Thermo Scientific ISE User manual

User Guide
Fluoride Ion
Selective
Electrode

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Table of Contents
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .1
Required Equipment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .3
Electrode Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Electrode Preparation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .5
Checking Electrode Operation (Slope) . . . . . . . . . . . . . . . . . . . . .6
Measurement Units . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Sample Requirements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .7
Measuring Hints . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .8
Electrode Storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .9
Electrode Maintenance. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Analytical Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . .13
Direct Calibration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16
Low-Level Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . .20
Single Known Addition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .23
Titrations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .29
Fluoride in Acid Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .31
Fluoride in Alkaline Solutions . . . . . . . . . . . . . . . . . . . . . . . . . . .32
Electrode Characteristics . . . . . . . . . . . . . . . . . . . . . . . . .33
Electrode Response . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Reproducibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Limits of Detection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .33
Temperature Effects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
Interferences . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .34
pH Effects. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .35
Complexation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .36
Theory of Operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .37
Troubleshooting. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .39
Assistance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40
Warranty . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .40
Troubleshooting Checklist . . . . . . . . . . . . . . . . . . . . . . . . . . . . .41
Ordering Information . . . . . . . . . . . . . . . . . . . . . . . . . . . . .42
Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .43

1Fluoride Ion Selective Electrode User Guide
Introduction
This user guide contains information on the preparation,
operation and maintenance for the fluoride ion selective
electrode (ISE). General analytical procedures, electrode
characteristics and electrode theory are also included in this
user guide. Fluoride electrodes measure free fluoride ions in
aqueous solutions quickly, simply, accurately and economically.
The measurement of fluoride in drinking water and wastewater
using an ion selective electrode is an EPA approved test
procedure. EPA-approved ISE test procedures for fluoride in
drinking water are ASTM D1179-04 (B) and Standard Methods
4500-F- C (21st edition). EPA-approved ISE test procedures for
fluoride in wastewater are ASTM D1179-93, 99 (B) and Standard
Methods 4500-F- C-97 (online) or 4500-F- B (18, 19, 20th
editions). Approved test method lists are updated periodically.
Check with your regulatory agency for latest approve method
revisions.
Technical Support Chemists can be consulted for assistance
and troubleshooting advice. Within the United States call
1.800.225.1480 and outside the United States call 978.232.6000
or fax [phone]. In Europe, the Middle East and Africa,
contact your local authorized dealer. For the most current
contact information, visit www.thermoscientific.com/water.
Fluoride ionplus®Sure-Flow®Solid State Combination ISE
The reference and sensing electrodes are built into one
electrode, which decreases the amount of required solution
and reduces waste. The built-in Sure-Flow reference junction
prevents electrode clogging and provides fast and stabile
readings. The fluoride ionplus combination ISE is available with
a waterproof BNC connector, Cat. No. 9609BNWP. Electrodes
with a waterproof BNC connector can be used on any ISE meter
with a BNC connection.
Fluoride Solid State Half-Cell ISE
The fluoride half-cell electrode must be used with the single
junction reference electrode, Cat. No. 900100. The fluoride
half-cell is available with a BNC connector, Cat. No. 9409BN;
and a screw cap connector, Cat. No. 9409SC. Electrodes with a
screw cap connector require a separate cable.

2Fluoride Ion Selective Electrode User Guide
cap
cable
spring
O-ring
filling hole
inner cone
sensing crystal
reference
element
filling solution
chamber
outer body
inner
body
Figure 1
9609BNWP Fluoride Combination Electrode

3Fluoride Ion Selective Electrode User Guide
Required Equipment
1. Thermo Scientific Orion ISE meter, such as the Orion 4-Star
pH/ISE meter, Orion 5-Star pH/ISE/DO/conductivity meter
or Orion DUAL STAR meter.
Fluoride electrodes can be used on any ISE meter with
a BNC connection. The electrodes can also be used on
meters with a variety of inputs when an adapter cable is
used. Visit www.thermoscientific.com/water for details.
2. Thermo Scientific Orion fluoride ion selective electrode.
The 9409BN and 9409SC fluoride half-cell electrodes
require a separate reference electrode,
Cat. No. 900100.
3. Magnetic stirrer or Orion stirrer probe, Cat. No. 096019.
The Orion stirrer probe can be used with 3-Star, 4-Star, 5-
Star benchtop meters, and Dual Star meter.
4. Volumetric flasks, graduated cylinders and beakers.
Plastic labware is highly recommended for fluoride analysis.
5. Distilled or deionized water.
6. Fluoride electrode filling solution.
Use Optimum Results A filling solution, Cat. No. 900061,
for the 9609BNWP fluoride electrodes.
Use single junction reference electrode filling solution,
Cat. No. 900001, for the reference electrode used with the
9409BN and 9409SC fluoride electrodes.
7. Fluoride calibration standards.
Cat. No. Description
940906 0.1 M NaF, 475 mL bottle
940907 100 ppm F¯, 475 mL bottle
040908 10 ppm F¯ with TISAB II, 475 mL bottle
040907 2 ppm F¯ with TISAB II, 475 mL bottle
040906 1 ppm F¯ with TISAB II, 475 mL bottle

