Cyclic Voltammetry An Example of Voltaic Methods March/2007, Prof. S. Shippy and M-J Lu Objective This experiment is designed to acquaint the student with the techniques of cyclic voltammetry for the study of redox systems. The "computer age" has provided us the opportunity to control the experiment and process the results with very little effort. What is meant by control? How does this apply to electrochemical analysis? How does this apply to chemical analysis? Answers to some of the questions are provided by the BAS- 100 Electrochemical analyzer, a sophisticated instrument which provides both diverse control and data reduction functions for the study of redox phenomena. Theory Cyclic Voltammetry (CV) is perhaps the most effective and versatile electroanalytical technique available for the mechanistic study of redox systems. It enables the electrode potential to be rapidly scanned in search of redox couples. Once located, a couple can then be characterized from the potentials of peaks on the cyclic voltammogram and from changes caused by variation of the scan rate. CV is often the first experiment performed in an electrochemical study. Figure 1. Typical excitation signal for cyclic voltammetry. The repetitive triangular potential excitation signal for CV (Figure 1) causes the potential of the working electrode to sweep back and forth between two designated values (the switching potentials). To obtain a cyclic voltammogram, the current at the working electrode is measured during the potential scan (Figure 2). Figure 2. Cyclic voltammogram of Fe2+ in 1M H2SO4. During the scan +250 to +750 mV, the applied potential becomes sufficiently positive at 400 mV to cause oxidation of Fe2+ to occur at the electrode surface. This oxidation is accompanied by anodic current, which increases rapidly until the surface concentration of Fe2+ approaches zero, as signaled by peaking of the current at point c in Figure 2. The current then decays (after c) as the solution surrounding the electrode is depleted of Fe2+ due to its oxidation to Fe3+. This depletion of Fe2+ and accumulation of Fe3+ near the electrode is depicted by concentration-distance profiles a-e as shown in Figure 2. The magnitude of the current is related to the slope of the c-x profile for Fe2+, as described by the following equation: where: It = Current at time t, (Amperes). n = Number of electrons, eq/mole. F = Faraday's constant, 96,485 e/eq. A = Electrode area, cm2. C = Concentration of oxidized species, mol/cm3. (not mol/L!) Do = Diffusion coefficient of oxidized species, cm2/s. t = Time (s). X = Distance from the electrode (cm). The product Do (∂Co/∂X) at x = 0,t is the flux or the number of moles of oxidized species diffusing per unit time to unit area of the electrode in units of mol/cm2s. During the positive scan in which Fe2+ is oxidized to Fe3+, the depletion of Fe2+ in the vicinity of the electrode is accompanied by an accumulation of Fe3+. This can be seen by the concentration distance profiles at various potentials in the shown figure. After the direction of the potential scan is switched at 750 mV to a negative scan, oxidation continues (as is evident by the anodic current and the C-X profile, (e), as seen in Figure 2, until the applied potential becomes sufficiently negative to cause reduction of the accumulated Fe3+. Reduction of Fe3+ is signaled by the appearance of cathodic current. Once again, the current increases as the potential becomes increasingly negative until all of the Fe3+ near the electrode is reduced. When the concentration of Fe3+ is significantly depleted, the current peaks, and then decreases. See f, g, and h in Figure 2. Thus the physical phenomena that caused a current peak during an oxidation cycle also cause a current peak during the reduction cycle. This can be seen by comparing the concentration-distance profiles for the two scans. Simply stated, in the forward scan Fe3+ is electrochemically generated, as indicated by the anodic current. In the reverse scan, this Fe3+ is reduced back to Fe2+, as indicated by the cathodic current. Thus, CV is capable of rapidly generating a new species during the forward scan and then probingits fate on the reverse scan. The