1.What is cyclic voltammetry, and why is it so commonly used?
In a cyclic voltammetry experiment, as in other controlled potential experiments, a potential is applied to the system, and the faradaic current response is measured (a faradaic current is the current due to a redox reaction). The current response over a range of potentials (a potential window) is measured, starting at an initial value and varying the potential in a linear manner up to a pre-defined limiting value. At this potential (often referred to as a switching potential), the direction of the potential scan is reversed, and the same potential window is scanned in the opposite direction (hence the term cyclic). This means that, for example, species formed by oxidation on the first (forward) scan can be reduced on the second (reverse) scan. This technique is commonly used, since it provides an fast and simple method for initial characterization of a redox-active system. In addition to providing an estimate of the redox potential, it can also provide information about the rate of electron transfer between the electrode and the analyte, and the stability of the analyte in the electrolyzed oxidation states (e.g., do they undergo any chemical reactions).
2. What is potentiostatic control of an electrode?
A control voltage is fed into the control input CI. This control voltage forces a current through the counter electrode exactly as high as to achieve the wanted potential difference between working electrode and reference electrode. The control voltage may be produced by the internal potential control source of the potentiostat, or by an external signal generator, e.g. a ramp generator or a sine wave generator. While for a constant potential the internal potential control source is sufficient, time-dependent signals are to be fed from external sources (except our scanning potentiostats that also have built-in voltage scanners).
3.Why are the peaks in a cyclic voltammogram asymmetric (tailed)? The asymmetry is due to the diffusion of analyte from the bulk solution to the electrode surface during the experiment. In experiment where the analyte is attached (adsorbed) to the electrode surface, the peaks are symmetric. Actually, even for monolayer modified electrodes the peaks are not symmetrical, unless the system is completely reversible.
4. Why can cyclic voltammograms of the same system look inverted when done by different groups?
This is due to the existence of different conventions for plotting current and potential. For any voltammogram, current is plotted along the y-axis, and potential along the x-axis. In one convention (called classical or polarographic), potentials become more negative along the (positive) x-axis, and reduction (cathodic) currents are positive. In the other convention (IUPAC), the opposite is applies. If you don't want problems with your papers, you have to use the IUPAC convention: anodic current is positive and positive potentials are right-hand plotted.
5. What are the characteristic parameters of a cyclic voltammogram?
The parameters of a cyclic voltammogram are the peak potential and the peak current (the peak width at half the peak current is also sometimes measured). Since there are two peaks associated with a redox reaction (one on the forward scan and one on the reverse scan), it is standard practice to quote the difference of the peak potentials and the ratio of the peak currents.
6. How can I measure a redox potential from a cyclic voltammogram?
For a reversible system, it is common to quote the mean of the peak potentials as the redox potential. If the diffusion coefficients of the oxidized and reduced species are similar, this is a good approximation to the formal redox potential. If the system is irreversible, the approximation is not valid, particularly if there is only one peak.
7. What is meant by reversible?
For a redox reaction to be reversible, the concentrations of the oxidized and reduced species at the electrode surface must be maintained at the values required by the Nernst equation. In practical terms, a redox reaction is reversible if the rate of electron transfer is fast relative to the scan rate (the rate of change of potential) and if the oxidized and reduced species are stable on the experimental time scale (i.e., they do not undergo any significant chemical reactions).
8. How can we tell from a cyclic voltammogram whether the system is reversible or irreversible?
One advantage of cyclic voltammetry is that it is relatively easy to see from a cyclic voltammogram whether a system is reversible. The peak potential difference for a reversible system is about 60 - 70 mV (the theoretical value is about 58–59 mV, depending on the temperature and the switching potential), and the peak currents are equal. In addition, the peak potential difference does not change with scan rate. If the electron transfer kinetics are slow, the peak potential separation is larger, and increases with increasing scan rate. It should be noted that this behavior is also characteristic of uncompensated solution resistance; the two causes can be distinguished by running cyclic voltammograms at different analyte concentrations. If the redox reaction is coupled to chemical reactions, both the peak currents and peak potentials can be affected. For example, if there is a chemical reaction following the electron transfer reaction, the peak current on the reverse scan is decreased.
9. Why are different scan rates used?
The reversibility of a system (i.e., the effect of electron transfer kinetics and coupled chemical reactions) depends upon the time scale of the experiment, which is determined by the scan rate. Therefore, for example, increasing the scan rate can decrease the effect of a coupled chemical reaction, since the reaction has less time to occur. Reversibility can also vary with temperature.
10. Can I obtain electron transfer and chemical reaction rate constants directly from a cyclic voltammogram?
This is possible for very simple redox mechanisms. However, for more complicated mechanisms, it is generally not possible to separate the effects of different variables. For such systems, simulation software (e.g., DigiSim) has to be used, where the user proposes a mechanism, and the PC calculates what the cyclic voltammogram should look like.