4Fluoride Ion Selective Electrode User Guide
8. Total Ionic Strength Adjustment Buffer (TISAB),
which provides a constant background ionic strength,
decomplexes fluoride ions and adjusts the solution pH.
Cat. No. Description
940909 TISAB II, 3.8 L bottle
940999 TISAB II, 4 x 3.8 L bottle
940911 TISAB III concentrate, 475 mL bottle
Low-Level TISAB
For use when measuring in samples containing less than 0.4
ppm (2 x 10-5 M) fluoride and no fluoride complexing agents such
as iron or aluminum are present.
To prepare low level TISAB: Place 500 mL of distilled water in
a 1 liter beaker. Add 57 mL of glacial acetic acid and 58 g of
reagent grade sodium chloride to the beaker. Place the beaker
in a water bath for cooling. Immerse a calibrated pH electrode
into the solution and slowly add 5 M NaOH until the pH is
between 5.0 and 5.5. Cool the solution to room temperature.
Pour the solution into a 1 liter volumetric flask and dilute to the
flask mark with distilled water. All reagents must be as pure as
possible to keep the fluoride level low in the buffer.
Use as directed for TISAB II; combine equal volumes of low level
TISAB and sample or standard before taking any measurements.
TISAB IV
TISAB IV will complex more than 100 ppm of iron or aluminum
in the presence of 1 ppm fluoride. A measurement of 1 ppm
fluoride will be in error by 5% in the presence of 200 ppm iron
or aluminum.
To prepare TISAB IV: Place 500 mL of distilled water in a 1 liter
volumetric flask. Add 84 mL of concentrated HCl (36 to 38 %),
242 g TRIS (hydroxymethyl) aminomethane and 230 g sodium
tartrate (Na2C4H406-2H2O) to the flask. Stir to dissolve the solids
and cool the solution to room temperature. Dilute to the flask
mark with distilled water.
Use as directed for TISAB II; combine equal volumes of TISAB IV
and sample or standard before measurements.

5Fluoride Ion Selective Electrode User Guide
Electrode Setup
Electrode Preparation
9409BN and 9409SC Fluoride Half-Cell Electrode– Remove the
protective shipping cap from the sensing element and save the
cap for storage.
900100 Single Junction Reference Electrode– Prepare the
reference electrode according to the reference electrode user
guide. Fill the reference electrode with single junction reference
filling solution, Cat. No. 900001.
9609BNWP Fluoride Combination Electrode– Remove the
protective shipping cap from the sensing element and save
the cap for storage. Fill the electrode with Optimum Results A
filling solution, Cat. No. 900061.
Note: Optimum Results A filling solution minimizes junction
potential issues and fluoride contamination in the sample. The
use of any other filling solution will void the electrode warranty.
Electrode Filling Instructions
1. Lift the flip spout on the filling solution bottle to a vertical
position.
2. Insert the spout into the filling hole on the outer body of the
electrode and add a small amount of filling solution to the
reference chamber. Invert the electrode to moisten the top
O-ring and then return the electrode to the upright position.
3. Hold the electrode body with one hand and use your thumb
to push down on the electrode cap to allow a few drops of
filling solution to drain out of the electrode.
4. Release the electrode cap. If the sleeve does not return to
its original position, check if the O-ring is moist and repeat
steps 2-4 until the sleeve returns to the original position.
5. Add filling solution to the electrode up to the filling hole.
Note: Add filling solution each day before using the electrode.
The filling solution level should be at least one inch above the
level of sample in the beaker to ensure a proper flow rate. The
fill hole should always be open when taking measurements.

