What is voltammetry?
Why is there a current response to the applied potential?
How are the energies of Fermi level and the frontier orbitals determined?
Do all molecules have a measurable redox potential?
What equipment is required for voltammetry experiments?
Is the solution stirred?
Do I need to deoxygenate the solution?
In a voltammetric experiment, a potential is applied to a system (e.g., a transition metal complex in solution) using two electrodes (a working electrode and a reference electrode), and the current response is measured using the working electrode and a third electrode, the auxiliary electrode.
The current arises from transfer of electrons between the energy levels of the working electrode and the molecular energy levels of the system under study. This current is often referred to as the faradaic current. Transfer of electrons from filled electrode orbitals to vacant molecular orbitals is referred as reduction, whereas transfer of electrons from filled molecular orbitals to vacant electrode orbitals is referred to as oxidation. Whether oxidation or reduction can occur depends upon the relative energies of the Fermi level of the electrode (i.e., the energy of the highest occupied electrode orbital) and the frontier molecular orbitals; for example, reduction can occur if the Fermi level is higher than the lowest unoccupied molecular orbital, whereas oxidation requires that the Fermi level is lower than the highest occupied molecular orbital.
The Fermi level is determined by the potential applied to the electrode; that is, varying the applied potential changes the oxidizing/reducing ability of the electrode. For example, more negative potentials increase the reducing ability of the electrode. In contrast, the energies of the molecular frontier orbitals are determined by the molecular structure and can be considered to be constant. Therefore, a common approach in voltammetry experiments is to vary the applied potential, and to record the potential at which a current response is detected; that is, the energy at which oxidation or reduction occurs. The redox potential is a measure of this energy.
Although all molecules do have frontier orbitals, in practice these are not always accessible in a voltammetry experiment. Molecules for which a redox potential can be measured are referred to as electrochemically active. Examples of electrochemically active molecules include organic molecules with extended p-systems (e.g., aromatic molecules) and transition metal complexes. It should also be noted that some systems have the ability to undergo more than one oxidation or reduction, and hence have more than one redox potential.
First, a potentiostat is required for controlling the applied potential, and a current-to-voltage converter is required for measuring the current. These are both contained within the epsilon. A user interface is required to define the way the potential is applied - the potential waveform. There are a number of different potential waveforms, and these are referred to by characteristic names; for example, cyclic voltammetry, and differential pulse voltammetry. These different potential waveforms (or techniques) are discussed in more detail in the appropriate section. The epsilon must be connected to the electrochemical cell. This contains the three electrodes immersed in an electrolyte solution of the molecule.
Stirring the solution has a significant effect on the current response, since it affects the rate at which electroactive molecules are brought from the bulk solution to the electrode surface (this process is referred to as mass transport). In many voltammetry experiments, there is no stirring, and the only form of mass transport is diffusion (this gives rise to the tailed peak shape observed in cyclic voltammetry). These are referred to as stationary solution techniques. In other experiments, the solution is stirred, either by a stir bar or a rotating electrode (the latter is preferable, due to the more precise control of the rate of rotation). These are referred to as hydrodynamic techniques.
Oxygen is electroactive, and can be reduced quite easily. Therefore, it must be removed from the solution if the system under study is reducible. Oxygen is typically removed by bubbling an inert gas (e.g., nitrogen or argon) through the solution for about 10 minutes. If a stationary solution experiment is to be performed, it is important that the stirring is stopped and the solution is allowed to become quiescent before the experiment is started (although a blanketing layer of inert gas over the solution can be maintained during the experiment.