Determine the concentration of fluoride in a surface water sample with an ion-selective electrode. The sensitivity...
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Determine the concentration of fluoride in a surface water sample with an ion-selective electrode. The sensitivity of the method is extended by the method of standard addition. Introduction The small size of the fluoride anion and its resulting mobility, even through solids, makes it a candidate for potentiometric methods using ion-selective electrodes. Fluoride readily permeates a thin slice of lanthanum fluoride crystal, which is typically used as the membrane in an ion selective electrode for fluoride. The only major direct interference is hydroxide, another small, mobile anion, which readily replaces fluoride in many of its minerals (the typical "fluoride treatment" of dental enamel depends upon the replacement of hydroxide ions by fluoride ions). Thus, it is important to keep the pH below 7, to keep the instrument from reading hydroxide as a false indication of fluoride. The pH should not be too low, however, since hydrofluoric acid is a weak electrolyte, in equilibrium with hydrogen and fluoride ions. Beyond causing a loss of fluoride ions by its formation, HF further interferes with F by formation of the hydrofluoride ion HF2. For high fluoride concentrations, where formation of HF is more serious than interference by OH, the pH should not be lower than 5. Typically, the pH useful for potentiometric fluoride analysis is between 5 and 7. The sensitivity of potentiometric electrodes to ionic strength is a further complication. The activity of an ion depends upon the total ionic strength of a solution. Concentration gradients of even spectator ions can produce large, uncontrolled potentials if they contribute significantly to the total ionic strength. Furthermore, polyvalent cations of silicon, aluminum, and iron will complex fluoride. All of these effects are mitigated by a Total Ionic Strength Adjustment Buffer (TISAB). This is usually a 5-5.5 pH citrate/acetate buffer with standard amounts of chloride, generally prepared from the free acids and the sodium salts (why are the salts necessary?). Additionally, a cation complexing agent such as CDTA (1,2-diaminecyclohexane-N,N,N',N'-tetracetic acid) prevents secondary interferences from silica, alumina and ferric ions (how?). Here, a standard "shot" of concentrated TISAB is added to each solution under test. Even with all of these controls, the concentration of fluoride cannot be read directly in terms of the potential across the cell. In addition to the membrane potential (which, in principle, follows the Nernst equation here), there are still several uncontrolled sources of potentials. The numerous concentration gradients set up their own "Donnan equilibrium potentials", while the possibility for small temperature gradients can produce "thermocouple" potentials. (If two different conducting materials are part of a circuit and experience different temperatures at opposite ends of the circuit, a potential will result! This is how thermocouples are used to probe temperature. The opposite is also true and is called the "thermoelectric effect".) Finally, the lanthanum fluoride membrane does not always act as an ideal fluoride-semipermeable membrane. For all these reasons, it is usually best to determine fluoride concentration potentiometrically by adding known amounts of fluoride to the sample being analyzed, the "Method of Standard Additions". We will analyze the data obtained by the "Method of Standard Additions", after appropriate linearization, using a least-squares fit. Fluoride Determination Since fluoride is readily absorbed by glass, after you make standard solutions in glass volumetric flasks, you should immediately transfer them to plastic containers for storage during the completion of the experiment (See Potentiometry1-3 for pictures of the glass- and plasticware and potentiometry setup needed for this experiment). All measurements will be completed using plastic beakers, which will be provided at the workstations. The potentiometers are high-impedance (greater than 100 megaohm input resistance) instruments, similar to those used for pH measurements. (Why does this high impedance help in these experiments?) You will want to read the values directly in millivolts. Potentiometry-2 Procedure Standards Record the temperature of the lab in your notebook. Bring a laptop, and plot your data as you are collecting it. (If you and/or your partner do not have a laptop, there are some computers in lab you can use, but by bringing your own, you won't have to transfer any files later.) Have an Excel spreadsheet ready with necessary formulas set up to save time. Show the plot for the Blank, QC, and Unknown water sample to your TA before finishing the experiment. This will ensure you have enough time to re-collect data if there is a major problem. Known Addition Solution Make up a "KNOWN ADD" solution to approximately 10 mg/L in fluoride. You will need about 100 mL of this solution, but the easiest way to do this is to add 221 mg NaF to 1 liter of water, making a 