6Fluoride Ion Selective Electrode User Guide
Checking Electrode Operation (Slope)
These are general instructions that can be used with most
meters to check the electrode operation. Refer to the meter
user guide for more specific information.
This procedure measures electrode slope. Slope is defined
as the change in millivolts observed with every tenfold change
in concentration. Obtaining the slope value provides the best
means for checking electrode operation.
1. If the electrode has been stored dry, prepare the electrode
as described in the Electrode Preparation section.
2. Connect the electrode to the meter. Set the meter to the
mV mode.
3. Place 50 mL of distilled water and 50 mL of TISAB II (Low-
level TISAB or TISAB IV) into a 150 mL beaker. Stir the
solution thoroughly.
If using TISAB III, place 90 mL of distilled water and
10 mL of TISAB III into a 150 mL beaker. Stir the solution
thoroughly.
4. Rinse the electrode with distilled water and place the
electrode into the solution prepared in step 3.
5. Select either a 0.1 M sodium fluoride or 100 ppm fluoride
standard. Pipette 1 mL of the standard into the beaker and
stir thoroughly. When a stable reading is displayed, record
the electrode potential in millivolts.
6. Pipette 10 mL of the same standard into the same beaker
and stir thoroughly. When a stable reading is displayed,
record the electrode potential in millivolts.
7. There should be a 54 to 60 mV difference between the
two millivolt readings when the solution temperature is
between 20 to 25 ˚C. If the millivolt potential is not within
this range, refer to the Troubleshooting section.

7Fluoride Ion Selective Electrode User Guide
Measurement Units
Fluoride concentration can be measured in moles per liter (M),
parts per million (ppm) or any convenient concentration unit.
Table 1
Concentration Unit Conversion Factors
Moles/Liter (M) ppm
1.0 19000
10-1 1900
10-2 190
10-3 19
10-4 1. 9
Sample Requirements
The epoxy body of the fluoride electrode is resistant to damage
by inorganic solutions. The electrode may be used intermittently
in solutions that contain methanol or acetone. Contact Technical
Support for information on using the electrode for specific
applications.
Samples and standards should be at the same temperature.
The solution temperature must be less than 100 °C.
In all analytical procedures, TISAB must be added to all samples
and standards before measurements are taken.

8Fluoride Ion Selective Electrode User Guide
Measuring Hints
• Add 50 mL of TISAB II (Low-level TISAB or TISAB IV) to
every 50 mL of standard or sample. Add 10 mL of TISAB III
to every 90 mL standard or sample. Once TISAB II (Low-
level TISAB or TISAB IV) or TISAB III is selected, it should be
added to all samples and standards so the dilution ratio of
TISAB to solution remains the same.
• Stir all standards and samples at a uniform rate.
• Always use fresh standards for calibration.
• Always rinse the electrode with deionized water between
measurements and shake the electrode to remove the
water and prevent sample carryover. Do not wipe or rub
the electrode sensing element.
• Allow all standards and samples to come to the same
temperature for precise measurements.
• Place a piece of insulating material, such as Styrofoam or
cardboard, between the magnetic stirrer and beaker to prevent
measurement errors from the transfer of heat to the sample.
• Verify the electrode calibration every two hours by placing
the electrode in a fresh aliquot of the least concentrated
standard used for calibration. If using more than 2
standards, use a mid-level standard. If the value has
changed by more than 2%, recalibrate the electrode.
• After immersing the electrode in a solution, check the
electrode sensing surface for air bubbles and remove air
bubbles by reimmersing the electrode in the solution.
• For high ionic strength samples, prepare standards with
a background composition similar to the sample or use
manual known addition procedure or a Dual Star meter.
• Adjust highly acidic or highly basic solutions to pH 5 - 6
before adding TISAB.
• Remove the filling hole cover during measurements to
ensure a uniform flow of filling solution.
• If the electrode is used in dirty or viscous samples or the
electrode response becomes sluggish, empty the electrode
completely, hold the junction open and flush the junction
with deionized water. Empty any water from the electrode
and refill it with fresh filling solution. Press down on the
electrode cap to let a few drops of the filling solution flow
out of the electrode and then replenish any lost solution.

