Introduction
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 25°C < 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 80°C
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 25°C : 54±5 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.

Interference:
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.

Procedure for determining the concentration of FLUORIDE (F^-) in WATER

Analytical Requirements:

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

Reference electrode: single junction silver chloride (ELIT 001)

Dual electrode head (ELIT 201)

Standard solution: 1000 ppm F as NaF

Buffer solution: TISAB = Total Ionic Strength Adjustment Buffer

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

100 or 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 200, 100, 10, 1, and 0.1ppm F. If the approximate range of concentrations of the samples is known, and this is within the specified linear range of the ISE, then it is only necessary to make two solutions which span this range: e.g. if samples are known to lie between, say, 30 and 130ppm then you could use standards of 10 and 200ppm or even 20 and 150ppm.

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.

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.

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.