100 mg/L fluoride stock solution. Then dilute 10:1 by adding 10 mL of this solution to 10 mL of TISAB in a 100 mL volumetric flask, making sure to include any pendant drop from the pipette, and then add water to the mark (see Potentiometry6 and Potentiometry 7). Store in plastic containers. You will use this solution as your standard, adding known amounts to solutions to determine the voltage for the blank solution, the quality control (QC) solution, and the unknown solution. It is a good idea to measure and take note of the temperature of a small aliquot of the KNOWN ADD solution. Comparing Electrodes You will be supplied a commercial fluoride electrode. Be sure to record its serial number. You will be comparing your electrode to that of the other group, so note the serial number of theirs, as well. Start by measuring the potential of a standard solution in the range of 10-4 to 10-5 M fluoride by placing 50 mL of the solution into a plastic beaker along with a stirring magnet. (See Potentiometry8 and Potentiometry9 for pictures of the fluoride-selective electrode and its lanthanum fluoride membrane.) Lower the electrodes into the solution, and carefully start the magnetic stirrer, making sure not to damage the electrodes by contact with the spinning magnet. Turn off the stirrer momentarily so that vibrations in the stir plate and the rotating magnetic field do not cause large errors in the potential measured by the electrodes. Read the voltage in millivolts for both solutions (see Potentiometry10 and Potentiometry11). Notice that a significant amount of time (about a minute) is required for the voltage to equilibrate (why?). Repeat this measurement using the other group's commercial fluoride electrode and record all values and serial number of the electrode. Rinse off the electrodes with deionized water, taking care to blot and not harshly wipe the lanthanum fluoride membrane, before proceeding to avoid cross-contamination of your samples (see Potentiometry12). Potentiometry-3 External Standard Run Although you will be using the Method of Standard Additions to determine fluoride in the Unknown, it is a good idea to do an External Standards calibration starting with a blank to check that the potentiometer is working properly and that you understand how to plot the data to obtain a linear standardization curve. Make 100 mL of a "blank" by measuring out 10 mL of water and 10 mL of TISAB then adding 80 mL of water. (What type of blank is this?) Note the volume of this BLANK. Measure the potential between the fluoride electrode and your reference electrode dipped in after totals of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mL of KNOWN ADD solution have been added, milliliter by milliliter, to this BLANK solution (see Potentiometry13). Notice that the sample without any fluoride, containing zero KNOWN ADD, has a poorly defined potential. Allow plenty of time for the electrodes to reach equilibrium (~ 1 minute!). [Question 1: What should the voltage be ideally/hypothetically when the sample contains zero (i.e., only trace amounts of) fluoride in solution, if the solution inside the fluoride- selective electrode contains 0.1 M fluoride? Why do you think this value not actually achieved?] Quality Control Next do a QC run by making a 0.2 mg/L standard QC solution, mixing 1 mL of your 100 mg/L fluoride stock solution with water in a 50 mL volumetric flask, and then mixing 10 mL of this with 10 mL of TISAB in a 100 mL volumetric flask, adding water to the mark (a ~500:1 dilution overall). Make sure to note the volume of your QC. Record the voltage between the fluoride electrode and reference electrode after 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mL of KNOWN ADD have been added to exactly 100 mL of this QC solution, similarly to what you did in Part I. Method of Standard Additions for Unknown sample of surface water You will be given a sample of water from a body of water in Oregon. Measure out 90 mL of it, and add 10 mL of TISAB, noting the actual final volume. Determine the fluoride concentration in a manner similar to that used in determining the fluoride in the QC solution. Use a linear regression to report the concentration of fluoride in your water sample, as described below in "Data Linearization", and think about uncertainty contributions for your determination. Data Linearization Harvey, pp. 164-167, gives a very general demonstration of how to use the Method of Standard Additions to determine the concentration of analyte in an initial solution that works well Potentiometry-4 for many analytical techniques. For Potentiometry, we introduce a slight variation that is needed due to the exponential part of the Nernst equation, and you can view a more full explanation in the separate document "Potentiometry Help Sheet". In any case, plot (Vs + Vadd)e-EnF/RT vs. total volume of fluoride added for when 0, ..., 10 mL of KNOWN ADD was mixed with the QC solution. Here E is the cell potential, Vs is the QC solution volume, and Vadd is the total volume of the standard added to the sample when the voltage reading is taken. If the slope of the trend line through your