9Fluoride Ion Selective Electrode User Guide
Electrode Storage
9409BN and 9409SC Fluoride Half-Cell Electrode
The fluoride half-cell electrodes should be rinsed thoroughly
with distilled water and stored dry in the air at all times. When
storing the electrode for long periods of time, cover the sensing
element with the protective shipping cap.
900100 Single Junction Reference Electrode
The single junction reference electrode may be stored in the
single junction electrode filling solution, Cat. No. 900001,
between sample measurements and up to one week. A 0.01 M
or 100 ppm fluoride standard is also an acceptable storage
solution. The filling solution inside the electrode should not be
allowed to evaporate, as crystallization will result.
For storage longer than one week, drain the reference electrode,
flush the inside with distilled water and store the electrode dry.
9609BNWP Fluoride Combination Electrode
For storage between measurements and up to one week, store
the electrode in electrode fill solution, Cat. No. 900061, with
fluoride added. The fluoride concentration of the storage solution
should be close to least concentrated fluoride calibration
standard. Do not add TISAB to the storage solution. The filling
solution should not be allowed to evaporate, as crystallization
will result. If fill solution is unavailable, store in 2 M potassium
nitrate with fluoride added.
For storage longer than one week, drain the electrode. Flush
the chamber with distilled water. Disassemble the electrode as
described in the electrode maintenance section. Rinse the inner
sleeve with distilled water and blot dry with a Kimwipe. Do not
touch the reference element other than to blot dry. Let air dry
and then reassemble for storage. Use the protective shipping
cap to cover the sensing element.

10 Fluoride Ion Selective Electrode User Guide
Electrode Maintenance
Polishing the Sensing Surface of the Fluoride Combination
Electrodes and Fluoride Half-Cell Electrodes
The sensing surface of the fluoride electrode might need
restoration over time as deposits coat the sensing surface. The
sensing crystal can be restored using the following procedure.
Toothpaste that contains fluoride will be needed for this
procedure.
1. Dispense a small amount of fluoridated toothpaste on
a kimwipe. Add a few drops of deionized water. A soft
toothbrush can also be used in placed of a kimwipe.
2. Rub the toothpaste on the sensing element on the bottom
of the electrode in a circular motion for around one minute.
3. Rinse the electrode thoroughly with DI water, flush the
electrode fill solution out (if using a combination fluoride
electrode) by pressing on the cap of the electrode) then
refill with fresh fill solution.
4. Soak the electrode in 100 ppm fluoride standard for 2 hours.
Flushing the 9609BNWP and 900100 Single Junction
Reference Electrode
If the area between the electrode sleeve and inner cone
becomes clogged with sample or precipitate, flush the area with
filling solution or distilled water.
1. Hold the electrode body with one hand and use your thumb
to push down on the electrode cap to drain the chamber.
Push down on the cap until all the filling solution is drained
from the chamber.
2. Fill the electrode with distilled water and then push down
on the cap until all the water is drained from the chamber.
3. Fill the electrode with fresh filling solution up to the fill
hole. Push down on the cap to allow a few drops of filling
solution to drain out of the electrode and then refill any lost
filling solution.

11Fluoride Ion Selective Electrode User Guide
Disassembling the Fluoride Combination Electrodes and
Single Junction Reference Electrode
Note: Disassembly is usually not required and should not be
done unless a thorough cleaning is required.
1. Tip the electrode so the filling solution moistens the O-ring
on the electrode body. Hold the electrode body with one
hand and use your thumb to push down on the electrode
cap to drain the chamber.
2. Unscrew the cap counter clock-wise and then slide the cap
and the spring up the cable.
3. Hold the outer sleeve with one hand and firmly push down
on the threaded portion with the thumb and forefinger to
separate the inner body from the sleeve.
4. Grasp the cone with a clean, lint-free tissue and withdraw
the body from the sleeve using a gentle twisting motion.
Do not touch the reference electrode element (coiled wire
in combination electrode, pellet in single junction reference)
above the cone, it will damage to the reference element.
Rinse the outside of the electrode body and the entire
sleeve with distilled water. Blot dry with a Kimwipe and
allow to air dry if needed.
Reassembling the Fluoride Combination Electrodes and
Single Junction Reference Electrode
1. Moisten the O-ring on the electrode body with a drop of
filling solution. Insert the screw-thread end of the electrode
body into the tapered, ground end of the sleeve.
2. Push the body into the sleeve using a gentle twisting
motion until the bottom surface of the inner cone is flush
with the tapered end of the sleeve.
3. Place the spring onto the electrode body and screw on the
cap. Refill the electrode with filling solution.

