Chemistry

Experiment 6

DETERMINATION OF FLUORIDE BY ION SELECTIVE ELECTRODE

Part A:

Introduction:

In recent years ion-selective electrodes (ISE) have found use in many fields such as medicine

and environmental monitoring. They are versatile and easy to use. The fluoride content can be

measured in dental products such as toothpaste and mouthwash samples using a fluoride ion-

selective combination electrode. Two important classes of ISEs are the glass electrode and the

solid state electrode. Best known of these is the glass pH electrode, which measures the

hydrogen ion’s activity.

All ISEs are based on the Nernst equation.

E = Eo + 2.303 (RT/F) log A ion

The activities “A “of ions are related to concentrations of ions but depend upon ionic strength.

Thus, if we prepare our standards and unknown such that they have basically the same ionic

strength then the potentiometric measurements will correspond to their concentrations. The

Total Ion Strength Adjustment Buffer (TISAB), is added to accomplish this and to complex any

metal ions (c.a. Fe +3

, Al +3

) which might interfere with the F – ion. The solid state F

– ISE is

constructed of an ion sensing membrane containing conducting crystals in a inert matrix. The F –

ion electrode is based upon a crystal of LaF3 that is doped with Eu +2

to improve conductivity.

As in all electrochemical measurements a working electrode and a reference electrode must be

used. In both types of modern ISEs the working and the reference electrodes are self contained

and combined into a single electrode package.

Procedure

1. Be sure to clean, rinse with dI-water, and dry, several plastic polyethylene (PE) beakers! 2. Prepare a 100 ppm standard stock solution of F- ion: Begin by weighing by difference ~0.22

g of NaF to the nearest ± 0.0001 g into a clean PE beaker. Use the dried NaF stored in the

desiccator (this should have been dried @ 110C for ~2 hrs, consult TA).

Caution: NaF is highly toxic and poisonous, if touched, wash exposed hands or skin

immediately with copious amounts of water! It is very important to clean up your spills to

avoid harm to other lab personnel.

Quantitatively transfer to a 1L volumetric flask and dilute to volume with dI water. Mix

well by several inversions. Do not store F – ion solutions for long periods in glass

containers!

3. From the 100 ppm standard solution, prepare F-ion calibration standards of 0.50, 1.00, 2.00, 5.00, 10.00, and 20.00 ppm. To prepare a 20.00 ppm F

– ion standard, add 10.0 mL

of TISAB, use a volumetric pipette to add 20.00 mL of 100 ppm standard to a clean 100

mL volumetric flask, dilute to volume with dI water. Mix well. Do not store for long

periods in glass containers! Use pipettes provided and prepare the other standards in the

same likewise manner. Place about 20 mL of each standard into a clean dry PE beaker.

4. Pipet out 10 mL of mouthwash sample, add 10 mL of TISAB, and dilute to 100 mL volumetric flask. Place about 20 mL of this sample in PE beaker.

5. See Instructor for your assigned fluoride unknown. Pipet out 10 mL of unknown sample, add 10 mL of TISAB, and dilute to 100 mL volumetric flask. Place about 20

mL of unknown in PE beaker.

6. The pH of you standards and unknowns should be checked with a pH electrode. The optimal working range for the fluoride ISE is 5-9 pH. The fluoride ISE is not valid for

concentrations below 10 -6

M F¯ ion. You should calibrate the pH electrode as was done

for experiment #3. Your instructor will show you how to use the pH meter and F-ion

selective electrode. Read each solution using clean and dry PE beakers. Immerse the

electrode tip and allow ~ 30 sec for it to stabilize before recording instrument readings.

Rinse electrodes and dry electrodes between reading each solution. Make sure the

electrolyte salt solution inside the electrode is at the proper level, (the instructor may

need to add additional electrolyte filling solution).

Provided that the pH of your solutions are acceptable, proceed with the rest of

experiment.

The Fluoride Ion Selective electrode is not a pH electrode and will not measure

pH! There are no calibration buffers for the fluoride ISE instead you have

prepared a series of F¯ ion standards.

7. See instructor to check out Fluoride ISE electrode

Instrumental setup for the Fluoride ISE:

a. If not already connected to a pH\mV meter, connect the Fluoride ISE cable connection (BIN) to a pH\mV meter (one which has the mV readout mode option).

b. Make sure the filling solution level is just below the filling hole. c. Remove the protective end cap off of the electrode tip, (the electrode tip should have

been stored dry!).

d. Be careful not to damage the electrode tip, it is fragile. e. Hold onto ISE while performing a measurement. f. Do not let it rest on the bottom of the beaker. g. Avoid the tip’s contact with organic solvents. h. Avoid the tip’s contact with strong base. i. Read the standards in order from lowest concentration to highest. If you stir your

solutions with a magnetic stir bar while making measurements, avoid letting stir bar

bump the electrode tip.

j. Allow the reading to become stable, ~30 seconds. k. Rinse electrode tip with dI-water and blot dry with Kim-wipes® between

measurements.

l. When finished using the fluoride ISE, rinse and blot dry the tip, replace the protective end cap.

m. Turn off pH\mV meter. n. Rinse and dry off fluoride ion electrode. Be sure to store it dry for next user. Rinse

the PE beakers and leave at your work station.

o. Leave pH\mV meter and fluoride ISE at your work station

8. Helpful Hints: After the calibration standard measurements and prepare a rough calibration curve for

your standards in your lab notebook using excel on the lab computer. If the readings are

done correctly, it should be linear. If your unknown is outside the range for the curve,

consult your instructor. Different dilutions maybe required, (Try dilutions: e.g. 1 to 10,

1 to 100, etc.). If your unknown is significantly less than the least standard, then see

instructor.

NOTE:

Avoid contamination of solutions!

Use dry, clean glassware

Read all samples three times

Data processing

Use excel or similar program to make a graph as shown below to turn with lab report:

1) Prepare a linear calibration curve by plotting mV readings vs log of concentration of F -ion

in (ppm).

2) Obtain the linear regression equation of this calibration curve to find the concentration of the

unknowns. Remember any dilutions.

3) Report the F- ion concentration in ppm for your unknowns.

4) Recalculate the concentrations of your standards expressed in moles F-ion /Liter.(molarity).

Graph these values with mV readings of standards.

5) Report the slope of the this curve? Does your electrode follow the Nernst Equation

prediction? (Is you slope close to 59.16?)

Other Questions:

1. If you did not use TIASB what would happen to your results?

2. For the instrument, are two electrodes needed?

3. What two interferences does the TIASB buffer prevent? (hint: TIASB contains a compound

similar to EDTA salts.)

4. If NaCl is added and why doesn’t it affect the fluoride determination?

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