1225 Lab # 11 Fluoride Speciation and Analysis by Flow Injection using Ion-Selective Electrode: Measurement of Total Fluoride in Water

Transcription

1 1225 Lab # 11 Fluoride Speciation and Analysis by Flow Injection using Ion-Selective Electrode: Measurement of Total Fluoride in Water Introduction Fluoride is recognized world- wide to help build stronger tooth enamel, stop or reverse the tooth decay process and prevent loss of important minerals. Tooth enamel is basically the mineral fluorapatite. This mineral is developed prior to teeth breaking through the gums and continues to develop and be replenished by dietary fluoride throughout the lifetime of the teeth 1. According to the Salt Lake Valley Health Department 2 the optimal level of fluoride is 0.7 to 1.2 ppm in drinking water. In 2003 the State of Utah started adding fluoride to tap water. Then, in 2008, Utah counties voted on addition of fluoride resulting in a variation of water treatment from county to county. At present, the state has about 50% of the population receiving fluoride treated water with the aim of providing the 1ppm fluoride level. The fluoride ion- selective electrode (ISE) with the use of appropriate Total Ionic Strength Adjustments Buffers (TISAB) has been used to measure fluoride in water samples since 1968 providing a versatile tool especially for chemical speciation studies of fluoride 3. Electrochemical cell: A system consisting of electrodes in contact with electrolyte and in which a chemical reaction either uses or generates an electric current. There are two types of cells possible, these are, voltaic and electrolytic cells. Voltaic cell: In a voltaic cell a spontaneous reaction generates a voltage. Electrical current, and therefore, energy is obtained out of the cell. Also, information such as the concentration of chemical species can be obtained with chemical sensors based on this type of cell. Examples of voltaic cells are batteries, fuel cells, and potentiometric measurements such as sensing the ph of aqueous solutions. Potential Difference: The potential difference is the difference in electric potential between two points. For a cell, it is between the electrodes. It is measured in units of volts. It is the measured voltage of a cell. Electrode Potential: The electrode potential is the potential of an electrode under nonstandard conditions. The electrode potential is denoted E. For example, the potential of Zn electrode will be different in 0.1 M Zn 2+ solution compared to 1 M Zn 2+ Notation for voltaic cells: To draw sketches of cells is tedious so an accepted shorthand notation is used. Rules 1. Phase boundaries are vertical lines, 2. Salt bridge or solution junction parallel lines,

2 Reference electrodes and Ion-Selective Electrodes (ISE): Reference electrodes: Reference electrodes have constant potential, which is achieved by isolation of an electrode in a constant concentration solution via salt bridge (or a liquid junction). The Ag AgCl system is commonly used. In this case a silver wire coated in solid silver chloride, which is in contact with a saturated potassium chloride solution, is used. The short hand notation of the electrode is as follows: Ag/AgCl saturated (4M) KCl (aq) A schematic diagram follows: Notes on Reference Electrodes: The saturated KCl is used to guarantee constant concentration of chloride ions [Cl - ]. A small potential difference exists at the liquid junction point due to the diffusion of potassium and chloride ions. These two ions have slightly different size and diffuse at different rates setting up a small charge separation, that is, a potential difference, at the junction point. It is called the liquid junction potential. To keep this constant the fill hole is opened during measurements so that a constant, but small flow occurs through the junction. A blocked junction is a common cause of needless error in many labs. Generally chemical ion sensors of this type obey the following form of Nernst equation: E = E I + (.0592/z) log [ X z ] Where, X z = ion with charge z, For example: H3O +, z = +1, Cl -, z = -1, Cu 2+, z = +2, etc.