12 Fluoride Ion Selective Electrode User Guide

13Fluoride Ion Selective Electrode User Guide
Analytical Techniques
A variety of analytical techniques are available to the analyst.
The following is a description of these techniques.
Direct Calibration is a simple procedure for measuring a
large number of samples. Only one meter reading is required
for each sample. Calibration is performed using a series of
standards. The concentration of the samples is determined by
comparison to the standards. TISAB is added to all solutions to
ensure that samples and standards have similar ionic strength.
Incremental Techniques provide a useful method for measuring
samples, since calibration is not required. As in direct
calibration, any convenient concentration unit can be used. The
different incremental techniques are described below. They can
be used to measure the total concentration of a specific ion in
the presence of a large (50-100 times) excess of complexing
agents.
• Known Addition is useful for measuring dilute samples,
checking the results of direct calibration (when no
complexing agents are present), or measuring the total
concentration of an ion in the presence of an excess
complexing agent. The electrode is immersed in the
sample solution and an aliquot of a standard solution
containing the measured species is added to the sample
when performing single known addition. From the change
in potential before and after the addition, the original
sample concentration is determined. Double known
addition is recommended for complex sample matrices.
• Known Subtraction is useful as a quick version of
a titration, or for measuring species for which stable
standards do not exist. It is necessary to know the
stoichiometric ratio between standard and sample. For
known subtraction, an electrode sensing the sample
species is used. Stable standards of a species reacting
completely with the sample in a reaction of known
stoichiometry are necessary.
• Analate Addition is often used to measure soluble solid
samples, viscous samples, small or very concentrated
samples, to diminish the effects of complex sample
matrices, or to diminish the effects of varying sample
temperatures. This method is not suitable for dilute or low
concentration 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.

14 Fluoride Ion Selective Electrode User Guide
• Analate Subtraction is used in the measurement of ions
for which no ion-selective electrode exists. The electrode is
immersed in a reagent solution that contains a species that
the electrode senses, and that reacts with the sample. It
is useful when sample size is small, or samples for which
a stable standard is difficult to prepare, and for viscous
or very concentrated samples. The method is not suited
for very dilute samples. It is also necessary to know the
stoichiometric ratio between standard and sample.
• Titrations are quantitative analytical techniques for
measuring the concentration of a species by incremental
addition of a reagent (titrant) that reacts with the sample
species. Sensing electrodes can be used for determination
of the titration end point. Ion selective electrodes are
useful as end point detectors, because they are unaffected
by sample color or turbidity. Titrations are approximately
10 times more precise than direct calibration, but are more
time-consuming.

15Fluoride Ion Selective Electrode User Guide
Typical Calibration Curve
In the direct calibration procedure, a calibration curve is
constructed either in the meter memory or in an electronic
spreadsheet and graphing the log of the fluoride concentration
against the millivolt value readings. Electrode potentials of
standard solutions are measured and plotted on the linear
axis against their concentrations on the log axis. In the linear
regions of the curves, only two standards are needed to
determine a calibration curve. In non-linear regions, more points
must be taken. These direct calibration procedures are given for
concentrations in the region of linear electrode response. Low
level measurement procedures are given in the next section for
measurements in the non-linear region.
10- 5
0.1
180
160
140
120
100
80
60
40
20
0
-20
-40
-60
1 10 100 1000
10- 410 - 310 - 210- 1
molarity
ppm fluoride as F-
10-fold change
electrode
potential
(mV)
-54 to -60 mV
Figure 2 Typical Calibration Curve

16 Fluoride Ion Selective Electrode User Guide
Direct Calibration
Setup
1. Prepare the electrode as described in the Electrode
Preparation section. If using the 9409BN or 9409SC half-
cell fluoride electrode with the 900100 reference electrode,
fill the reference electrode with Cat. No. 900001. If using
the 9609BNWP combination fluoride electrode, fill the
electrode with Cat. No. 900061.
2. Connect the electrodes to the meter.
3. Prepare at least two standards that bracket the expected
sample range and differ in concentration by a factor of ten.
Standards can be prepared in any concentration unit to suit
the particular analysis requirement. All standards should
be at the same temperature as the samples. For details on
temperature effects on electrode performance, refer to the
Temperature Effects section.
Serial Dilutions
Serial dilution is the best method for the preparation of
standards. Serial dilution means that an initial standard is
diluted, using volumetric glassware, to prepare a second
standard solution. The second standard is similarly diluted to
prepare a third standard, and so on, until the desired range of
standards has been prepared.
1. To prepare a 10-2 M NaF standard (190 ppm fluoride)–
Pipette 10 mL of the 0.1 M NaF standard into a 100 mL
volumetric flask. Dilute to the mark with deionized water
and mix well.
2. To prepare a 10-3 M NaF standard (19 ppm fluoride)–
Pipette 10 mL of the 10-2 M NaF standard into a 100 mL
volumetric flask. Dilute to the mark with deionized water
and mix well.
3. To prepare a 10-4 M NaF standard (1.9 ppm fluoride)–
Pipette 10 mL of the 10-3 M NaF standard into a 100 mL
volumetric flask. Dilute to the mark with deionized water
and mix well.