps 1 through 3 for additional samples.
Results
Five aliquots of diluted toothpaste were measured for fluoride. The data in the table below is representative of
the results expected for fluoride in toothpaste.
Toothpaste with Sodium Fluoride
Fluoride (% w/v)
Sample # 1:
0.111
Sample # 2:
0.110
Sample # 3:
0.111
Sample # 4:
0.110
Sample # 5:
0.110
Mean:
0.110
Standard Deviation:
0.001
%CV:
0.50
Electrode Storage
For short-term storage between sample measurements, store the electrode in the 10 ppm (0.0010 % w/v)
fluoride standard. The filling solution in the fluoride electrode should not be allowed to evaporate, which
would cause crystallization. For long-term storage over one week, refer to the electrode use guide. Fluoride
electrodes, whose response may become sluggish with time, can often be restored to working order by brushing
the sensing element with fluoride toothpaste and a soft brush.
Equipment Setup
Electrode Setup – Fluoride Electrode
1. Remove the protective shipping cap that covers the electrode tip.
2. Attach the flip-spout cap to the Optimum Results A filling solution bottle and lift the spout to a
vertical position.
3. Insert the spout into the fill hole in the outer sleeve of the electrode, add a small amount of filling solution
to the chamber, tip the electrode upsidedown to wet the O-ring and return the electrode to its original,
vertical position.
4. Holding the electrode by the body with one hand, use the thumb to push down on the electrode cap and
allow a few drops of filling solution to drain wetting the inner cone.
5. Release the electrode cap and allow the outer sleeve to return to its original position. If the outer sleeve
does not return to its original position immediately, check to see if the O-ring is moist and repeat steps 3
through 5 until the outer sleeve returns to its original position.
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6. Add filling solution to the electrode until the amount of filling solution within the electrode body is level
with the bottom edge of the electrode fill hole.
Meter Setup – 4-Star Plus Benchtop pH/ISE Meter
For initial meter setup, follow the steps in the quick start guide that is included with the meter. The quick start
guide also contains a layout of the meter keypad for reference. The words in all capital letters, such as POWER,
indicate a key on the meter and words in quotations, such as “UnIt”, indicate a display on the meter screen.
1. Connect the electrode to the meter.
2. Connect the stirrer to the meter.
3. Press the POWER key on the meter to turn the meter on.
4. Note that the arrow icon on the left of the screen indicates the active line. If the top line is not active, press
the LINE SELECT key to change the selected line to the top line. Press the UP or DOWN ARROW key
to change the measurement mode of the top line to ISE.
5. Press the SETUP key. Press the UP or DOWN ARROW key until “ISE” is displayed on the top line.
6. Press the LINE SELECT key to select the middle line. Press the UP or DOWN ARROW key to select
“rES”, which is for ISE resolution and the number of significant digits that will be displayed on the meter.
Press the LINE SELECT key to select the bottom line. Press the UP or DOWN ARROW key to select “3”
for “rES”. Press the LINE SELECT key to accept the setting.
7. Press the LINE SELECT key to select the middle line. Press the UP or DOWN ARROW key to select
“nLIn”, which is an abbreviation for non-linear blank correction. Press the LINE SELECT key to select
the bottom line. Press the UP or DOWN ARROW key to select “OFF” for “nLIn”. Press the LINE
SELECT key to accept setting.
8. Press the LINE SELECT key to select the middle line. Press the UP or DOWN ARROW key to select
“rAng”, which is an abbreviation for the measurement range type. Press the LINE SELECT key to select
the bottom line. Press the UP or DOWN ARROW key to select “HIgH” for “rAng”. Press the LINE
SELECT key to accept setting.
9. Press the LINE SELECT key to select the middle line. Press the UP or DOWN ARROW key to select
“UnIt”. Press the LINE SELECT key to select the bottom line. Press the UP or DOWN ARROW key to
select “PEr” for “UnIt”. Press the LINE SELECT key to accept setting.
10. Press the MEASURE key to return to the measurement mode.
11. If all steps were followed correctly, the meter display will show three digits in the top line and “ISE: %” to
the right of the top line. The meter and electrode are now ready for calibration.
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Calibration and Analysis
1. Allow all the standards and the samples to reach room temperature for precise measurements, since fluoride
measurements are temperature sensitive.
2. Prepare the 0.002 % w/v (1 ppm) and 0.020 % w/v (10 ppm) fluoride standards as described in the Calibration Standard
Preparation section and stir the standards at a moderate and uniform rate.
3. Place the electrode and stirrer into the beaker with the 0.002 % w/v standard so the electrode tip is fully immersed in
the solution. The stirrer should be positioned slightly below the tip of the electrode. Press the STIR key on the meter to
turn the stirrer on. Press the CALIBRATE key.
4. Wait for a stable reading, usually in 1 to 2 minutes. Enter the standard value of “0.002” using the DIGIT key and
UP and DOWN ARROW keys. Press the DIGIT key until the first digit to be changed is flashing, press the UP and
DOWN ARROW keys to change the value of the flashing digit and continue to change the digits until the meter
displays the correct value of the standard. Once the value is set, press the DIGIT key until the decimal point is in the
correct location.
5. Press the CALIBRATE key to accept the 0.002 % w/v standard and press the STIR key to turn the stirrer off.
6. Rinse the electrode and stirrer thoroughly with deionized water. Gently remove excess solution from the outer sleeve of
the electrode by dabbing it with a clean paper tissue. Do not wipe or rub the sensing element of the electrode.
7. Place the electrode and stirrer into the beaker with the 0.020 % w/v standard so the electrode tip is fully immersed in
the solution. The stirrer should be positioned slightly below the tip of the electrode. Press the STIR key on the meter to
turn the stirrer on.
8. Wait for a stable reading, usually in 1 to 2 minutes. Enter the standard value of “0.020” using the DIGIT key and UP
and DOWN ARROW keys.
9. Press the MEASURE key to accept the 0.020 % w/v standard and return to the measurement mode. Press the STIR key
to turn the stirrer off.
10. Rinse the electrode and stirrer thoroughly with deionized water. Gently remove excess solution from the outer sleeve of
the electrode by dabbing it with a clean paper tissue. Do not wipe or rub the sensing element of the electrode.
11. Prepare a sample as described in the Sample Preparation section.
12. Place the electrode and stirrer in a prepared sample with the electrode tip fully immersed in the solution. Press the
MEASURE key on the meter and the stirrer will turn on. The ISE:% icon will flash as the measurement is being made.
The ISE:% icon will become solid and the display value will freeze when a stable reading is achieved. The stirrer turns
off automatically.
13. Repeat steps 10 through 12 for additional samples. When all samples have been measured, rinse the electrode with
deionized water and store the electrode according to the Electrode Storage section.
©2008 Thermo Fisher Scientific Inc. All rights reserved. All trademarks are the property of Thermo Fisher Scientific Inc.
and its subsidiaries. ROSS and the COIL trade dress are trademarks of Thermo Fisher Scientific Inc.
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