Electroanalytical methods

Electroanalytical methods are essential tools in the field of analytical chemistry. These techniques allow scientists to study a specific substance known as an analyte. To perform these studies, researchers use an electrochemical cell. This cell contains the analyte and allows for the measurement of electrical properties. Specifically, scientists measure the potential, which is expressed in volts. They also measure the current, which is expressed in amperes. By observing these electrical changes, they can identify what is in a solution.

These methods are organized into three main categories. The first category is potentiometry, which focuses on measuring potential. The second category is amperometry, which uses electric current as the primary signal. The third category is coulometry, which measures the total charge passed through the cell. Each category controls different aspects of the electrochemical cell. Each also measures different electrical outputs to reach a conclusion about the analyte.

Potentiometry is a passive way to measure a solution. This means the process affects the solution very little. It uses two different electrodes to find the answer. One is the reference electrode, which maintains a constant potential. The other is the indicator electrode, which changes based on the sample's composition. The difference in potential between these two electrodes reveals the sample's makeup. Because this is a non-destructive measurement, the solution remains largely unchanged.

Indicator electrodes in potentiometry are often very selective. For example, a fluoride selective electrode only responds to fluoride ions. This ensures the potential depends only on the specific ion of interest. In aquatic environments, scientists often use platinum for these electrodes. Platinum has high electron transfer kinetics, which helps the process move quickly. Sometimes, electrodes made of several metals are used to improve these kinetics. One of the most common tools in this category is the glass-membrane electrode used in pH meters.

Amperometry involves measuring current as a function of an independent variable. This variable is usually time or electrode potential. One specific technique is chronoamperometry. In this method, a sudden step in potential is applied to the working electrode. Scientists then measure the current over a specific period of time. This is usually done in an unstirred solution to avoid convection. To prevent consuming too much of the analyte, researchers use microelectrodes. These experiments are very fast, typically lasting between 20 ms and 1 s.

A related technique is voltammetry, which is a subclass of amperometry. In voltammetry, scientists apply a constant or varying potential to the electrode surface. They then use a three-electrode system to measure the resulting current. This method reveals the electrochemical reactivity and the reduction potential of an analyte. While it consumes some analyte at the electrode surface, it is considered non-destructive in practice. A standard experiment might use 1 to 10 mL of solution. The analyte concentration usually ranges between 1 and 10 mmol/L.

History provides interesting specialized versions of these methods. In 1879, H. F. Weber initiated a variant called chronopotentiometry. This method uses a constant current to measure potential as a function of time. Another specific type of voltammetry is called polarography. Polarography is unique because it uses a dropping mercury electrode as the working electrode. More advanced voltammetric techniques can now work with microliter volumes. They can even detect concentrations as low as nanomolar levels.

Coulometry is the third major method and works differently. It uses applied current or potential to change an analyte completely. This process converts the analyte from one oxidation state to another. This is known as a redox reaction. Scientists measure the total current passed to determine the number of electrons transferred. This number can reveal the concentration of the analyte. If the concentration is already known, the measurement shows the number of electrons in the reaction. Common forms include bulk electrolysis and various coulometric titrations.