important parameters of cyclic voltammetry are the magnitude of the peak currents, Ipa and Ipc, and the potentials at which peaks occur, Epa and Epc. Difficulty inobtaining accurate peak currents is perhaps the biggest liability of CV. A redox couple in which both species rapidly exchange electrons with the working electrode is termed an electrochemically reversible couple. The following equation applies to a system that is both electrochemically and chemically reversible: ΔEp = Epa - Epc ~ 0.059/n (at 25ΕC) where n = number of electrons transferred. The values of Ipa and Ipc are similar in magnitude for a reversible couple with no kinetic complications. In most CV experiments there is little advantage to be gained by carrying on the potential scan for more than two to three cycles (Note: The first voltammogram is not quite the same as the reproducible curve obtained after several cycles.) Apparatus Instrument for CV such as BAS 100B Electrochemical cell Platinum auxiliary electrode (red wire) Ag/AgCl reference electrode (white wire) Carbon plaster working electrode (black wire) Pt working electrode (black wire) Au working electrode (black wire) N2 tank with regulator Rubber Gloves: For good results you must wear rubber gloves when handling the electrodes. The electrodes are extremely sensitive to contamination from any source and especially from your fingers. Operation of the BAS 100B I. Start-Up Procedure 1. Turn the Isobar surge suppressor located on top of the BAS 100B system unit. 2. Turn on the BAS 100B with the red power switch located on the back of the system unit. 3. Turn on the cell stand power with black power switch on the back of it 4. Open the N2 gas tank located on the left hand side corner. 5. Turn on the Gateway computer with the power switch on the front panel. 6. Turn on the monitor. 7. If it is not already on, turn on the laser printer located by the balance with its power switch located on the left side of the printer. 8. Log into Windows NT after the computer has finished booting: This is done by entering CTRL+ALT+DEL when prompted to do so. The system will then display a login box. The user name is chem 421 and there is no password so just press ENTER or click OK 9. If this is your first time using this computer create a sub-directory to store your data inside the directory for your section before you open the BAS 100B program. 10. To launch the BAS 100B program :double click on BAS 100 W icon, after BAS logo is on screen, press any key or click on the graphic image to continue the work. II. Establishing a directory The first time you use the system you must create a directory to store all of your data. 1. Click on my computer and go to C drive. Find myfiles folder and click on. 2. Create your own folder under chem421 folder. 3. Close the windows. After each scan be sure to save your data to your folder. III. Setting the parameters 1. On the toolbar, located on the top of the BAS window, click on Method 2. Choose Select Mode 3. The next screen that should appear is the 'Mode of Operation.' On this screen, the category highlighted should be Sweep Techniques and the techniq ue that should be highlighted is Cyclic Voltammetry. Then click OK 4. The next screen should be 'CV General Parameters.' This is where you want to enter the appropriate parameters for each run, then click OK 5. To change the parameters for each run, click on method, which is located on the toolbar at the top. 6. Choose General Parameters, then the 'CV General Parameters' screen will appear and you are able to make changes to the parameters for each scan. IV. Running a scan: 1. Before a scan can be completed, the solution in the cell should be stirred and purged before each run. 2. After the solution is stirred and purged, for about five minutes, then pull the nitrogen gas line out of the solution, so that the nitrogen gas is blowing over the solution, but not producing any bubbles on the surface of the solution. This prevents oxygen from reentering the solution. 