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Method for determining the concentration of FLUORIDE (F-)
in Aqueous Solutions

Electrode Specifications       Links to publications on the WWW     Go to Ion Selective Electrodes page

Extra analytical details for PLANT MATERIAL and TOOTHPASTE

Analytical Requirements:

Ion-Selective Electrode for fluoride ion (ELIT 8221 crystal membrane)

Reference electrode: single junction silver chloride (ELIT 001n)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm F as NaF

Buffer solution: TISAB = Total Ionic Strength Adjustment Buffer - Dissolve 57ml acetic acid + 45g Sodium Chloride + 4g CDTA (1,2-diamino cyclohexan N,N,N,N-tetra acetic acid) in 500ml distilled water. Adjust pH to 5.5 by adding drops of 5M NaOH, then make up to 1L with water.

ELIT Computer Interface/Ion Analyser, or Ion/pH/mV meter.

150 ml polypropylene beakers, 100ml volumetric flask, 1, 2, 5, 10, 25ml pipettes.


Before use, the electrodes must be calibrated by measuring a series of known standard solutions, made by serial dilution of the 1000ppm standard solution. For a full calibration, prepare 100ml of solutions containing 1000, 100, 10, 1, and 0.1ppm F. If the approximate range of concentrations of the samples is known, and this is within the linear range of the ISE, then it is only necessary to make two solutions (preferably a decade apart) which span this range.

NB: If the samples to be measured are expected to have a total ionic strength of greater than 0.01 Molar, or a pH outside the range 4 to 8, or contain fluoride locked in complex ions then all standards and samples must be mixed with equal volumes of buffer solution (say 25ml of each) and mixed thoroughly to compensate for any differences in activity coefficients between sample and standards, ensure correct pH level for electrode operation and release any fluoride from complex ions.

Follow the instructions in the General Operating Instructions to measure these standard solutions and prepare a calibration graph.

Sample Preparation:

For low ionic strength samples with pH between 4 and 8, no sample preparation is necessary. Simply take approximately 50 to 100 mls of sample in a plastic beaker, or even immerse the electrodes directly in a lake or river (but take care to avoid losing the electrodes!). For samples with high ionic strength and/or pH > 8, or pH < 4 take 25mls of sample and add 25 mls of buffer solution and mix well before measurement.

Sample Measurement:

Follow the instructions in the electrode operating instructions to measure a series of samples and record the results. Briefly, it is important to note that, if measuring in beakers, the electrodes must be washed and dried between each sample, to avoid cross contamination, and sufficient time must be allowed (2 or 3 minutes), before taking a reading after immersion, to permit the electrode signal to reach a stable value. For the highest precision, frequent recalibration is recommended (see operating instructions).


The results will be displayed as ppm and mol/l. If buffer solution has been added equally to standards and samples then these figures will not need adjusting because they will all be affected by the same dilution factor.

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Procedure for determining FLUORIDE (F-) in PLANT MATERIAL

Because of the high ionic strength and complex chemical composition in plant extracts F is best measured using the Standard Addition method whereby a large volume of sample is measured before adding a smaller volume of standard and remeasuring. In this way a calibration is performed essentially at the same time as every sample measurement and they are both made at the same temperature, Ionic Strength and background composition.


Before use, the electrodes must be calibrated to determine the slope, which must be entered into the Standard Addition calculations. This can be done by making one standard solution which lies in the middle of the range expected for the samples and measuring it by the SA method. Enter a previously measured value for the slope (or the nominal 55 mV/dec for a univalent ion) and calculate the ppm. If this is not correct then recalculate using different slope values until it is correct. Then use this slope value to calculate the sample results.

Note that the calibration standard and all sample solutions must be mixed 50/50 with TISAB before measuring.

Sample Preparation

The plant material should be washed with deionised water and then dried at 80C for 24 hours to remove all moisture. It should then be ground to pass a 60 mesh sieve.
Weigh out 2g of the powdered material into a 150ml plastic beaker and add 100ml of 0.1M perchloric acid. Stir continuously on a magnetic stirrer for at least 20 mins. Divide the suspension into two and retain 50mls for a duplicate analysis if necessary. If further replicate analyses are anticipated then use a correspondingly larger sample weight and extraction volume.

