Voltammetry

Voltammetry is a category of electroanalytical methods used in chemistry and many industrial processes. It is a dynamic electrochemical method because the applied potential varies over time. Scientists use it to study how much of a specific substance, called an analyte, is present in a solution. This is done by measuring the electrical current that results from changing the electrical push, or potential. The final data is presented as a voltammogram. This is a graph that plots the measured current against the applied potential of the working electrode.

To perform these experiments, scientists use an electrochemical cell. This cell contains an analyte solution and an ionic electrolyte to help electricity move. The cell requires electrodes to drive redox reactions, which are reactions involving the transfer of electrons. In these reactions, a species is either oxidized by losing electrons or reduced by gaining them. As a species is oxidized, the electrons move through an external circuit. This movement creates a faradaic current. These currents follow Faraday's law. This law states that the number of moles of a substance produced or consumed is proportional to the electric charge passed through the electrode. Therefore, measuring the current allows scientists to calculate the exact concentration of the analyte.

Modern experiments typically use a three-electrode system to ensure accuracy. The first is the working electrode, which makes contact with the analyte. This electrode must apply the desired potential and facilitate charge transfer. The second is the reference electrode, which has a known, constant reduction potential. Its only job is to provide a baseline so the potential of the working electrode can be measured and controlled. The third is the auxiliary electrode, also called the counter electrode. This electrode completes the circuit by passing the current required to balance the reaction at the working electrode.

There are different ways to set up these electrodes depending on the experiment. While a two-electrode system is possible, it is difficult to maintain a constant potential while passing current. The three-electrode system solves this by separating the tasks of providing a reference and supplying current. Some advanced setups use even more electrodes. For example, a rotating ring-disk electrode uses two separate working electrodes: a disk and a ring. This allows a researcher to scan or hold potentials independently on each part. In some cases, such as with microelectrodes, the counter and reference electrodes might be combined. This happens when the generated current is too small to affect the reference potential.

The shape of a voltammogram depends on how the experiment is run. One major factor is mass transfer, or how the analyte moves through the liquid. This can be controlled by stirring the solution or leaving it quiescent, which means still. If the analyte is continuously stirred, the diffusion layer stays a constant width. This produces a voltammogram that reaches a constant, flat level called a limiting current.

If the mixture is not stirred, the diffusion layer grows wider over time. This results in a graph that shows a maximum peak current, known as the peak current.

A third shape measures the change in current rather than the current applied.

Scientists use several mathematical models to interpret these results. The Nernst equation is used to relate the electrochemical cell potential to the concentration ratio of the oxidized and reduced species. However, the Nernst equation does not include a time component. Because voltammetry varies potential over time, other models are needed. The Butler-Volmer equation relates concentration, potential, and current as a function of time. It helps predict how forward and backward redox reactions affect the cell's reactivity. At high overpotentials, this simplifies into the Tafel equation. The Tafel equation relates electrochemical currents to the overpotential exponentially to calculate reaction rates.

Finally, voltammetry is closely linked to the study of how particles move, known as diffusion. As redox species react, material accumulates at the electrode/electrolyte interface. This creates a concentration gradient between the electrode and the rest of the solution. Scientists use Fick's laws of diffusion to relate this movement to the faradaic current. This helps explain how the diffusion of species affects the overall process. By combining these electrical measurements with mathematical laws, voltammetry provides a deep look at chemical behavior at a molecular level.