3. To run a scan, press the F2 key, or click on Control, which is on the toolbar at the top, then choose, start run. 4. After the scan is complete, save the data to your file by clicking on the following in this order: File (which is on the toolbar) Save Data C Drive MYFILE chem421 click on your own folder give the file a name Click ok 5. After your data is saved, then print your data, by pressing F8 6. If you are prompted by a window that does not allow you to print, do the following: Click on Print (which is on the toolbar) Choose Print Setup Reselect the same printer Click Ok Then your data should be sent to the printer By pressing F8, you can print out additional copies If you still encounter printer problems, notify the TA. The above procedure might need to be done each time you want to print a new voltammogram. 7. Once your data is printed, then you are able to go onto the next scan. V. Shut down procedure 1. Go to File and then exit from the BAS 100W software. 2. Click on Start, in the lower left hand corner, then click on Shut Down, make sure that Shut down the computer is highlighted and then click OK. 3. Wait for the computer to tell you that it is ok to turn off the computer before proceeding any further. 4. Once the computer screen reads that it is now safe to shut off the computer, press the power button on the Gateway Tower 5. Turn off the monitor 6. Turn off the power to the BAS 100B electrochemical analyzer, by pressing the red POWER button on the side of the BAS 100B. 7. Turn off the power to the surge suppressor, which is located on top of the BAS 100B Electrochemical analyzer. 8. Turn off the printer, if and only if all the other groups are also done printing their data. VI. Hints: .. Do not stir any of the solutions when running a cyclic voltammogram. But, always stir the solution for at least 10 seconds before scanning. .. Do not fill the cell any higher than 1 cm below the Teflon cover. .. Make sure that the working electrode and the reference electrode are approximately at the same height. .. Be sure that the surface of the working electrode is clean. It may be polished briefly after cleaning by rubbing on a clean sheet of unprinted paper. Part I Dissolved Oxygen, Potential Limits & Surface Effects When investigating a chemical system for the first time with CV, there are several experimental conditions that need to be established. You need to choose the appropriate solvent, electrode, and supporting electrolyte, and determining whether O2 is an interferent. This effort is justified because it demonstrates how to design an efficient CV experiment for almost any chemical system of interest. Reagents 25 ml of 1.0 M H2SO4 500 ml of 0.1 M Phosphate Buffer (pH = 7.0) KH2PO4 and K2HPO4 .. Hint for buffer preparation: Buffer preparation is based on Henderson-Hasselbalch equation: HA .. H+ + A- pH = pka + log ([A-]/[HA]) where pKa for KH2PO4 is 6.86 Procedure A three electrodes system is used in this section: 1. Au electrode/Pt electrode/Carbon electrode (black wire) 2. Pt auxiliary electrode (red wire) 3. Ag/AgCl reference electrode (white wire) 1. Start with using the gold working electrode, a cyclic voltammogram of 0.1 M phosphate buffer with initial scan limits of 700 mV and -1000 mV is run. This is initiated at 400 MV in the negative direction with a scan rate 50 mv/s. 2. Before running a cyclic voltammogram, be sure that there are not any bubbles on the bottom of the electrodes. Also, never stir the solution when running a cyclic voltammogram. Fill the cell with 12-13 ml of solution. Do not overfill the cell! 3. Run the cyclic voltammogram. After viewing the results adjust the scan limits so that the current at the ends of the scan do not exceed 10 μA. 4. Stir the solution for 10 seconds. Rescan the solution. When you obtain satisfactory results. Print the graph and store your data in your sub-directory on disk. 5. After finishing gold working electrode, switch to platinum electrode, run a cyclic voltammogram of the same solution and repeat step 2~4. 6. Switch to carbon electrode, run a cyclic voltammogram of the same solution and repeat step 2~4. 7. After the first three cyclic voltammograms are acquired, deoxygenate the solution for ten minutes by bubbling nitrogen gas through it. After approximately ten minutes, raise the nitrogen tubes out of the solution so that the nitrogen gas is blowing over the solution. This will prevent oxygen from reentering the solution. 