This guide contains the basic information for proper use of your new Fluoride Ion-selective electrode.

The Ion Selective Electrode (ISE) is a sensor which converts the activity of a specific ion e.g. Fluoride (dissolved in a solution) into a voltage (potential), which can be measured by mV meter or Ion meter or Titrator. The voltage is theoretically dependent on the logarithm of the ion activity, as described by the Nernst Equation.

E = E0 + (2.303*RT/nF) log(A)

The sensing part of the electrode is usually made from an ion specific membrane, coupled together with a reference electrode (either separate or as a combination).

IONODE’s IJ-F ISE is a Crystalline or solid state membrane electrode, which is made from relatively insoluble, ionically conducting inorganic salts. These are available in homogeneous and heterogeneous forms.  They have good selectivity since only ions which can introduce themselves into the crystal lattice can interfere.


The IONODE’s IJ-F ISE responds reversibly to Fluoride ions. The limit of detection is 0.02 ppm F-. The response is theoretical down to approximately 0.2 ppm.

Preparation of electrode –

IJ series electrodes are shipped without sleeve electrolyte, and must be filled prior to use. To fill, hold the electrode by the sleeve and gently ease off the rubber wetting cap.

Prepare as follows:

Invert the electrode. Hold the electrode just below the sleeve and with careful rotation and pulling along the axis of theelectrode, remove the sleeve. DO NOT BEND.


Fill the annular space with gel or electrolyte to approximately half to three quarter full.



Slide the sleeve back onto the electrode ensuring the black O-ring is well seated within the electrode body. Do not exert sideways force. Any excess electrolyte will be expelled from the end of the electrode through the ground junction. Ensure there are no air bubbles in the sleeve. Wash off any excess electrolyte before use.



Electrolyte Replacement

The IJ-F has an inbuilt double junction Ag/AgCl with a replaceable sleeve electrolyte. Saturated potassium chlorideis suitable for most applications.Whenever choosing a filling solution/ sleeve electrolyte, it is important to select one that is close to being equitransferant to minimize junction potential errors. The E0 factor in the Nernst equation is the sum of all the liquid junction potentials present in the system, and any variation in this during analysis can introduce major sources of potential drift and error in measurements.

Calibration Standards

Standard solutions of Fluoride Ion should preferably bracket the expected measurement range. For example, to determine Fluoride ions in drinking water (expected value around 1 ppm), it is normally sufficient to calibrate with 0.2 and 2.0 ppm Fluoride standards.

Calibration Steps

  • Make up F- calibration standards in distilled water bracketing the expected measurement range.
  • Starting with the lowest concentration first, measure out 20ml of standard and add 20ml TISAB.
  • Insert the electrode after rinsing, stir, and note the potential in mV.
  • Perform this (carefully rinse between measurements to eliminate carry over) on each of the standards.
  • At room temperature, a slope should be obtained of around -55mV/decade. (from 0.2ppm upwards)

Typical Calibration Curve

Sample Pre-Treatment

It is important to note that Ion selective electrodes measure the activity of the ionic analytes in solution. If the ion to be measured is complexed or bound, sample pre-treatment may be required before analysis. This can involve preliminary steps such as drying, washing and grinding, as well as extraction or dry ashing. This is particularly the case with samples such as plant material and soils. In the event of interferences and oxidation effects, adding an appropriate reagent to both samples and standards can be beneficial. (Sample preparations for application examples – Annexure I)

Interfering Ions

The major interference is OH- . For best results, ensure measurement is done in the pH range of 5 –7 pH (optimally 5.2-5.5pH). In common applications such as drinking water analysis, this is easily achieved with the use of TISAB (Total Ionic Strength Adjuster Buffer).

Sample Measurement

Follow the same basic procedures as calibration, substituting the calibration standard for your sample. It is important to use the same stirring conditions, temperature, etc for best results. Use the same ratio of sample/ISA as used in the calibration step.

ISE Analytical Techniques

Direct Calibration

Simple procedure used for measuring a large number ofsamples. This can also easily be automated with sample changers for unattended analysis. Calibration is performed using a series of standards, preferably bracketing the expected sample range. ISA is added to both samples and standards in the same ratios, and the concentration of the samples is determined by comparison to the standards.

Standard Addition

The electrode is immersed in the sample solution and an aliquot of a standard solution containing the measured species is added to the sample. From the change in potential before and after the addition, the original sample concentration is determined.