Sample Measurement

Run the Ion Analyser software for Standard Addition and enter the recently determined slope factor. If the likely range of concentrations of the samples in the solution is known then enter the average value in the appropriate box; the software will then calculate a suitable volume and concentration of standard to be added. If these figures are inconvenient then the nearest convenient values can be entered i.e. to suit the value of your pipette or concentration of existing standards. If the sample is completely unknown then try using 1ml of 100ppm standard as a first attempt (this will increase the sample concentration by approximately 2ppm; ignoring the 2% dilution factor). NB: it is important to keep the volume of standard low so as to avoid too much dilution of the sample matrix and avoid any significant changes in activity coefficients.

After entering all the details required by the software, immerse the electrodes in the required volume of standard (continuing stirring) and wait for a stable reading before taking the first measurement. Then add the specified aliquot of standard and again wait for a stable reading. On taking the second measurement, the software will immediately calculate the concentration in the solution and also give some comment as to the quality of the determination. The measurement can be repeated with a different concentration of standard if the ratio of the sample concentration to the amount of standard added was not optimum.


The results will be displayed as ppm ( micrograms per ml).and mol/l in the solution. This value must be multiplied by the volume and divided by the sample weight to get the concentration in the sample in micrograms per gram.

Procedure for determining FLUORIDE (F-) in TOOTHPASTE

Toothpaste is measured in exactly the same way as plant material except that it is dispersed in pure water. Ideally, the sample weight should be chosen to give an F concentration between 10 and 100 ppm. For example, if the sample contains around 2000 ppm F then 2g dissolved in 100ml will give a solution concentration of about 40 ppm.

However, if it is inconvenient/difficult to control the weight of paste dispensed and the resultant solution ends up with more than 100ppm F then the samples can be measured by the identical Sample Addition method where a small volume of sample is added to a larger volume of standard - details also in the Standard Addition link above. But note that the maximum solution concentration for the F electrode is around 2000ppm.

Technical Specifications
for the Fluoride Ion-Selective Electrode
(ELIT 8221)

Click here to download a printer-friendly (pdf) Specification Sheet.

The Fluoride Ion-Selective Electrode has a solid-state mono-crystalline membrane. The electrode is designed for the detection of fluoride ions (F- ) in aqueous solutions and is suitable for use in both field and laboratory applications.
The Fluoride Ion is a monovalent anion .
One mole of ( F-) has a mass of 18.998 grams; 1000 ppm is 0.053 M
Dissolve 2.210g anhydrous sodium fluoride (NaF) in 1 litre water.

Physical Specifications
Length of body excluding gold contact = 130 mm
Length of body including gold contact = 140 mm
Diameter of body = 8 mm
DC resistance at 25C < 2.5 MOhm
Minimum feasible sample volume = 5 ml

Chemical / Operational Specifications
Preconditioning / Standard solution : Normally 1000 ppm F- as NaF
(But see General Operating Instructions)
Preconditioning time : 5 minutes
Optimum pH range : pH 4 to pH 8
Temperature range : 0 to 80C
Recommended ISAB : SPECIAL TISAB (1:1 v/v)
(Dissolve 57ml acetic acid + 45g Sodium Chloride + 4g CDTA (1,2-diamino cyclohexan N,N,N,N-tetra acetic acid) in 500ml distilled water. Adjust pH to 5.5 by adding drops of 5M NaOH, then make up to 1L with water.)
Recommended reference electrode : Single Junction AgCl (ELIT 001)
Electrode slope at 25C : 545 mV/decade
Concentration range : 0.02 to 1,900 ppm (1x10-6 to 0.1 Molar)
Response time : < 10 seconds
Defined as time to complete 90% of the change in potential after immersion in the new solution.
Time for stable reading after immersion : < 1 to > 5 minutes
Depending on concentration, use of ISAB, nature of sample and stabilisation time of liquid junction potential of reference electrode.
Potential drift (in 1000 ppm) < 3 mV/day (8 hours)
Measured at constant temperature and with ISE and Reference Electrode continually immersed.

The only interference on the fluoride electrode is from the hydroxyl ion (OH-) with a selectivity coefficient of about 0.1. This is eliminated by ensuring that the pH is kept below 8. The other function of the TISAB buffer, apart from equalizing the ionic strengths, is as a de-complexing agent to release any fluoride ions which may be bound up in complex molecules.

Note low pH range (4 to 8) for this electrode.

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