Thermo Fisher Scientific Ion Selective Electrodes Fluoride User Manual

EUTECH INSTRUMENTS PTE LTD.

INSTRUCTION MANUAL

FLUORIDE ION ELECTRODE

TABLE OF CONTENTS

GENERAL INSTRUCTIONS ..........................................................................................................1

Introduction...................................................................................................................................................1

Required Equipment............................................ ... .... .... ... .... ... .... ... .... .... ... .... ... .... .... ....................................1

Required Solutions................................................. .... .... ... .... ... .... ... .... .... ... .... ... .... .... ... .... .. ...........................1

GENERAL PREPARATION............................................................................................................2

Electrode Preparation ....................................................................................................................................2

Electrode Slope Check (with pH/mV meter).................................................................................................2

Electrode Slope Check (with ion meter) .......................................................................................................3

MEASUREMENT..............................................................................................................................3

Measuring Hints............................................................................................................................................3

Sample Requirements....................................................................................................................................4

Units of Measurement...................................................................................................................................4

MEASUREMENT PROCEDURE ...................................................................................................4

Direct Measurement ......................................................................................................................................4

Direct Measurement of Fluoride (using a pH/mV meter) .............................................................................4

Direct Measurement of Fluoride (using an ion meter)................................................. .... ... .... .... ... .... ... ........6

Direct Measurement of Fluoride in Water (ASTM D1179, METHOD B) ............................................... ....6

Direct Measurement of Fluoride in Acid Solutions ......................................................................................7

Direct Measurement of Fluoride in Alkaline Solutions ................................................................................7

Low Level Fluoride Measurements (using a pH/mV meter).........................................................................8

Low Level Fluoride Determination (using an ion meter)..............................................................................8

Titration.........................................................................................................................................................9

Titration Procedure for Fluoride Determination............................................................................................9

ELECTRODE CHARACTERISTICS...........................................................................................10

Reproducibility............................................................................................................................................10

Interference..................................................................................................................................................10

Complexation ..............................................................................................................................................11

Temperature Influences...............................................................................................................................11

Electrode Response .....................................................................................................................................11

Limits of Detection......................................................................................................................................12

pH Effects....................................................................................................................................................12

Electrode Life..............................................................................................................................................13

Electrode Storage ........................................................................................................................................14

ELECTRODE THEORY................................................................................................................14

Electrode Operation.....................................................................................................................................14

TROUBLESHOOTING GUIDE....................................................................................................15

Meter ...........................................................................................................................................................15

Plastic-ware.................................................................................................................................................15

Electrodes....................................................................................................................................................15

Standards and Reagents................................... .... ... .... ... .... .... ... .... ...............................................................16

Sample.........................................................................................................................................................16

Technique....................................................................................................................................................16

TROUBLESHOOTING HINTS.....................................................................................................17

SPECIFICATIONS..........................................................................................................................19

ORDERING INFORMATION.......................................................................................................19

Instruction Manual Fluoride Electrode

EUTECH INSTRUMENTS PTE LTD.

FLUORIDE ION ELECTRODE

INSTRUCTION MANUAL

GENERAL INSTRUCTIONS

Introduction

Eutech Instruments Fluoride Ion Electrode is used to measure fluoride ions in aqueous solutions quickly, simply, accurately, and economically.

Required Equipment

1. A pH/mV meter or an ion meter, either line operated or portable.

2. Semi-logarithmic 4-cycle graph paper for preparing calibration curves when using the meter in the mV mode.

3. A magnetic stirrer.

4. Eutech Fluoride Ion Combination Epoxy-body Electrode, Code no. EC-FO-03.

5. Lab-ware made of plastic, not glass.

6. Polishing Paper, Code no. EC-MIS-PP, to polish dirty or etched electrode membranes.

Required Solutions

1. Deionized or distilled water for solution and standard preparation.

2. Eutech Fluoride Standard Solution, 0.1 M NaF, Code No. EC-SCS-FL1-BT. To prepare this solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 4.2 grams of reagent-grade sodium fluoride. Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.

3. Eutech Fluoride Standard, 1,000 ppm F solution from your own laboratory stock, half fill a one liter volumetric flask with distilled water and add 2.21 grams of reagent-grade sodium fluoride. Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.

4. Eutech Fluoride Standard, 100 ppm F solution from you own laboratory stock, half fill a one liter volumetric flask with distilled water and add 0.22 grams of reagent-grade sodium fluoride. Swirl the flask gently to dissolve the solid. Fill the flask to the mark with distilled water, cap, and upend several times to mix the solution.

