Electroanalytical method

The electroanalytical methods are a class of techniques in analytical chemistry, which study an analyte by measuring the electrical potential (volts) and/or electrical current (amps) in an electrochemical cell, which contains the analyte. These methods can be divided into several categories depending on which aspects of the cell are controlled and which are measured. The three main categories are: potentiometry (potential difference across the electrode is measured), coulometry (cell current is measured over time), and voltammetry (cell current is measured while the cell potential is actively altered). the cells).

Potentiometry

Potentiometry passively measures the potential of a solution between two electrodes, affecting the solution very little in the process. The potential is then related to the concentration of one or several analytes. The cell structure used is often referred to as an electrode even though it actually contains two electrodes: an indicator electrode and a reference electrode (other than the reference electrode used in the three-electrode system). Potentiometry generally uses selectively constructed electrodes sensitive to the ions of interest, such as a selective fluoride electrode. The most common potentiometric electrode is the glass membrane electrode used in a pH-meter.

Coulometry

Coulometry uses applied current or potential to completely convert an analyte (by oxidation or electrode reduction) from one oxidation state to another. In these experiments, the total current that passes is measured directly or indirectly. Knowing the number of electrons that have passed can tell us the concentration of the analyte or, when the concentration is known, the number of electrons transferred in the redox reaction. Common forms of coulometry include potentiostatic coulometry or controlled potential coulometry and constant intensity coulometry , as well as a variety of coulometry titrations.

Voltammetry

Voltammetry applies a constant and/or variable potential to the surface of an electrode and measures the resulting current with a three-electrode system. This method can reveal the reduction potential of an analyte and its electrochemical reactivity. This method, in practice, is a non-destructive method since only a very small amount of the analyte is consumed on the two-dimensional surface of the working and auxiliary electrodes. In practice, solutions of the analyte are usually removed, since it is difficult to separate the analyte from the supporting electrolyte and the experiment requires only a small amount of analyte. A normal experiment may involve between 1-10 mL with a concentration of analytes between 1-10 mM.

Polarography

Polarometry is a subclass of voltammetry that uses a mercury drop electrode as the working electrode. The auxiliary electrode is often the resulting mercury cuvette. Concern over mercury toxicity has caused the use of mercury electrodes to be greatly reduced. Alternative electrode materials, such as noble metals and crystalline carbon, are affordable, inert, and easy to clean.

Amperometry

Most amperometry is now a subclass of voltammetry in which the electrode is held at constant potentials for various periods of time. The distinction between amperometry and voltammetry is primarily historical. There was a time when it was difficult to switch between "holding" and "scan" a potential. This function is trivial to modern potentiostats, and today there is little difference between techniques that either 'hold', 'scan', or do both in a single experiment. However, the terminology is still confusing, for example, differential pulse voltammetry is also known as differential pulse amperometry. This experiment can be seen as combining linear scanning voltammetry and chronoamperometry, hence the confusion as to which category it should be named.

One advantage that distinguishes amperometry from other forms of voltammetry is that in amperometry, measured currents are averaged (or summed) over time. In most voltammetry, the measured currents must be considered independently at individual time intervals. The averaging used in amperometry gives these methods a higher precision than most individual measurements of (other) voltammetric techniques.

Not all experiments that were historically amperometry are now included in the realm of voltammetry. In an amperometric titration, current is measured, but this would not be considered voltammetry since the entire solution is transformed during the experiment. Amperometric titrations are instead a form of coulometry.