8. Now, acquire a cyclic voltammogram of the deoxygenated solution using the carbon electrode. 9. Replace the carbon electrode with the platinum electrode. Bubble nitrogen gas through the solution for another five minutes. Now, acquire a cyclic voltammogram of the deoxygenated solution using the platinum electrode. 10. Replace the platinum electrode with the gold electrode. Deoxygenate the solution for five minutes. 11. Now, acquire a cyclic voltammogram of the deoxygenated solution using the gold electrode 12. The 7th and last voltammogram to be acquired is the Gold electrode in deoxygenated H2SO4.Replace the solution with 1.0 M H2SO4. Deoxygenate the solution for ten minutes. Now, acquire a cyclic voltammogram of the deoxygenated solution using the gold electrode Questions 1. Discuss any interference of O2 apparent from series of voltammograms. 2. What considerations for the CV experiment requires choosing the appropriate solvent, supporting electrolyte, and working electrode? 3. Why isn't the solution stirred when running a cyclic voltammogram? 4. Based on your results, what are the useful potential ranges for each of the four degassed systems studied here? 5. Why is phosphate buffer used as a background electrolyte? List the characteristics of a good background electrolyte. Part II The electroactive of Dopamine Electrochemical method had been used to analyze electroactive materials for years. The aim of this experiment is to use cyclic voltammatry to determine the electroactivity of dopamine (DA). The measurement involves studying the current flow from and to the dissolved analyte. The oxidation reaction of dopamine is in the following: Based on previous work from Part I of this laboratory, you continue the experiment by using an appropriate solvent, electrode and chemical environment to measure the electroactivity of DA by CV. Reagents .. 100 ml stock solution of 50 mM 4-(2-aminoethyl)benzene-1,2-diol (Dopamine, DA), prepared in 0.1 M phosphate buffer (pH=7.0) .. Prepare 25 ml each of 5, 10, 15, 20 and 25 mM DA solutions in 0.1 M phosphate buffer from stock solution. .. Prepare an unknown DA solution Procedure A three electrode system is used for this section. 4. Pt working electrode/area = 2.5 mm2, (black wire) 5. Pt auxiliary electrode (red wire) 6. Ag/AgCl reference electrode (white wire) 1. The top of the electrode holder has four holes to accommodate the three electrodes and a tube for deoxygenating by bubbling with a stream of N2. 2. The cell is assembled and filled with about 12-13 ml of 0.1M phosphate buffer. The system is deoxygenated by purging with N2 for 10 min. Following deoxygenation, N2 is allowed to flow over the solution to prevent O2 from reentering the cell for the remainder of the experiment. 3. While the system is being deoxygenated, the scan parameters can be set. The working electrode (black wire) should be disconnected during this procedure. The initial potential is set at 400 mV and the scan limits are based on the optimized condition you found in part 1 by using Pt working electrode. All scans are initiated in the negative direction with a scan rate of 20 mV/s. Use the default value for the sensitivity. 4. After allowing the current to attain a constant value (quiet time = 10 s), the potential scan is initiated and a background CV of the supporting electrolyte solution is obtained. 5. After turning off the working electrode, the cell is cleaned and refilled with 15 mM DA prepared in 0.1 M phosphate buffer. Following the same procedure as above, a CV of the DA/DA ortho-quinone is obtained. The effect of the sweep rate (n) on the voltammograms is observed by using this same solution and recording CV's at the following rates: 20, 50, 75, 100, 125, 150, 175, and 200 mV/s. 6. Obtaining scans of 5, 10, 15, 20 and 25 mM DA using a sweep rate of 20 mV/s to construct calibration curve. 7. Run cyclic voltammatry for unknown DA. Note: .. Make sure that there are no bubbles on the electrodes, and do not stir the solution when running the cyclic voltammogram.) Calculations: 1. Determine the diffusion coefficient of the system using Ipa and Ipc for each concentration at 20 mV/s. Ip = (2.69 x 105)・n 3/2 ・A・Do . ・C・v 1/2 Where: n = Number of electrons transferred A = Area of the electrode Do = Diffusion coefficient C = Concentration of the solution (mol/cm3) v = Scan rate (V/s). 