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, Code No. EC-SCS-FL2-BT. To prepare this

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, Code No. EC-SCS-FL3-BT. To prepare this

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Instruction Manual Fluoride Electrode

5. Eutech Total Ionic Strength Adjuster Buffer (TISAB 1), Code No. EC-ISA-FL1-BT. TISAB 1 is used to adjust the pH of the solution, de-complex fluoride and provide a constant background ionic strength. To prepare this solution from your own laboratory stock, half fill a four liter beaker with distilled water. Place the beaker on a magnetic stirrer, add a large stirring bar, and begin stirring. Slowly add 230 ml of concentrated acetic acid, 232 grams of reagent-grade sodium chloride, and 16 grams of reagent-grade CDTA. After the solids have dissolved, allow the solution to cool to room temperature. Slowly add 150 grams of reagent-grade sodium hydroxide. After the solids have dissolved, allow the solution to cool to room temperature. Calibrate a pH electrode and adjust the pH to 5.25 with small addition of 5 M NaOH. Fill to the mark with distilled water.

6. Eutech Low Level Total Ionic Strength Adjuster Buffer (TISAB 2), Code No. EC-ISAFL2-BT. Use when measuring in samples containing less than 2X10

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M (0.4 ppm) fluoride and containing no fluoride complexing agents. To prepare this solution from your own laboratory stock, place about 2'000 ml distilled water in a four liter beaker. Add 57 ml glacial acetic acid and 58 grams sodium chloride. Place the beaker on a magnetic stirrer, add a stirring bar and begin stirring. Immerse a calibrated pH electrode into the solution. Slowly add 5 M NaOH until the pH is 5.25. Allow the solution to cool and fill to the mark with distilled water.

GENERAL PREPARATION

Electrode Preparation

Remove the rubber caps covering the electrode tips and the rubber insert covering the filling hole of the reference electrode. Fill the combination electrode or the reference electrode with the filling solution shipped with the electrode to a level just below the fill hole. No preparation is required with a sealed reference electrode. Connect the electrodes to the proper terminals of the meter as recommended by the meter manufacturer.

Electrode Slope Check (with pH/mV meter)

(Check electrodes each day)

1. To a 150 ml plastic beaker, add 50 ml of distilled water and 50 ml of TISAB. Place the beaker on a magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is in the millivolt mode, lower the electrode tips into the solution.

2. Using a pipet, add 1 ml of 0.1M, 1,000 ppm, or 100 ppm fluoride standard to the beaker. When the reading has stabilized, record the mV reading.

3. Using a pipet, add 10 ml of the same fluoride standard used above to the beaker. When the reading has stabilized, record the mV reading.

4. Determine the difference between the two readings. The electrode is operating correctly if the millivolt potential has changed by 57±2 mV, assuming the solution temperature is between 20o and 25oC. See the

TROUBLESHOOTING section if the potential change is not

within this range.

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Instruction Manual Fluoride Electrode

Slope

is defined as the change in potential observed when the concentration changes by a factor of

10.

Electrode Slope Check (with ion meter)

(Check electrodes each day)

1. Prepare standard fluoride solutions whose concentrations vary by tenfold. Use either the

0.1M, 1,000 ppm, or 100 ppm fluoride standard. Use the serial dilution method for this preparation.

2. To a 150 ml beaker, add 50 ml of the lower value standard and 50 ml of TISAB. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution. Assure that the meter is in the concentration mode.

3. Adjust the meter to the concentration of the standard and fix the value in the memory according to the meter manufacturer's instructions.

4. Rinse the electrodes with distilled water and blot dry.

5. To another 150 ml beaker, add 50 ml of the higher value standard and 50 ml of TISAB. Place the beaker on the magnetic stirrer and begin stirring at a constant rate. Lower the electrode tips into the solution.

6. Adjust the meter to the concentration of the standard and fix the value in the memory.

7. Read the electrode slope according to the meter instructions. Correct electrode operation is indicated by a slope of 90-100%. See the

TROUBLESHOOTING section if the slope is not

within this range.

MEASUREMENT

Measuring Hints

All samples and standards should be at the same temperature for precise measurement. A difference

C in temperature will result in a 2% measurement error.

of 1

Constant, but not violent, stirring is necessary for accurate measurement. Magnetic stirrers can generate sufficient heat to change the solution temperature. To counteract this effect, place a piece of insulating material, such as a styrofoam sheet, between the stirrer and the beaker.