 Analate Addition

Often used to measure viscous samples, small or very concentrated samples. It can be used to overcome the effects of complex sample matrics, but is not suitable for dilute samples.Total concentration is measured even in the presence of complexing agents. The electrode is immersed in a standard solution containing the ion to be measured, and an aliquot of the sample is added to the standard. The original sample concentration is determined from the change in potential before and after the addition.

Titration Indicators

colour or turbidity. A common example is use of a chloride ISE (or a silver billet electrode) for determining the salt content of dairy products by silver nitrate titration. Titrations are much more precise (approx. 10x) than direct ISE calibration, but they are more time consuming and they have a narrower dynamic range. ISE’s can also be used in titrations to determine ions for which there is no current ISE. For example, Lanthanum ions can be determined in this way using a Fluoride ISE as indicator.


Titration is a very accurate determination of fluoride. The IONODE’s IJ-F ISE can be used as highly sensitiveendpoint detectors for titrations of a fluoride-containing sample.

Though titrations are more time consuming than direct ionmeasurements, the results are more accurate and reproducible.

Titrations accurate to +0.2% of the total fluoride concentrationof the sample can be performed using lanthanum nitrate as thetitrant. Total fluoride concentration should be at least 1.0X10-3Mfor endpoint detection. Low results are given if aluminum, iron,or trivalent chromium are present at a level of 1% or higher.Special titration procedures for aluminum, lithium, lanthanum, andthorium also make use of the fluoride electrode as an endpointindicator.

Titration Procedure for Fluoride Determination

  • Dissolve 43.3 grams of reagent grade lanthanum nitrate, La(NO3)3.6H2O, in about 500 ml distilled water in a 1liter volumetric plastic flask. Fill to the mark with distilled water. This 0.1M lanthanum solution will be used for all titrations.
  • Using the 0.1M fluoride standard, standardize the lanthanum nitrate by titration. To a 150 ml plastic beaker, add approximately 10.0 ml of fluoride standard (accurately measured) and about 50 ml of distilled water. Place the beaker on the magnetic stirrer and begin stirring. Lower the electrode tip into the solution.
  • Using a 10 ml plastic burette, add the La(NO3)3 titrant in 0.5-1.0 ml increments. Record the mV reading against the volume of titrant added. As the mV potential change increases, add smaller increments, down to 0.1-0.2 ml increments. Continue to add titrant and record the mV potential against the volume until little change is noted in the mV reading even when adding 0.5-1.0 ml increments.
  • Using linear graph paper, plot the mV readings (y-axis) against the volume (x-axis). The end-point is determined as the steepest slope on the titration curve. Record the endpoint.
  • To a 150 ml plastic beaker, add approximately 9.0 ml of sample solution (accurately measured) and about 50 ml of distilled water. Place the beaker on a magnetic stirrer and begin stirring. Lower the rinsed, dried electrode tip into the solution.
  • Titrate the sample as in step 3 above.
  • Calculate the sample concentration.

A typical titration curve


If the membrane is poisoned by interferences, the surface may be cleaned carefully by polishing with a fine polishing agent such as 3μm diamond paste. Polishing the membrane gently with fluoridated toothpaste can also restore sensitivity.

Organic contaminants can be removed with ethanol. DO NOT use the electrode in chlorinated hydrocarbons.

Routinely remove the sleeve and replace the potassium chloride electrolyte.


User maintenance and troubleshooting

Symptom Possible cause Remedy
Drift Junction blocked Remove and clean sleeve
Membrane unclean or contaminated Remove sleeve. Gently polish membrane with fine polishing agent
Noisy Poor connection to meter Check connection
Junction not immersed fully Lower electrode into solution below junction
Insufficient electrolyte or bubble Refill electrolyte
Inaccurate readings or poor reproducibility Contaminated standard(s ) Make up fresh calibration standards and recalibrate
Drift since last calibration Recalibrate using freshly prepared standards
Improperly made standards Recalibrate using freshly made standards
Temperature differences between calibration and sample measurement Temperature differences between calibration and sample measurement
No TISAB used Add TISAB buffer
Incorrect Ph Ensure pH is between 5 -7
Variations in measurement procedure between calibration and measurement Ensure stirring rates, temperature, and other conditions are the same in calibration and measurement steps

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