data is negative, change the sign of the potential; a line through your data should have positive slope. [Question 2: Why does the equation above include both Vs and Vadd? What will be the result of the calculation if the term Vadd is missing? Be specific about issues that can occur with data if this quantity is not accounted for.] Fit a straight line through the BLANK data, and observe where it hits the x-axis - this should be at negative fluoride volume. (Again, your data should have a positive slope. Higher concentrations should correspond to higher values of e-F/RT. Otherwise, you may have the wrong sign convention on the potential E, in which case, change it!) The absolute value of the distance from this intercept on the x-axis to the origin is denoted |fluoride intercept] (i.e. absolute value of fluoride intercept). The concentration of fluoride in the BLANK should in principle be zero, but your intercept may not correspond to exactly zero fluoride concentration. (Why?) If it is not exactly zero, you should account for it for both your QC and unknown data (see below). Similarly, the concentration of fluoride in the QC is determined by calculating |fluoride intercept * Cstd/ Vs for the QC solution, and it should match the known concentration for the QC solution. Should the slope of your calibration curve for the QC match that of the BLANK? Do a similar analysis for your unknown by plotting (Vs + Vada)e- e-EnF/RT vs. total added fluoride volume with Vs now being the initial volume of the unknown (Part I). Use this method ("Standard Additions") to compute the concentration of fluoride in your original unknown. [Question 3: Once the concentration of the Unknown water sample is determined, plot E vs logio (total concentration of F) for each addition of Known Add. What is the slope? (The units of the slope are called "volts per decade of fluoride concentration"; do you see why?) What is its meaning? Do you get the same slope for your External Standards data?] [Question 4: Show the work-up of your QC and unknown data according to the "Data Linearization" section.] [Question 5: Fill out your Arrow Diagram with all significant sources of uncertainty that you can identify, justifying each, and show your calculation for the resulting total estimated uncertainty.] [Question 6: Give two reasons why the Method of Standard Additions is a good idea for the analysis of fluoride in surface water with a fluoride-selective electrode.] [Question 7: Are the potentials you measure for the 104 and 105 M fluoride solutions the same for the two reference probes (your fluoride electrode and the other team's electrode)? Determine the concentration of fluoride in a surface water sample with an ion-selective electrode. The sensitivity of the method is extended by the method of standard addition. Introduction The small size of the fluoride anion and its resulting mobility, even through solids, makes it a candidate for potentiometric methods using ion-selective electrodes. Fluoride readily permeates a thin slice of lanthanum fluoride crystal, which is typically used as the membrane in an ion selective electrode for fluoride. The only major direct interference is hydroxide, another small, mobile anion, which readily replaces fluoride in many of its minerals (the typical "fluoride treatment" of dental enamel depends upon the replacement of hydroxide ions by fluoride ions). Thus, it is important to keep the pH below 7, to keep the instrument from reading hydroxide as a false indication of fluoride. The pH should not be too low, however, since hydrofluoric acid is a weak electrolyte, in equilibrium with hydrogen and fluoride ions. Beyond causing a loss of fluoride ions by its formation, HF further interferes with F by formation of the hydrofluoride ion HF2. For high fluoride concentrations, where formation of HF is more serious than interference by OH, the pH should not be lower than 5. Typically, the pH useful for potentiometric fluoride analysis is between 5 and 7. The sensitivity of potentiometric electrodes to ionic strength is a further complication. The activity of an ion depends upon the total ionic strength of a solution. Concentration gradients of even spectator ions can produce large, uncontrolled potentials if they contribute significantly to the total ionic strength. Furthermore, polyvalent cations of silicon, aluminum, and iron will complex fluoride. All of these effects are mitigated by a Total Ionic Strength Adjustment Buffer (TISAB). This is usually a 5-5.5 pH citrate/acetate buffer with standard amounts of chloride, generally prepared from the free acids and the sodium salts (why are the salts necessary?). Additionally, a cation complexing agent such as CDTA (1,2-diaminecyclohexane-N,N,N',N'-tetracetic acid) prevents secondary interferences from silica, alumina and ferric ions (how?). Here, a standard "shot" of concentrated TISAB is added to each solution under test. Even with all of these controls, the concentration of fluoride cannot be read directly in terms of the potential across the cell. In addition to the membrane potential (which, in principle, follows the Nernst equation here), there