Electroanalytical method


Electroanalytical method

Electroanalytical methods are a class of techniques in analytical chemistry which study an analyte by measuring the potential (Volts) and/or current (Amps) in an electrochemical cell containing the analyte. [Bard, A.J.; Faulkner, L.R. Electrochemical Methods: Fundamentals and Applications. New York: John Wiley & Sons, 2nd Edition, 2000.] [Skoog, D.A.; West, D.M.; Holler, F.J. Fundamentals of Analytical Chemistry New York: Saunders College Publishing, 5th Edition, 1988.] These methods can be broken down into several categories depending on which aspects of the cell are controlled and which are measured. The three main categories are Potentiometry (the difference in electrode potentials is measured), Coulometry (the cell's current is measured over time), and Voltammetry (the cell's current is measured while actively altering the cell's potential).

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 more 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" (distinct from the reference electrode used in the three electrode system). Potentiometry usually uses electrodes made "selectively" sensitive to the ion of interest, such as a fluoride-selective 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 from one oxidation state to another. In these experiments, the total current passed is measured directly or indirectly to determine the number of electrons passed. Knowing the number of electrons passed can indicate 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 bulk electrolysis, also known as "Potentiostatic coulometry" or "controlled potential coulometry", as well as a variety of coulometric titrations.

Voltammetry

Voltammetry applies a constant and/or varying potential at an electrode's surface 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 practical terms is nondestructive since only a very small amount of the analyte is consumed at the two-dimensional surface of the working and auxiliary electrodes. In practice the analyte solutions is usually disposed of since it is difficult to separate the analyte from the bulk electrolyte and the experiment requires a small amount of analyte. A normal experiment may involve 1-10 mL solution with an analyte concentration between 1-10 mM.

Polarometry

Polarometry is a subclass of voltammetry that uses a dropping mercury electrode as the working electrode. The auxiliary electrode is often the resulting mercury pool. Concern over the toxicity of mercury has caused the use of mercury electrodes to decrease greatly. Alternate electrode materials, such as the noble metals and glassy carbon, are affordable, inert, and easily cleaned.

Amperometry

Most of Amperometry is now a subclass of voltammetry in which the electrode is held at constant potentials for various lengths of time. The distinction between "amperometry" and "voltammetry" is mostly historic. There was a time when it was difficult to switch between "holding" and "scanning" a potential. This function is trivial for modern potentiostats, and today there is little distinction between the techniques which either "hold", "scan", or do both during a single experiment. Yet the terminology still results in confusion, for example, differential pulse voltammetry is also referred to as and "differential pulse amperometry". This experiment can be seen as the combination of linear sweep voltammetry and chronoamperometry thus the confusion in which category it should be named.

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

Not all of the experiments which were historically amperometry now fall under the domain of voltammetry. In an amperometric titration, the 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.

References

Bibliography

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