3 E I = sum of all other potential in measurement system, includes reference electrode and potentials inside ion sensor electrode (these should be kept constant). The Fluoride Ion-Selective Electrode (ISE): The fluoride ISE is constructed very similarly to a ph glass electrode. The difference is the sensing membrane and internal solution. The shorthand notation is as follows: Ag/AgCl wire 1 M NaCl + 1 M NaF LaF3 crystal membrane Below is a schematic diagram of the ISE: The electrode responds to the activity (voltage) of free F - ions in aqueous solution in the same manner as a ph electrode. Activity is directly proportional to concentration when ionic strength of all solutions is constant, including samples and standards, and therefore the ISE can be used to measure concentrations. The LaF3 crystal membrane is very selective, the only interferent is the hydroxide ion, OH -, which can be kept negligible and controlled by a ph buffer. (In our experiment this is part of TISAB II) The Fluoride electrode can be used to measure Total Fluoride and its ionic forms in water samples and thereby provide information on the amount of the different fluoride species in water samples, such as free F -, HF and F - bound to transition metals. Measurement of the Concentration Free fluoride: To measure just the free fluoride (F - ) in a water sample one adjusts ionic strength of sample to that of standards, then directly measures the concentration of the free fluoride ions. Ionic strength adjustment is done using NaCl, since it does not react with F - ions or change the sample s ph. (In our experiment this is part of TISAB II) The ISE is very selective and one only needs to be aware that a high ph of a sample will cause OH - ions to interfere. If the sample has a ph over 9, significant error results and another method of analysis is required, such as spectroscopy. This is only a problem where just the free F - ion concentration is to be measured. Most water samples have a ph of less than 9.

4 Measurement of Concentration Free fluoride plus HF To measure fluoride present as HF it needs to be converted to free F - : HF H + + F - This is achieved by raising the ph to a value where all HF is converted to free F-, but not so high as to have OH- interfere. At ph = 5, all HF is converted to F -, yet [OH - ] = 10-9 M, and this is close enough to zero so the problem of hydroxide interference is negligible. Therefore if we use a ph 5 buffer which also provides constant ionic strength (In our experiment this is part of TISAB II) to treat the water sample, then both free F - and HF are measured as F - at the electrode. Measurement of Concentration Total fluoride: In many water samples there are traces of metals present. In normal potable water, Fe and Al are present. Fluoride ions, acting as ligands, easily complex these metals forming coordination complex ions in solution. The Fluoride electrode cannot detect these species since it responds only to free fluoride ions. Fe F - [FeF 4 ] - Al F - [AlF 4 ] - To measure the Total Fluoride content, these complexes need to have the fluoride ions released into the solution by the use of a stronger ligand (complexing agent) than fluoride. A reagent called CDTA (cyclohexane diamine tetra acetic acid) is added to the buffer (In our experiment this is part of TISAB II) used in the treatment of samples and standards before measurement with the electrode. CDTA is a polydentate ligand. The CDTA releases free fluoride ions into solution where they can respond at the electrode and we can measure the total amount of free F -, HF and the metal bound F -. The equations for release of F - away from metals are shown below. Total Fluoride: [FeF 4 ] - + CDTA [Fe CDTA ] F - [AlF 4 ] - + CDTA [Al CDTA ] F - In this lab, we will be using TISAB II, which stands for Total Ionic Strength Buffer, and the II refers to the fact that there is CDTA added. This will allow for measurement of all species of Fluoride in the water at once: Total F - = free F - + HF + metal complex F- References: 1. Frieden, E. Biochemistry of the Essential Ultratrace Elements, Plenum Press, New York, Salt Lake Valley Health Department, Frant, M. S. and Ross, J. W. Jr, Use of a Total Ionic Strength Adjustment Buffer for Electrode Determination of Fluoride in Water Supplies, Anal. Chem. 40 (7) ; , 1968.

5 Flow Injection Analysis Set-up Diagram (Figure-3) $ ' $ 5. % ( 6 $! 078$5.9(7!:,'#$!!!!!1A%97B$!! 4'()!5$''!!! ;,<.(<!!?@=?! "#$%&'()* +,-.$! =95%(;,>! /0123! 00! MicroLab/Computer Set-Up 1. Turn on laptop and open the MicroLab application by double clicking on the icon. 2. Choose an Experiment Type box will open. Double click on MicroLab Experiment icon. (See Figure-2) Figure-2

