Cyclic Voltammetry: an ec Mechanism

Cyclic voltammetry (CV) is a very nice diagnostic method to determine the mechanism and rate of chemical reaction (c) that may follow an electron transfer (e) step. The ec mechanism is illustrated below for the oxidation of species R to form species Ox, and Ox undergoing a chemical reaction to form product P:

If the R/Ox electrode reaction is reversible, that is the heterogeneous electron transfer step is fast, and k_f = 0 so that the follow-up chemical reaction does not occur, the cyclic voltammogram shown in Figure 1A, below, is observed.

The anodic peak current, I_pa, and cathodic peak current, I_pc, are equal in magnitude with the transport of species R and Ox in the solution to and from the electrode controlled only by diffusion. We are assuming that the electron transfer reaction occurs at a planar (flat) electrode immersed in a quiet, unstirred solution. The electrode potential, E_0.85, that is the potential found at 85% up the CV wave to I_pa (or E_pa), is equal to the reversible potential, E⁰. In this example, the E⁰= + 0.25 V so that the oxidative wave is seen in the potential range of the forward scan, going from 0.0 V to 0.5 V. The Ox species is reduced back to R during the reverse scan from 0.5 V back to the initial potential of 0.0 V.

If k_f of the c step is finite so that Ox is being removed to P, there will be less Ox remaining near the electrode so that the magnitude of I_pc diminishes. The time window to capture Ox is determined by the scan rate. The consequence of an ec mechanism, where k_f >> k_b is illustrated in curve B of Figure 1. The parameters used in computer simulations of the theoretical CV waves, shown in curves A and B in Figure 1, are listed under the captions.

If Ox is long-lived, an I_pc appears even at slow scan rates. If Ox is very short-lived, it may not be seen even at very fast scan rates. CV is unique for being able to adjust the time window of observations by the scan rate. The two Cypress Systems' instruments, the 66-EI400 and 66-CS1200 are capable of scanning at very high rates so that k_f values as high as
10⁵ - 10⁶ l/m/s can be determined.

The current ratio, I_rev/I_fwd, can be conveniently measured by the empirical method of Nicholson [3, 4], that requires the evaluation of I_pa, I_pc, and I_λ, where I_λ designates the switching potential, as illustrated in Figure 2. These quantities are then used in equation 3 to obtain the current ratio.

The ratio of the current is used to calculate the apparent rate constant, k_f, for the follow-up chemical reaction, Ox -> P from the theoretical working curve.

We can apply this CV scheme to determine the ec oxidative characteristics of the catecholamines, dopamine and norepinephrine, whose structures are shown below. These compounds are known to be neurotransmitters - involved in the chemical transmission of nerve impulses in the mammalian brain, and are often monitored in vivo electrochemically with carbon microfiber electrodes.

The catecholamines are easily oxidized to the corresponding open-chain quinone, as illustrated below for dopamine. Following the intial oxidation, a pH dependent, 1,4-addition reaction can occur, forming a cyclization product called a leucochrome. At low pH values, the open-chain quinones are protonated to a great extent, and the cyclization reaction is unfavorable. At higher pH values, a sufficient amount of unprotonated quinone is available so that cyclization is observed.

1. 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!
2. Place about 10 ml of this solution into an electrochemical cell and deoxygenate for ~ 10 minutes with nitrogen or argon.
3. 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 (e.g. trace a figure 8) on a polishing pad which 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. {See discussion by Dr. David Weiss about activation of glassy carbon electrodes.}
4. Record cyclic voltammograms for this solution at scan rates of 50 mV/s 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.
5. 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.
6. Degas the solution and repolish the electrode as previously instructed (see #3 above).
7. 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.
8. 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.
9. Degas the solution and repolish the electrode. Record CV scans 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.
   

Table 1

Theoretical Values for the Ratio of Reverse to Forward Peak Currents for Charge Transfer Followed by an Irreversible Chemical Reaction

Treatment of Data

1. Use the Nicholson equation to calculate the value of i_rev/i_fwd 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.
2. You will need to determine the time, t, in seconds that it takes to scan the potential from the E _½ value to the switching potential, E_λ, 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 E _½ value is the potential at _½ the peak current.
3. The i_rev/i_fwd values as a function of the theoretically calculated k_ct values, as determined by Nicholson, are listed in Table 1. Plot the (i_rev/i_fwd) vs log(k_ct) to make a working curve. Interpolate the points to obtain a smooth curve.
4. Expand the appropriate region of the working curve corresponding to each experimental value of (i_rev/i_fwd) and measure the value of log(k_ct) for each of your current ratio from the working curve. Next, use the experimentally determined value for t at each of your scan rates to calculate a value for k_c. Calculate the average value of k_c for DA and NE.
5. The literature values for k_c are 0.038 s^-1 and 0.36 s^-1 for DA and NE, respectively.¹

How close are your values to those in the literature?

1. M.D. Hawley, S.V. Tatawawadi, S. Pierkarski, R. N. Adams, J. Am. Chem. Soc., 89 (1967) 447-450.
2. A. W. Sternson, R. McCreery, B. Feinberg, R. N. Adams, J. Electroanal. Chem., 46 (1973) 313-321.
3. R. S. Nicholson, Anal. Chem., 38 (1966) 1406.
4. R. S. Nicholson, I. Shain, Anal. Chem., 36 (1964) 706-723.
5. W. E. Geiger, "Instructional Examples of Electrode Mechanisms in Transition Metal Complexes," in Laboratory Techniques in Electroanalytical Chemistry, 2nd Ed., Editors P. T. Kissinger and W. R. Heineman; Marcel-Dekker, NY 1996, pp. 683-717.
   

Postscripts:
* 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.

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