Fluoride detection in toothpaste

MARUM - Sediment Geochemistry - Leobener Str - D-28359 Bremen - Germany

Marine Geochemistry - Laboratory Methods

method

Fluoride in toothpaste is detected using a ion selective electrode

electrode

ORION F-combination electrode 96-09

solutions

electrolyte

INGOLD #9811 3M KCl, AgCl saturated

standard stock

:Merck #19814, 1000 mg F/L

work standard

100 ppm F by dilution with pure water

buffer

TISAB III (Total Ionic Strength Adjustment Buffer):
dissolve 300 g Na-citrate x 2H₂O 22g Titriplex IV (1,2 Cyclo-hexylen-dinitrilo-tetra-acidic acid) and 60 g NaCl in pure water and fill up to 1L

dilution solution

MilliQ 18 MOhm water

calibration standards

appropiate amounts of work standard are diluted with MilliQ pure water. Add 10% TISAB (50 mL standard + 5 mL TISAB)

general conditions

Ionic strength should be similar in all standards and samples. pH should be in the range of pH 5 - pH 8. High Mg concentrations considerably affect analysis and have to be compensated for by using either Mg background standards or standard addition.

buffer dose

10% of sample volume (e.g. 1 mL TISABIII for 10 mL of sample)

equations

The log of fluoride concentration is proportional to the measured electrode potential
For fluoride - a monovalent anion - the electrode potential should decrease by roughly one Nernst voltage (59.2 mV@25°C) as the concentration is increased by a factor of 10.

log(c) = E₀ + m*E_measured with m =( log c₂- log c₁) / (E₂-E₁) and E₀ = log c₁- m E₁

thus c = 10^{(E0 + m*Emeasured)}

for measurement of fluoride in toothpaste, a known amount (M) of toothpaste is dissolved in a known volume of water (V_liquid)
The concentration in toothpaste (c_liquid) is calculated from

c_toothpaste [mg/g] = c_liquid [mg/L]* V_liquid [L] / M_toothpaste [g]

example calibration

c₁ = 1 mg/L; E₁ = 334.6 mV ; c₂ = 10 mg/L; E₂ = 389.4 mV thus m = -0.0182 ; E₀ = -6.1 mV

example measurements

sample	measured E [mV]	log c	c _liquid [mg/L]	sample V [L]	amount M_toothp. [g]	c _toothp.[mg/g]
calib 1	-334.6	0.000	1.00	0.05	-	-
calib2	-389.4	1.000	10.0	0.05	-	-
A	-350.2	0.285	1.93	0.05	0.190	0.51
B	-381.2	0.850	7.09	0.05	0.410	0.86
C	-364.0	0.536	3.44	0.05	0.095	1.81
D	-387.2	0.960	9.12	0.05	0.050	9.12

sample volume

We work with sample volumes as low as 1.5 mL (+ 150 µL TISAB) if necessary. ORION combination electrodes have a flat membrane and a concentric diaphragm such that solution level of a few mm is sufficient

storage and conditioning

Store in 1000 ppm standard solution (without TISAB !)
Do not immerse into solutions containing TISAB for extended periods.
Do not store solutions containing fluoride ions in glass containers. Rinse all glassware immediately after use

tips + tricks

This electrode leaks electrolyte (on purpose) -> electrolyte level needs to be controlled frequently:electrolyte level should be 2.5 cm above solution level.

page created by M.Kölling & T.Feseker