6 3. Once MicroLab application is open, click on Add Sensor in the Data Sources / Variables box. 4. Choose Sensor Box will open. Under the title Sensor, you will see a drop down box. Click on the down arrow and select MilliVolts/Redox/ISE. 5. On the picture of the MicroLab, a red box will surround the ph/redox/do sensor. Click in the middle of the red box. 6. Under Range/Sensitivity make sure +/ mv is selected. Click Factory Calibration. 7. Under Data Source/ Variables, a millivolts (2500mV) label should show up. Click, hold, and drag it to the y-axis on the on-screen graph. 8. Click, hold, and drag the same millivolts (2500mV) label to box B1 in the table below the graph. 9. Click again on Add Sensor in the Data Sources / Variables box. 10. Choose Sensor Box will open. Under the title Sensor you will see a drop down box. Click on the down arrow and select Time. On the picture of the MicroLab, three red square boxes will surround Timers. Click on the #1 box, then click Set Timer Options. 11. Under Select the type of timer, make sure Automatic operation is selected. 12. Under Select the timer units, make sure Seconds is selected, then click Finish 13. Under Data Source / Variables, a Time 1 label should show up. Click and hold label and then drag it to the x-axis on the on screen graph. 14. Click, hold and then drag the same Time 1 label to box A1 in the table below the graph. System Set-Up 1. Clean a 250mL beaker, a 100mL beaker and six 50mL beakers by rinsing each 3 times with tap water only (no soap). Then rinse 3 times with Distilled water. Then rinse again 3 times with ultra pure water. 2. Pour approximately 200mL of TISAB II into the clean 250mL beaker. Cover beaker with parafilm. This is your carrier solution. 3. Fill a 100mL beaker with ultra pure water. Put the end of the tubing coming off of the pump into the beaker so that the tubing is in the solution. Cover beaker with parafilm. (This step is only to clean the system). 4. Place the tubing attached to the injection valve with blue connector and the tubing attached to the flow cell with green connector into a 600mL beaker. Because this is your overflow/waste beaker, it does not need to be cleaned. 5. Set pump speed to 25 (2.5mL/minute) by pushing the + or buttons on front of pump.

7 6. To start pump, push the arrow that is going counterclockwise. You should see the water coming up through the tubing and out to the waste beaker. Let it run while you finish your set-up. ***DO NOT LET YOUR BEAKER RUN DRY!!!! ENSURE THERE IS ALWAYS WATER GOING INTO THE SYSTEM. AIR BUBBLES IN THE SYSTEM WILL NEGATIVELY EFFECT YOUR RESULTS*** 7. Clean the syringe attached to injection valve by drawing ultra pure water into it then, discarding the water into the sink. Do this 3 times. 8. Compare your set-up to the provided diagram (figure-3) to ensure your set-up is correct. Preparation of Standards and Unknown Solutions: 9. Add 2mL of TISAB II into a clean 50mL beaker 10. To the 2mL of TISAB II, add 2mL of a standard F- Solution. Do this for each F- standard and both of your unknowns. Procedure: 1. Stop the pump by pressing Stop. Remove the tubing from the water and place it in the 250 ml beaker with TISAB II. Ensure that tubing is submerged in solution. Cover with parafilm. 2. Restart pump as before. ***REMEMBER TO ENSURE THAT THE BEAKER WITH TISAB IN IT DOES NOT RUN DRY!!! IF IT BEGINS TO GET LOW ON SOLUTION, SIMPLY ADD MORE TISAB INTO BEAKER AS SYSTEM CONTINUES TO RUN*** 3. On the laptop in the MicroLab program, click on Start in Data Source/Variables box. Watch on the graph as a baseline is being established. (Allow graph to continue to establish baseline while you finish cleaning) ***At this point, if the electrode area is leaking or if the graph is not slowly leveling out, then the o-ring may not be securely in place. Ask you instructor for assistance. 4. Fill syringe with 3mL ultra pure water. Reattach to injection valve. 5. The injection valve has a metal dial on the back that functions as a Load/Inject valve. Make sure the valve is in Load mode. (It is marked with an L for Load and I for Inject on the back of valve) 6. Load all 3mL of the ultra pure water (from the syringe) into the valve by pushing in the syringe. (This step of loading the valve with ultra pure water is only to wash out the valve.) Measurement of Standards 1. Fill Syringe with 3mL of lowest concentration standard. Reattach to injection valve. 2. Click on Stop on graph. Restart as above. Allow baseline to establish for seconds before proceeding. **Then allow it to run for the entirety of the experiment. Do not click on Stop again until all standards and unknowns have been run.** 3. With the valve in Load mode, push 1mL of the solution into the valve.