11. Why my cyclic voltammogram particularly at a slow potential scan rate has some oscillations?
This is not actually noise, but raise instability of the cell/potentiostate system. Such instability results from several reasons related to your reference electrode (mainly from too high resistance of the reference). The links below describe some methods of troubleshooting for this problem. If you find these explanations too complicated, just try to decrease resistance between the working and reference electrodes. It should help.
12. Why does the peak current increase with increasing the scan rate? The current is directly proportional to the rate of electrolysis at the electrode surface. Electrolysis occurs at the electrode surface in response to a change in potential in order to try and maintain the surface concentrations of the oxidized and reduced species at the values required by the Nernst equation. Therefore, the faster the rate of change of potential (i.e., the scan rate), the faster the rate of electrolysis, and hence the larger the current.
13. How is the redox potential related to the HOMO and LUMO frontier orbitals?
In a reduction process, an electron is added to the LUMO (Lowest Unoccupied Molecular Orbital), and whereas in an oxidation process, an electron is removed from the HOMO (Highest Occupied Molecule Orbital). Therefore, there should be a correlation between changes in the frontier orbitals and redox potentials due to changes in substitutents.
14. What is charging current, and why is it so important?
The interface between the electrolyte and the working electrode acts like a capacitor. Therefore, a current is required to change the potential applied to the working electrode, and this is referred to as the charging current. Since the potential in a cyclic voltammetry experiment is constantly changing, there is an (approximately) constant charging current throughout the experiment that makes a major contribution to the background current. This charging current restricts the detection limit for cyclic voltammetry to about 10-5 M, which limits its value as a technique for quantitative concentration measurements. In addition, the charging current directly proportional to the scan rate, whereas the peak current is proportional to the square root of the scan rate. Therefore, the charging current also limits the maximum scan rate that can be used.
15. How can charging current be minimized?
Charging current is minimized using a potential step; that is, the potential is stepped from an initial value to a new value, and it is held at this new value for a set time period. There is a charging current spike following the initial potential step; however, since there is no further change, the charging current decays exponentially. Since the faradaic current decays more slowly (e.g., with 1/(square root of time) for a diffusion-controlled process), the charging current can be eliminated by measuring the current after the charging current has decayed to zero. This is the basis of pulse and square wave techniques. Phase-selective and second harmonic phase-selective AC voltammetry can also be used to decrease or eliminate charging current.
16. How do pulse techniques and potential step techniques (e.g., chronoamperometry and chroncoulometry) differ?
They differ in the size of the potential step. For chronoamperometry and chronocoulometry, the potential step can be up to several hundred mV; that is, the potential window of the experiment is defined by the potential step. For pulse (and square wave) techniques, the magnitude of the potential step (or potential pulse) is much smaller (typically 25 – 50 mV). These potential pulses are combined with a linear or staircase wave form in order to scan the potential range. Different pulse technique use different combinations of linear and pulse wave forms, and different current sampling protocols.
17. What detection limits are possible with pulse or square wave techniques?
Different pulse techniques decrease the charging current to different extents, and hence the detection limits differ. The lowest detection limits (10-7–10-8 M) are obtained using differential pulse voltammetry and Osteryoung square wave voltammetry, and therefore these techniques are most commonly used for quantitative concentration measurements (using either a calibration curve or the method of standard additions).
18.What are stripping techniques?
In a stripping experiment, the analyte is preconcentrated in the working electrode or on the surface of the working electrode. Therefore, the concentration of the analyte is higher at the working electrode than in the bulk solution. This concentration is measured by electrolyzing the analyte (stripping) from the electrode. Provided that the relationship between the electrode concentration and the solution concentration is constant, stripping techniques can be routinely used for quantitative measurement down to low parts-per-billion (ppb) concentrations.
19. What is electrochemical impedance spectroscopy (EIS)?
In a controlled-potential EIS experiment, the system is held at equilibrium at a fixed DC potential, and a small amplitude (5–10 mV) AC wave form is superimposed on the DC potential. The response of the system to this perturbation from equilibrium is measured in terms of the impedance Z of the system. The frequency of the AC wave form is varied, and hence the impedance of the system is obtained as a function of frequency.
20. What is imaginary impedance? Since an AC potential is applied to the cell, there will probably be a phase shift between the applied AC potential and the AC current response. Therefore, the impedance can be represented using a vector diagram displaying the in-phase (Z') and out-of-phase (Z") impedances, the total impedance, and the phase angle. Since complex number terminology is also used when analyzing impedance spectra, the in-phase and out-of-phase impedances are often referred to as real and imaginary impedances.
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