CHEMISTRY 220. CYCLIC VOLTAMMETRY EXPERIMENT
OBJECTIVE
(1) Introduction to the technique of cyclic voltammetry (CV)
(2) Examination of the redox chemistry of the ferricyanide/ferrocyanide redox couple
[Fe(CN)6]3-/4- by CV
(3) Study the dependence of the anodic and cathodic peak currents on the scan rate, and
concentration of analyte.
(4) Experimentally determine the diffusion coefficients of the reduced and oxidized
forms of the analyte, using the CV data
(5) Examine the redox mechanism of acetaminophen at a Carbon electrode
EXPERIMENTAL
REAGENTS:
250 ml of 10 mM K3Fe(CN)6 in 0.1 M Sr(NO3)2 (this solution should be stored in the
dark when not in use)
250 ml 20 mM K3Fe(CN)6 in 0.1 M Sr(NO3)2 (leave it in the dark)
250 ml of 10 mM K4Fe(CN)6 in 0.1 M Sr(NO3)2
250 ml of 20 mM K4Fe(CN)6 in 0.1 M Sr(NO3)2
0.0700 M acetaminophen in 0.05 M sulfuric acid
Reference Electrode: Ag/AgCl or SCE
Auxiliary Electrode: Pt wire
Working Electrode: Pt (for part I), Glassy Carbon (for part II)
Argon gas tank
Glass Cells with cover
PROCEDURE:
Part 1. Ferri/Ferrocyanide redox couple.
1. Turn on the Potentiostat and the recorder.
2. Prepare the following concentrations of K3Fe(CN)6 and K4Fe(CN)6 from the provided
solutions by serial dilution: 2 mM, 5 mM, 10 mM, 15 mM, and 20 mM.
3. A Pt wire will be used as an Auxiliary electrode in your studies, clean it by
immersing it in 6 M nitric acid for 5 seconds, rinse it with double distilled water.
4. Ag/AgCl or SCE will be used as reference electrode, when not in use these are stored
in 3 M KCl. Rinse the electrodes with double distilled water.
5. A Pt electrode will be used as a working electrode for this part. Polish the surface of
the Pt on the provided cloth using Alumina Slurry (1 m and 0.05 m size particles).
You should polish in circular motion for 2-3 minutes, then rinse with double distilled
water.
6. Fill up the cell with solutions of the above concentrations, place the working,
auxiliary, and reference electrodes in the designated holes on the cover of the cell.
Place the gas tube in its designated hole, and insert it inside the solution.
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7. To drive away the Oxygen, open the gas tank, and gently bubble Ar (or N2) for ~10
minutes inside the solution, then remove the gas inlet tube from the solution, but
continue to bubble the gas gently above the solution, so a blanket of inert gas will
remain above the solution. After you stop bubbling the gas, press the cover of the cell
air-tight.
8. Connect the three electrodes to the potentiostat. The proper connection cable on the
potentiostat are labeled as REF = reference electrode, CE = auxiliary (also called
counterelectrode), and K1 = working electrode.
9. Be sure to remove any bubbles at the surface of the working electrode by gently
tapping the body of the electrode.
10. Your instructor will now show you the dials on the potentiostat panel and how to use
them to specify the scan rate, the scan limits, and the scan direction.
11. Set your scan limits on the potentiostat to scan between 600 mV and –150 mV.
Think of the direction in which you should be scanning the potential for both forms of
the redox couple.
12. Make sure before you scan to press the dial on the potentiostat from dummy to
normal electrodes.
13. Experiment with your potentiostat and recorder with the recorder pen up to specify
the X and Y scales on the recorder (X = current, and Y = voltage), without messing
up your chart paper. Make sure you write down the specific dial settings used since
you will need these to make a scale on your chart paper.
14. Scan the potential at the following scan rates for each of the 10 mM concentration:
50, 75, 100, 125, 150, 175, and 200 mV/s. Record the cyclic voltammogram for each
of the above scan rates (on the same or on separate sheets of paper).
15. Obtain a CV for each of the following concentrations 2 mM, 5 mM, 10 mM, 15 mM,
and 20 mM.
16. Clean the cell and the electrodes thoroughly, rinse them with double distilled water,
and dry them, before filling the cell with a new solution.
Part II. Acetaminophen
1. Use the same setup as before, except to replace the Pt working electrode with the GC
working electrode, after polishing its surface as specified above.
2. Place the Acetaminophen solution in the electrochemical cell.
3. Set the scan limits between 1000 and –200 mV. Scan the potential at 500 mV/s and
at 40 mV/s; and other scan rates you feel are necessary to help you in your discussion
of the redox mechanism of acetaminophen.
Clean the cell and rinse the electrodes. Place the reference electrode in the storage
solution. Turn off the gas tank.
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DATA ANALYSIS
1. Using the CVs, determine the separation between the peaks. Briefly discuss the
reversibility of this system, both chemical and electrochemical reversibility.
2. Create a table of the anodic and cathodic peak currents, obtained at each scan rate and
for each concentration studied.
3. Plot the anodic and cathodic peak currents vs the square root of the scan rate (V/s)1/2.
Using the slope of the plots, and the Randles-Sevčik equation, determine the values
for the diffusion coefficients for both forms of the redox couple. You can do so by
estimating the area of the electrode (you can also measure it using another couple of
known diffusion coefficient).
4. Plot the anodic and cathodic peak currents vs. the analyte concentration. Discuss the
use of CV as an analytical technique to determine concentrations of analyte species.
5. Discuss the effect of increasing the scan rate on the shape of the CV, both on peak
separation and peak height. Why does the peak current increase with scan rate?
6. The oxidation of acetaminophen is a pH-dependent, 2-electron process, which yields
N-acetyl-p-quinoneimine. Subsequent pH-dependent reactions of this reactive
species generate ultimately the hydroquinone/benzoquinone redox couple. Based on
this mechanism (given in class), interpret each peak in the CVs of acetaminophen
obtained at scan rates of 500 and 40 mV/s. Indicate which compounds are involved
in each redox process, and explain differences caused by changing the scan rate.
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