8 4. Turn dial to Inject mode (Right side down, left side up). This will inject the standard into the system. You will see a small negative peak on the graph as the electrode measures the concentration of F- ([F-]). 5. Record the time of injection as you inject (from the spreadsheet below the graph) and what Fluoride standard concentration [F-] is being injected on your data sheet. 6. Once the graph begins to go back up, you can close valve by turning it to Load. 7. Repeat this process 3 times for each standard. You will inject each standard 3 times to test for precision. You may load (not inject!) the next ml of standard once the graph begins to go back up. However, wait until graph is all the way back up to the baseline before injecting again. 8. Follow the same directions in steps 1-6 above (WITHOUT STOPPING GRAPH) until all 4 standards (0.4ppm, 1ppm, 1.4 ppm, and 2.0ppm) have been measured and recorded. Remember to record times and the concentration you are injecting on your data sheet. 9. After running the final standard, wash out syringe by filling with ultra pure water then expelling the water. Do this 3 times. 10. Wash injection valve again with ultra pure water by filling syringe with ultra pure water and loading all 3mL into injection valve. (DO NOT INJECT, ONLY LOAD) Measuring Unknown Samples: 1. After washing syringe and valve with ultra pure water, you will now test your unknown samples. 2. Repeat the injection/recording steps above as done for your standards with the exception of only one injection per unknown instead of three. 3. Once done gathering data on the first unknown sample, wash injection valve and syringe again with ultra pure water and then continue with next sample. 4. When final unknown sample measurement is recorded, click on Stop in Data Source/ Variables in MicroLab program. Clean up: 1. Wash injection valve again with ultra pure water as previously done. 2. Remove the tubing from the TISAB II beaker and put it in your beaker with the ultra pure water. This will wash out the system. Let it run while you continue clean up. 3. On your graph in MicroLab, use Select Domain to isolate each triplet of your graph. Take a screen shot of zoomed in graph using Snipping Tool, which is found in the START Menu. 4. Save your graphs to the desktop, remembering to name each based on it s concentration [F-] or unknown #. 5. You may the graph to yourself and your partners or print your graph at the printer at the front of the class. (Ensure that Fit Picture to Frame is not checked!) 6. Turn off Pump by pushing the stop button. 7. Clean beakers and return to cart or drawers. 8. Wipe down station Report: 1. Use a ruler to measure the peaks on your graph in mm. Start from the base of the peak and measure to the lowest point of peak. Find the average peak height in mm for each standard concentration and of the unknowns.

9 2. Next, find the ratio of Millivolts to Millimeters (mv/mm) by measuring the distance in mm between mv on the y-axis of the graph. For example, if from 150mV to 170mV measures 20mm, then the ratio is 20mV/20mm. Once you have the ratio, convert your standards avg. peak heights and unknown peak heights from mm to mv. For example, if my average peak height is 20 mm and I use the ratio above of mm/mv, then (20mm)(20mV/20mm) = 20mV. 3. Generate a calibration curve in Excel of peak height in mv (y axis) versus log [F - ] for the standards (x axis). Use this graph to determine the line equation and the fluoride ion concentration [F-] of the unknown samples. You may use either the line equation or graphical interpolation. Line equation should be in the following format: y = m log x + b. Relate your line equation to your variables as follows: mv = m log [F-] + b. (Go to for a tutorial if needed.) 4. Write results/discussion paragraph. State results and discuss what your results mean. If results were not as expected or there are errors, give possible reasons why.

10 DATA TABLES Concentration of Standard (ppm) Injection Time (s) #1 Injection Time (s) #2 Injection Time (s) #3 Concentration of Unknown (ppm) Standard #1 Standard #2 Standard #3 Standard #4 Standard #5 Unknown #1 Unknown #2 Concentration (ppm) Peak Height (mm) #1 Peak Height (mm) #2 Peak Height (mm) #3 Average Peak Height (mm) Millivolts (mv)/ Millimeter (mm) Ratio / Millivolts (mv) Log [F-] / / / / Unknown #1 / Unknown #2 / Average Peak Height Calculations:

11 Millimeter (mm)/millivolt (mv) Ratio Calculations: Line Equation Calculations: (mv = m log [F-] + b) Log [F-] Calculations: Discussion

12 Pre-lab - Chemistry 1225 Experiment #11 Name 1. What is the Nernst equation for the Fluoride Ion Selective Electrode (ISE)? How does this show that measured potential is proportional to Fluoride ion concentration? 2. What is a common cause of error when using a reference electrode? 3. Is the ph of solutions important when using the Fluoride ISE? If so, why? 4. What species of Fluoride will we be detecting in today s lab? What reagent will be used to ensure detection of Total Fluoride?