Always rinse the electrodes with distilled water and blot dry between measurements. Use a clean, dry tissue to prevent cross-contamination.

For samples with high ionic strength, prepare standards whose composition is similar to the sample.

Always check to see that the membrane is free from air bubbles after immersion into standard or sample.

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Instruction Manual Fluoride Electrode

Sample Requirements

All samples must be aqueous and not contain organics which can dissolve the epoxy electrode body and/or the cement bonding the sensing crystal to the electrode body. Inorganic solutions will not affect the electrode. Infrequent measurements in solutions containing methanol, acetone, or dioxane are permitted. Highly polar solvents, such as CHCl

or DMF, should not be contained in the

samples. Please check with Eutech Instruments Pte Ltd. before using the electrode in other organic solvents.

The addition of TISAB to samples and standards will adjust the pH to 5.0-5.5. Samples must be above pH 5 to avoid forming complexes with hydrogen ions and below pH 7 to avoid interference by hydroxide ions.

The temperature of the standard solutions and of the sample solutions should be the same and below

C. The use of TISAB 1 also preferentially forms complexes with aluminum and with iron,

80 breaking the complexes that fluoride forms with these ions. With 1 ppm fluoride present, up to 3-5 ppm aluminum or iron is complexed. If higher levels of aluminum or iron are present, use TISAB 3.

Units of Measurement

Fluoride concentrations are measured in units of ppm as fluoride, moles per liter, or any other convenient concentration unit. Table 1 indicates some concentration units and conversion factors.

TABLE 1

ppm F

190.0 1.0X10

19.0 1.0X10

1.9 1.0X10

: Concentration Unit Conversion Factors

-1

moles/liter

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-3

-4

MEASUREMENT PROCEDURE

Direct Measurement

Direct measurement is a simple procedure for measuring a large number of samples. A single meter reading is all that is required for each sample. The ionic strength of samples and standards should be made the same by adjustment with TISAB for all fluoride solutions. The temperature of both sample solution and standard solution should be made the same.

Direct Measurement of Fluoride (using a pH/mV meter)

1. By serial dilution, prepare three standard solutions from the 0.1M, 1,000 ppm, or the 100 ppm stock standard. The resultant concentrations should be 10

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M, 10-3M, and 10-4M or 100, 10, and 1 ppm. Add 50 ml of TISAB 1 or TISAB 2 to each 50 ml of standard. When calibrating, assume that the added TISAB has no effect on the standard concentration.

2. Place the most dilute solution on the magnetic stirrer and begin stirring at a constant rate. After assuring that the meter is in the mV mode, lower the electrode tips into the solution. After the reading has stabilized, record the mV reading.

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Instruction Manual Fluoride Electrode

3. Place the mid-range solution on the magnetic stirrer and begin stirring. After rinsing the electrodes with distilled water, blot dry, and immerse the electrodes in the solution. When the reading has stabilized, record the mV value.

4. Place the most concentrated solution on the magnetic stirrer and begin stirring. After rinsing the electrodes with distilled water, blot dry, and immerse the electrodes in the solution. When the mV reading has stabilized, record the mV value.

5. Using the semi-logarithmic graph paper, plot the mV reading (linear axis) against the concentration (log axis). Extrapolate the curve down to about 1.0X10

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M. A typical

calibration curve can be found in Figure 1.

A calibration curve is constructed on semi-logarithmic paper when using the pH/mV

meter in the mill ivolt mode. Th e measured electrode potential in mV (linear axis) is plotted against the standard concentration (log axis). In the linear region of the curve, only three standards are necessary to determine a calibration curve. In the non-linear region, additional points must be measured. The direct measurement procedures given are for the linear portion of the curve. The non-linear portion of the curve requires the use of low level procedures.

6. To a clean, dry, 150 ml plastic beaker, add 50 ml of sample and 50 ml of TISAB 1 or TISAB 2. Place the beaker on the magnetic stirrer and begin stirring. Rinse the electrodes with distilled water, blot dry, and lower the electrode tips into the solution. When the reading has stabilized, record the mV reading. Using the calibration curve, determine the sample concentration.

7. The calibration should be checked every 1-2 hours. Assuming no change in ambient temperature, place the electrode tips in the mid-range standard. After the reading has stabilized, compare it to the original reading recorded in Step 3 above. A reading differing by more than 0.5 mV or a change in the ambient temperature will necessitate the repetition of Steps 2-5 above. A new calibration curve should be prepared daily.

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