are still several uncontrolled sources of potentials. The numerous concentration gradients set up their own "Donnan equilibrium potentials", while the possibility for small temperature gradients can produce "thermocouple" potentials. (If two different conducting materials are part of a circuit and experience different temperatures at opposite ends of the circuit, a potential will result! This is how thermocouples are used to probe temperature. The opposite is also true and is called the "thermoelectric effect".) Finally, the lanthanum fluoride membrane does not always act as an ideal fluoride-semipermeable membrane. For all these reasons, it is usually best to determine fluoride concentration potentiometrically by adding known amounts of fluoride to the sample being analyzed, the "Method of Standard Additions". We will analyze the data obtained by the "Method of Standard Additions", after appropriate linearization, using a least-squares fit. Fluoride Determination Since fluoride is readily absorbed by glass, after you make standard solutions in glass volumetric flasks, you should immediately transfer them to plastic containers for storage during the completion of the experiment (See Potentiometry1-3 for pictures of the glass- and plasticware and potentiometry setup needed for this experiment). All measurements will be completed using plastic beakers, which will be provided at the workstations. The potentiometers are high-impedance (greater than 100 megaohm input resistance) instruments, similar to those used for pH measurements. (Why does this high impedance help in these experiments?) You will want to read the values directly in millivolts. Potentiometry-2 Procedure Standards Record the temperature of the lab in your notebook. Bring a laptop, and plot your data as you are collecting it. (If you and/or your partner do not have a laptop, there are some computers in lab you can use, but by bringing your own, you won't have to transfer any files later.) Have an Excel spreadsheet ready with necessary formulas set up to save time. Show the plot for the Blank, QC, and Unknown water sample to your TA before finishing the experiment. This will ensure you have enough time to re-collect data if there is a major problem. Known Addition Solution Make up a "KNOWN ADD" solution to approximately 10 mg/L in fluoride. You will need about 100 mL of this solution, but the easiest way to do this is to add 221 mg NaF to 1 liter of water, making a 100 mg/L fluoride stock solution. Then dilute 10:1 by adding 10 mL of this solution to 10 mL of TISAB in a 100 mL volumetric flask, making sure to include any pendant drop from the pipette, and then add water to the mark (see Potentiometry6 and Potentiometry 7). Store in plastic containers. You will use this solution as your standard, adding known amounts to solutions to determine the voltage for the blank solution, the quality control (QC) solution, and the unknown solution. It is a good idea to measure and take note of the temperature of a small aliquot of the KNOWN ADD solution. Comparing Electrodes You will be supplied a commercial fluoride electrode. Be sure to record its serial number. You will be comparing your electrode to that of the other group, so note the serial number of theirs, as well. Start by measuring the potential of a standard solution in the range of 10-4 to 10-5 M fluoride by placing 50 mL of the solution into a plastic beaker along with a stirring magnet. (See Potentiometry8 and Potentiometry9 for pictures of the fluoride-selective electrode and its lanthanum fluoride membrane.) Lower the electrodes into the solution, and carefully start the magnetic stirrer, making sure not to damage the electrodes by contact with the spinning magnet. Turn off the stirrer momentarily so that vibrations in the stir plate and the rotating magnetic field do not cause large errors in the potential measured by the electrodes. Read the voltage in millivolts for both solutions (see Potentiometry10 and Potentiometry11). Notice that a significant amount of time (about a minute) is required for the voltage to equilibrate (why?). Repeat this measurement using the other group's commercial fluoride electrode and record all values and serial number of the electrode. Rinse off the electrodes with deionized water, taking care to blot and not harshly wipe the lanthanum fluoride membrane, before proceeding to avoid cross-contamination of your samples (see Potentiometry12). Potentiometry-3 External Standard Run Although you will be using the Method of Standard Additions to determine fluoride in the Unknown, it is a good idea to do an External Standards calibration starting with a blank to check that the potentiometer is working properly and that you understand how to plot the data to obtain a linear standardization curve. Make 100 mL of a "blank" by measuring out 10 mL of water and 10 mL of TISAB then adding 80 mL of water. (What type of blank is this?) Note the volume of this BLANK. Measure the potential between the fluoride electrode and your reference electrode dipped in after totals of 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mL of KNOWN ADD solution