In your analytical chemistry courses, you have used electrochemistry in a quantitative manner to determine the concentration of analytes. In inorganic chemistry, electrochemistry is used mostly in a qualitative fashion. In quantitative electrochemistry, the current is the important quantity measured, indicating the concentration. In qualititative electrochemistry, the potential of a redox process and its reversibility are the important factors.
Potential
Two electrochemical processes will be discussed; oxidation (loss of electrons from the analyte molecule) and reduction (addition of electrons to the analyte molecule). On the scale used by the potentiostat, the more positive the potential, the more oxidizing the electrode becomes. Therefore, a molecule that oxidizes at very posititve potentials is very difficult to oxidize. Likewise, a molecule that is reduced at very negative potenitals is very difficult to reduce.
The potential measured by the potentiostat is versus a reference electrode. This is usually a Ag/AgCl or Ag/AgNO3 . The potential of the redox couple of the reference electrode is read as 0 V (the signifigance is that the location of 0 V depends on the type of reference electrode).
Oxidations can take place at negative potentials as well as positive potential (remember that negative and positive are just negative and positive compared to the reference electrode). The importance is the relative potential. In table 1, the relationship between the potential measured for a test compound is listed in relation to that of a reference compound. As you can see, if a reduction occurs at a more positive potential than the reduction in the reference compound, than the test compound is easier to reduce.
Table1. If a redox process of the test compound occurs at
| More Positive Potential than the Reference Compound | More Negative Potential. than the Reference Compound |
| more difficult to oxidize | easier to oxidize |
| easier to reduce | more difficult to reduce |
Cyclic Voltammetry
In cyclic voltammetry, the potential is varied in a cyclical manner as shown in figure 1. In this example, the potential is changed continuously from 0 V to -1 V and then back to 0 V. This constitutes one cycle. Several cycles can be run. During the cycle, the current flow through the sysem is monitored.
Two important pieces of information that can be gained from a cyclic voltammogram are the potential and the reversibility of the electrochemical process. The oxidation of ferrocene is shown in figure 2 as an example. As the potential is swept to positive potential, the ferrocene molecules near the electrode are oxidized to ferricinium (the negative current). On the return sweep to negative potentials, these ferricinium molecules are reduced back to ferrocene (the positive current). By comparing the current for the oxidation (i pa in figure 2) and reduction portion (i pc in figure 2), it is found that they are equal an the oxidation of ferrocene is a chemically reversible process . This means that the ferrocinium produced is stable enough to exist in solution until it was rereduced by the electrode and therefore the two currents are equal. The potential of this process is reported as the half-wave potential, E1/2. The E1/2 is calculated as the average of the two peak maxima E pa and Epc.
In a chemically irreversible process, the species produced (by oxidation or reduction) is not stable enough to survive until the reverse sweep of the electrode. An example of a chemically irreversible oxidation is shown in figure 3. Notice there is an oxidation maximum, Epa , but there is no corresponding reduction process on the return scan. That is because the oxidized product has decomposed and there is nothing to reduce. No E1/2 can be calculated for such a process, the Epa is reported instead.
Relation Between Redox Potentials and Frontier Orbitals
The frontier orbitals are the highest occupied molecular orbital (HOMO) which contains the highest energy electrons, the lowest unoccupied molecular orbital (LUMO), and orbitals close to the two previously mentioned orbitals. When electrons are added or removed from the molecule, they will be added to the LUMO and removed from the HOMO. Because of this, you might think that there should be a connection between the HOMO energy and the first oxidation potential of a molecule and the LUMO energy and the first reduction potential.
The electrodes used in electrochemistry are composed of metals, or metal-like materials (conductors of electricity). In these materials, there are not distinct orbitals, but continuous bands of energy that the electrons can occupy. Like the HOMO in a molecule, there is a Fermi level in a metal. All the energy levels below the Fermi level are occupied at 0 K. When you apply a potential to the metal, you change the value of the Fermi level (E F) (Figure 4). When a negative potential is applied, the Fermi level moves to higher energy, when a positivie potential is applied, it moves to a lower energy.
In Figure 5, a molecule in solution has been added to this picture. If the Fermi level of the electrode is made low enough in energy, it will become favorable for electrons to transfer from the molecule's HOMO to the electrode (oxidation). Likewise, if the Fermi level is made high enough, it will become favorable for electrons to be transferred from the electrode to the LUMO of the molecule (reduction). If the HOMO or LUMO energies are changed, this will change the potential of the first oxidation and reduction potentials of the molecule.
Reporting Electrochemical Data
When reporting electrochemical data, the conditions used in the experiment as well as the data itself need to be reported. Things that need to be reported include the solvent, the identity and concentration of the electrolyte, the working, auxilliary, and reference electrode types, the scan rate, and the potential of the electrochemical process (E1/2 , Epa, or Epc ).
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