Experiment
- Page ID
- 60925
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Equipment
- See the laboratory instructor about what potentiostat is available and the method of data acquisition (i.e., x-y recorder with analog potentiostat or printed output with a computerized potentiostat).
- Electrochemical cell and electrodes
- 1.0 or 3.0 mm diameter flat tipped glassy carbon electrode
- Pt auxiliary electrode
- Ag/AgCl reference electrode
- Small volume electrochemical cell
- Polishing kit
- Timer
Chemicals
- Dopamine [recommend the HCl salt of DA, F. W. 189.64]
- Norepinephrine [recommend the HCl salt of NE, F. W. 205.64]
- Na2HPO4 and citric acid for making McIlvaine buffer
- Sulfuric acid (1 M)
Procedure
- Prepare a solution of dopamine (DA) at a pH near 1.0 by adding approximately 10 mg of solid DA to a 50.00 ml volumetric flask and dissolving it in 1 M sulfuric acid. Record the actual mass of DA. Mix well by shaking the flask. Handle catecholamines with extreme care, as they can have severe physiological effects!
- Place about 10 ml of this solution into an electrochemical cell and deoxygenate for ~ 10 minutes with nitrogen or argon.
- Prepare a glassy carbon electrode (1 mM or 3 mM diameter) by polishing it for ~ 1 minute on 0.05 μm alumina. Use a gentle circular motion or trace a figure 8 on a polishing pad that has the alumina on the surface. Clean the electrode carefully with distilled water, sonicate for 10-15 seconds, and then touch the edge with a Kimwipe™ before introducing the electrode into the cell.
- Please ask laboratory instructor for the directions to set-up and operate the potentiostat.
- Record cyclic voltammograms for this solution at scan rates of 50 and 100 mV/s between an initial potential of 0.00 V and a positive potential limit of +1.00 V. Make duplicate runs at each scan rate. Measure the pH of the solution before discarding.
- Prepare a 1 mM solution of dopamine at ~ pH 7.0 by dissolving ~10 mg in a 50.00 ml volumetric flask. Record the actual mass of DA. Pipette 5.0 ml of 0.1 M citric acid, then dilute to mark with 0.2 M disodium phosphate. Together, these ingredients comprise the McIlvaine buffer.
- Degas the solution and repolish the electrode as previously instructed (see #3 above).
- Record cyclic voltammograms for dopamine in this pH 7.0 solution at scan rates of 50, 100, 150, 200, 250 and 300 mV/s, adjusting the x-axis (current) sensitivity scale as needed to record the entire I-t curves. Set the scan limits so that you start at -100 mV and scan the potential anodically to the limit of +700 mV, reverse back to -800 mV and end up at -100 mV [sequence of limits: -100, +700, -800 and stop at -100 mV]. Record the pH of the solution before discarding.
- Prepare a 1 mM solution of norepinephrine (NE) near pH 7.0 by dissolving ~11 mg in 50.00 ml of McIlvaine buffer. Record the actual mass of NE.
- Degas the solution and repolish the electrode. Record CV scans of NE at 50 mV/s and 400 mV/s with the same potential limits as in step #7. Run duplicates of the CV scans. Record the pH of this solution before discarding.
Calculations:
Table 1
Theoretical Values for the Ratio of Reverse to Forward Peak Currents for Charge Transfer Followed by an Irreversible Chemical Reaction
kft | irev/ ifwd |
---|---|
0.004 | 1.00 |
0.023
|
0.986 |
0.035 |
0.967
|
0.066 | 0.937 |
0.105
|
0.900
|
0.195 | 0.828 |
0.350 | 0.727 |
0.525 | 0.641 |
0.550 | 0.628 |
0.778 | 0.551 |
1.050 | 0.486 |
1.168 | 0.466 |
1.557 | 0.415 |
- Use the Nicholson equation to calculate the value of irev/ifwd from the CV scans for dopamine at pH 1.0 and at pH 7.0, recorded at the scan rates of 50, 100, 200 and 300 mV/s. Next, do the same calculations for the two CV scan rates with norepinephrine.
- You will need to determine the time, t, in seconds that it takes to scan the potential from the E½ value to the switching potential, Eλ, of each cyclic voltammogram. This value of t will be different for each of the scans (remember that the time it takes is dependent on the distance along the potential axis and the scan rate). The E½ value is the potential at ½ the peak current.
- Table 1 shows the irev/ifwd values as a function of the theoretically calculated kft values, as determined by Nicholson. Plot the (irev/ifwd) vs. log(kft) to make a working curve. Interpolate the points to obtain a smooth curve.
- Expand the appropriate region of the working curve corresponding to each experimental value of (irev/ifwd) and measure the value of log(kft) for each of your current ratios from the working curve. Next, use the experimentally determined value of t at each of your scan rates to calculate a value of kf. Calculate the average value of kf for DA and NE.
- The literature values for kf are 0.038 s-1 and 0.36 s-1 for DA and NE, respectively [ref. 8].
- How close are your values to those in the literature?
Postscript: The c step (cyclization) is first-order and irreversible for the oxidized product of both dopamine and norephenephrine. Other examples of this ec mechanism are the compounds of p-aminophenol and catechol – they undergo a 2-electron electrooxidation. The triol, produced by the c step in the case of catechol, is readily oxidized to the quinone form at potentials less positive than the parent.