have been added, milliliter by milliliter, to this BLANK solution (see Potentiometry13). Notice that the sample without any fluoride, containing zero KNOWN ADD, has a poorly defined potential. Allow plenty of time for the electrodes to reach equilibrium (~ 1 minute!). [Question 1: What should the voltage be ideally/hypothetically when the sample contains zero (i.e., only trace amounts of) fluoride in solution, if the solution inside the fluoride- selective electrode contains 0.1 M fluoride? Why do you think this value not actually achieved?] Quality Control Next do a QC run by making a 0.2 mg/L standard QC solution, mixing 1 mL of your 100 mg/L fluoride stock solution with water in a 50 mL volumetric flask, and then mixing 10 mL of this with 10 mL of TISAB in a 100 mL volumetric flask, adding water to the mark (a ~500:1 dilution overall). Make sure to note the volume of your QC. Record the voltage between the fluoride electrode and reference electrode after 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 mL of KNOWN ADD have been added to exactly 100 mL of this QC solution, similarly to what you did in Part I. Method of Standard Additions for Unknown sample of surface water You will be given a sample of water from a body of water in Oregon. Measure out 90 mL of it, and add 10 mL of TISAB, noting the actual final volume. Determine the fluoride concentration in a manner similar to that used in determining the fluoride in the QC solution. Use a linear regression to report the concentration of fluoride in your water sample, as described below in "Data Linearization", and think about uncertainty contributions for your determination. Data Linearization Harvey, pp. 164-167, gives a very general demonstration of how to use the Method of Standard Additions to determine the concentration of analyte in an initial solution that works well Potentiometry-4 for many analytical techniques. For Potentiometry, we introduce a slight variation that is needed due to the exponential part of the Nernst equation, and you can view a more full explanation in the separate document "Potentiometry Help Sheet". In any case, plot (Vs + Vadd)e-EnF/RT vs. total volume of fluoride added for when 0, ..., 10 mL of KNOWN ADD was mixed with the QC solution. Here E is the cell potential, Vs is the QC solution volume, and Vadd is the total volume of the standard added to the sample when the voltage reading is taken. If the slope of the trend line through your data is negative, change the sign of the potential; a line through your data should have positive slope. [Question 2: Why does the equation above include both Vs and Vadd? What will be the result of the calculation if the term Vadd is missing? Be specific about issues that can occur with data if this quantity is not accounted for.] Fit a straight line through the BLANK data, and observe where it hits the x-axis - this should be at negative fluoride volume. (Again, your data should have a positive slope. Higher concentrations should correspond to higher values of e-F/RT. Otherwise, you may have the wrong sign convention on the potential E, in which case, change it!) The absolute value of the distance from this intercept on the x-axis to the origin is denoted |fluoride intercept] (i.e. absolute value of fluoride intercept). The concentration of fluoride in the BLANK should in principle be zero, but your intercept may not correspond to exactly zero fluoride concentration. (Why?) If it is not exactly zero, you should account for it for both your QC and unknown data (see below). Similarly, the concentration of fluoride in the QC is determined by calculating |fluoride intercept * Cstd/ Vs for the QC solution, and it should match the known concentration for the QC solution. Should the slope of your calibration curve for the QC match that of the BLANK? Do a similar analysis for your unknown by plotting (Vs + Vada)e- e-EnF/RT vs. total added fluoride volume with Vs now being the initial volume of the unknown (Part I). Use this method ("Standard Additions") to compute the concentration of fluoride in your original unknown. [Question 3: Once the concentration of the Unknown water sample is determined, plot E vs logio (total concentration of F) for each addition of Known Add. What is the slope? (The units of the slope are called "volts per decade of fluoride concentration"; do you see why?) What is its meaning? Do you get the same slope for your External Standards data?] [Question 4: Show the work-up of your QC and unknown data according to the "Data Linearization" section.] [Question 5: Fill out your Arrow Diagram with all significant sources of uncertainty that you can identify, justifying each, and show your calculation for the resulting total estimated uncertainty.] [Question 6: Give two reasons why the Method of Standard Additions is a good idea for the analysis of fluoride in surface water with a fluoride-selective electrode.] [Question 7: Are the potentials you measure for the 104 and 105 M fluoride solutions the same for the two reference probes (your fluoride electrode and the other team's electrode)?
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