Cyclic Voltammetry

Electrochemistry Laboratory Information
Voltammetry

This document contains a list of tasks to complete, after which you should be fairly comfortable with the common electrochemical technique, cyclic voltammetry.

Cyclic Voltammetry:

One of the workhorses of electrochemical measurements is cyclic voltammetry, an electrochemical technique that is capable of providing a wealth of information about an electrochemical system. Information related to analyte concentration, electrode reaction kinetics, and diffusional contributions is all contained in a cyclic voltammogram. You will use the BAS 100 electrochemical system to complete this experiment.

For an introduction into cyclic voltammetry read: “Cyclic Voltammetry: The Electrochemical Equivalent of Spectroscopy” available from Dr. Lamp.

NOTE: all experiments described in this document should use a platinum wire auxiliary electrode and silver/silver chloride reference electrode.

General BAS 100 Operation:

1. To start the program, click on the BAS Icon on the screen.
2. In order to run a cyclic voltammetry experiment, click on the Method menu and choose Select Mode a dialog box should appear, in this box, choose 1. Sweep Techniques and Cyclic Voltammetry and then click OK.
3. To prepare the instrument for a specific experiment it will be necessary to adjust the potential window, scan rate, etc. by clicking on the Method menu and choosingGeneral Parameters. Doing this brings up the dialog box shown below. Once all values have been set, click on OK to continue.
4. You can now initialize a run by clicking Control Start Run or by pressing F2.

I. Electrode Preparation:

The response of a species at an electrode surface is strongly dependent on how the electrode has been prepared prior to running the experiment. Typically, electrodes are polished and rinsed before the start of the experiment.

Solution(s) Needed: 10 mM Potassium Ferricyanide (K₃Fe(CN)₆) in 0.10 M Potassium Chloride (it is easiest to prepare the KCl solution first). Prepare some extra 0.1 M KCl (~200 mL) for use in the rest of the experiment.

1. Find a BAS gold electrode in the Electrodes drawer. This will be your workingelectrode.
2. Immerse the working, reference, and auxiliary electrodes in a small vial and add a few mL of your KCl solution, without ferricyanide (you only need enough solution to cover the ends of the electrode (~1/2 inch in the bottom of the vial)).
3. Connect the leads of the BAS 100 as follows: BLACK to Working Electrode, RED to Auxiliary Electrode (platinum wire) and WHITE to Reference Electrode (silver/silver chloride)
4. Perform a cyclic voltammetry experiment using the following conditions: (This will be a Background scan containing no electroactive material)

   Initial E	800 mV	Initial Direction	Negative
   High E	800 mV	Number of Segments	2
   Low E	-100 mV	Sensitivity	10 uA/v
   Scan Rate	100 mV/s

    
   

   NOTE: You may need to adjust the sensitivity throughout the course of the experiment to maximize S/N for each voltammogram.

5. Save the CV using the File – Save command.
6. Disconnect and remove the electrodes from the cell, and rinse them with deionized water.
7. Immerse the working, reference, and auxiliary electrodes in a small vial and add a few mL of your ferricyanide solution.
8. Perform a cyclic voltammetry experiment using the conditions in step 4. Following completion of the CV, disconnect and remove the electrodes and rinse them.
9. Polishing procedure:

1. Affix three small pieces of polishing felt to a glass plate.
2. Deposit a pea-sized drop of 1-micron polishing compound on one of the pieces of felt.
3. Polish the gold electrode by applying moderate pressure while moving the electrode in a figure-eight pattern on the felt. Polish for about 30 seconds.
4. Rinse the electrode well with distilled-deionized water.
5. Repeat steps b-d using 0.30 micron, then 0.05 micron polishing compound, rinsing well after each step.

10. Rinse the electrode again and return it to the KCl solution and collect a Backgroundscan.
11. Transfer the electrodes to the ferricyanide solution and collect a CV using the conditions in step 4.
12. Disconnect and remove the gold electrode and immerse it in a dilute solution of Decanethiol for ~5 minutes (do this in the hood, it stinks!). After 5 minutes remove the electrode, and rinse it well with ethanol and water.
13. Collect a Background CV using the KCl solution.
14. Collect a CV in the ferricyanide solution
15. Repeat the entire polishing series using a Glassy Carbon working electrode instead of gold.

What changes in do you notice in the CV’s collected following various electrode treatments?

Suggest a reason for the differences between the polished, unpolished, and thiol-treated electrodes.

 

II. Scan Rate Dependence

The response observed during a voltammetry experiment depends strongly on the rate that material approaches the electrode surface. This task will introduce you to this dependence.

Solution Needed: 10 mM Potassium Ferricyanide (K₃Fe(CN)₆) in 0.10 M Potassium Chloride

1. Polish a BAS gold electrode as described in step 7 of the Electrode Pretreatmenttask.
2. Immerse the working, reference, and auxiliary electrodes in a small vial and add a few mL of your ferricyanide solution.
3. Connect the leads of the BAS 100 as follows: BLACK to Working Electrode, RED to Auxiliary Electrode (platinum wire) and WHITE to Reference Electrode (silver/silver chloride)
4. Prepare to perform a cyclic voltammetry experiment using the following conditions:
   
   

   Initial E	800 mV	Initial Direction	Negative
   High E	800 mV	Number of Segments	2
   Low E	-100 mV	Sensitivity	10 uA/V

    

5. Run a series of CV’s at scan rates ranging from 10 mV/s to 500 mV/s, save each CV using the File – Save command. NOTE: the Sensitivity may need to be adjusted as the scan rate changes in order to observe a reasonable CV.
6. Determine the peak potential and peak current for each peak on each CV by clicking on Analysis -Results Graph or by pressing F6. Do you observe any trends in the data? Make plots of peak current for the anodic and cathodic peaks as a function of scan rate and as a function of square root of scan rate. Which plots are linear? Which plots should be linear?

 

III. Concentration Dependence

The concentration of electroactive species present in a solution also plays a major role in determining the response observed in a voltammetric experiment.

Solutions Needed: Solutions of 1, 2, 5, and 10 mM Potassium Ferricyanide (K₃Fe(CN)₆) in 0.10 M Potassium Chloride. These can be prepared by first making the 10 mM solution and diluting aliquots of this solution with 0.10 M KCl to make the appropriate concentrations. You will also need to bring your instructor your 10 mM ferricyanide solution, your 0.1 M KCl solution, and a clean 25 mL flask to be used for an unknown. 

1. Polish a BAS gold electrode as described in step 7 of the Electrode Pretreatmenttask.
2. Immerse the working, reference, and auxiliary electrodes in a small vial and add a few mL of your ferricyanide solution.
3. Connect the leads of the BAS 100 as follows: BLACK to Working Electrode, RED to Auxiliary Electrode (platinum wire) and WHITE to Reference Electrode (silver/silver chloride)
4. Prepare to perform a cyclic voltammetry experiment using the following conditions:
    

   Initial E	800 mV	Initial Direction	Negative
   High E	800 mV	Number of Segments	2
   Low E	-100 mV	Sensitivity	10 uA/v
   Scan Rate	100 mV/s

    

5. Run series CV for each of the four concentrations you prepared, as well as your unknown. Save each CV using the File – Save command.
6. Determine the peak potential and peak current for each peak on each CV by clicking on Analysis -Results Graph or by pressing F6. Do you observe any trends in the data? Plot anodic and cathodic peak current as a function of concentration. Is the response what you would expect? Based on your results, determine the ferricyanide concentration in your unknown.

Based on your observation in the above experiments; is the ferri/ferrocyanide couple electrochemically reversible? Chemically reversible? Are we operating under diffusion controlled conditions?