2. Plot Ipa and Ipc vs n 1/2 (sweep rate mV/min) 1/2 for your 15 mM data. Obtain the slope of these plots and calculate the diffusion coefficient 3. Calculate the mean and standard deviation in the diffusion coefficient. How does this compare to typical diffusion coefficients? 4. What is the evidence indicating the reversibility of the system? 5. Show that Ipa and Ipc are directly proportional to concentration by plotting a graph of current vs. concentration at 20 mV/s. Does this graph demonstrate this concept well? 6. Determine the concentration of the unknown, if you were asked to perform an unknown. 7. Why is the Platinum electrode used instead of the carbon or gold electrodes? Part III Effect of Coupled Chemical Reactions Acetaminophen (APAP), the active ingredient in Tylenol, is commonly used as an aspirin substitute. This redox system is useful in demonstrating the mechanistic information that can be obtained from CV's. Reagents McIlvaine buffer: (table 1) 500 ml of pH 2.2 McIlvaine buffer (ionic strength = 0.5) 200 ml of pH 6.0 McIlvaine buffer (ionic strength = 0.5) 200 ml of 1.8 M H2SO4 0.070 M APAP stock solution is prepared in 0.05 M perchloric acid Unknowns: Tylenol tablet Procedure 1. Using the stock solution, 25 ml of an APAP solution is prepared in each of the three supporting electrolyte solutions. The concentration of these APAP solutions should be approximately 3 mM. 2. For the purpose of establishing a calibration curve, four additional APAP solutions in pH 2.2 buffer are needed (range from 0.1 mM to 5.0 mM); 25 ml of each are needed. An unknown is prepared from a solid sample by dissolving a tablet in 250 ml of pH 2.2 buffer. A workable concentration is obtained by diluting a 5 ml aliquot of this solution. If a liquid aspirin substitute is given, a suitable solution can be obtained by direct dilution. Table I. Preparation of Constant Ionic Strength McIlvaine Buffered Solutions Desired pH at 25EC Na2HPO4・12H2O (g/l) Citric acid, H3C6H5O7・H2O (g/l) Ionic strength KCl (g/l) to bring to 0.5 M ionic strength 2.2 1.43 20.6 0.0108 37.2 2.4 4.44 19.7 0.0245 35.4 2.6 7.8 18.7 0.0410 34.2 2.8 11.35 17.7 0.0592 32.9 3.0 14.7 16.7 0.0771 31.4 3.2 17.7 15.8 0.0934 30.3 3.4 20.4 15.0 0.112 28.9 3.6 21.5 14.2 0.128 27.6 3.8 25.4 13.6 0.142 26.7 4.0 27.6 12.9 0.157 25.5 4.2 29.7 12.3 0.173 24.4 4.4 31.6 11.7 0.190 23.1 4.6 33.4 11.2 0.210 21.6 4.8 35.3 10.7 0.232 19.9 5.0 36.9 10.2 0.256 18.2 5.2 38.4 9.75 0.278 16.5 5.4 40.0 9.29 0.302 14.8 5.6 41.5 8.72 0.321 13.3 5.8 43.3 8.32 0.336 12.2 6.0 45.2 7.74 0.344 11.6 Using 3 mM APAP solution in pH 2.2 buffer, carbon electrode, scan limits are established at 1000 mV and .200 mV. Scans are initiated in the positive direction at 0 mV. Cyclic voltammograms of the 3 mM APAP solution in each of the three buffers are then obtained at a scan rate of 40 mV/s and 250 mV/s. The solutions should be stirred between each run. Obtain voltammograms of four additional APAP solutions in pH 2.2 buffer and unknown solutions at a scan rate of 50 mV/s. Calculations: 1. Give a reasonable explanation to the following oxidation mechanism of APAP, and how it applies to this experiment. 2. How do you explain the large separation between the anodic and cathodic peak currents in the pH = 6 cyclic voltammograms? 3. Construct a calibration curve by plotting peak current vs concentration of APAP for the standard solutions of APAP. 4. From you calibration curve, determine the concentration of APAP in the diluted unknown solutions that you analyzed. 5. Calculate the weight of APAP in the unknown tablet and/or the concentration in the liquid unknown. Do your results agree with the expected value? 6. Explain why faster sweep rates are necessary to study electrochemical reaction mechanisms where the oxidized and/or the reduced species can participate in slower side reactions. 7. What problems can you anticipate encountering for very fast scan rates (>100 v/s)? 8. Why the carbon electrode is used in part III instead of the gold or platinum electrodes?