Electroanalytical methods are a class of techniques in
analytical chemistry
Analytical skill, Analytical chemistry studies and uses instruments and methods to Separation process, separate, identify, and Quantification (science), quantify matter. In practice, separation, identification or quantification may constitute t ...
which study an
analyte
An analyte, component (in clinical chemistry), titrand (in titrations), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The remainder of the sample is called the matrix. The procedure ...
by measuring the
potential
Potential generally refers to a currently unrealized ability. The term is used in a wide variety of fields, from physics to the social sciences to indicate things that are in a state where they are able to change in ways ranging from the simple r ...
(
volt
The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s) and/or
current
Currents, Current or The Current may refer to:
Science and technology
* Current (fluid), the flow of a liquid or a gas
** Air current, a flow of air
** Ocean current, a current in the ocean
*** Rip current, a kind of water current
** Current (hydr ...
(
ampere
The ampere ( , ; symbol: A), often shortened to amp,SI supports only the use of symbols and deprecates the use of abbreviations for units. is the unit of electric current in the International System of Units (SI). One ampere is equal to 1 c ...
s) in an
electrochemical cell
An electrochemical cell is a device that either generates electrical energy from chemical reactions in a so called galvanic cell, galvanic or voltaic cell, or induces chemical reactions (electrolysis) by applying external electrical energy in an ...
containing the analyte. 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),
amperometry
Amperometry in chemistry is the detection of ions in a solution based on electric current or changes in electric current.
Amperometry is used in electrophysiology to study vesicle release events using a carbon fiber electrode. Unlike patch clamp ...
(electric current is the analytical signal),
coulometry
In analytical electrochemistry, coulometry is the measure of charge (Coulomb, coulombs) transfer during an Electrochemistry, electrochemical Redox, redox reaction. It can be used for precision measurements of charge, but coulometry is mainly used ...
(charge passed during a certain time is recorded).
Potentiometry
Potentiometry passively measures the potential of a solution between two electrodes, affecting the solution very little in the process. One electrode is called the
reference electrode
A reference electrode is an electrode that has a stable and well-known electrode potential. The overall chemical reaction taking place in a cell is made up of two independent half-reactions, which describe chemical changes at the two electrodes. ...
and has a constant potential, while the other one is an indicator electrode whose potential changes with the sample's composition. Therefore, the difference in potential between the two electrodes gives an assessment of the sample's composition. In fact, since the potentiometric measurement is a non-destructive measurement, assuming that the electrode is in equilibrium with the solution, we are measuring the solution's potential.
Potentiometry usually uses indicator electrodes made ''selectively'' sensitive to the ion of interest, such as fluoride in
fluoride selective electrodes, so that the potential solely depends on the
activity of this ion of interest.
The time that takes the electrode to establish equilibrium with the solution will affect the sensitivity or accuracy of the measurement. In aquatic environments, platinum is often used due to its high
electron transfer
Electron transfer (ET) occurs when an electron relocates from an atom, ion, or molecule, to another such chemical entity. ET describes the mechanism by which electrons are transferred in redox reactions.
Electrochemical processes are ET reactio ...
kinetics, although an electrode made from several metals can be used in order to enhance the electron transfer kinetics. The most common potentiometric electrode is by far the glass-membrane electrode used in a
pH meter
A pH meter is a scientific instrument that measures the hydrogen-ion activity in water-based solutions, indicating its acidity or alkalinity expressed as pH. The pH meter measures the difference in electrical potential between a pH electro ...
.
A variant of potentiometry is chronopotentiometry which consists in using a constant current and measurement of potential as a function of time. It has been initiated by
Weber.
Amperometry
Amperometry indicates the whole of electrochemical techniques in which a current is measured as a function of an independent variable that is, typically, time (in a chronoamperometry) or electrode potential (in a voltammetry). Chronoamperometry is the technique in which the current is measured, at a fixed potential, at different times since the start of polarisation. Chronoamperometry is typically carried out in unstirred solution and at the fixed electrode, i.e., under experimental conditions avoiding convection as the mass transfer to the electrode. On the other hand, voltammetry is a subclass of amperometry, in which the current is measured by varying the potential applied to the electrode. According to the waveform that describes the way how the potential is varied as a function of time, the different voltammetric techniques are defined.
Chronoamperometry
In a chronoamperometry, a sudden step in potential is applied at the working electrode and the current is measured as a function of time.
Since this is not an exhaustive method, microelectrodes are used and the amount of time used to perform the experiments is usually very short, typically 20 ms to 1 s, as to not consume the analyte.
Voltammetry
A voltammetry consists in applying a constant and/or varying potential at an electrode's surface and measuring the resulting current with a three-electrode system. This method can reveal the
reduction potential
Redox potential (also known as oxidation / reduction potential, ''ORP'', ''pe'', ''E_'', or E_) is a measure of the tendency of a chemical species to acquire electrons from or lose electrons to an electrode and thereby be reduced or oxidised respe ...
of an analyte and its
electrochemical reactivity. This method, in practical terms, is non-destructive since only a very small amount of the analyte is consumed at the two-dimensional surface of the
working and
auxiliary electrode
Auxiliary may refer to:
In language
* Auxiliary language (disambiguation)
* Auxiliary verb
In military and law enforcement
* Auxiliary police
* Auxiliaries, civilians or quasi-military personnel who provide support of some kind to a military se ...
s. In practice, the analyte solution 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 and 10 mmol/L. More advanced voltammetric techniques can work with microliter volumes and down to nanomolar concentrations. Chemically modified electrodes are employed for the analysis of organic and inorganic samples.
Polarography
Polarography is a subclass of voltammetry that uses a
dropping mercury electrode
A liquid metal electrode is an electrode that uses a liquid metal, such as mercury, Galinstan, and NaK. They can be used in electrocapillarity, voltammetry, and impedance measurements.
Dropping mercury electrode
The dropping mercury electro ...
as the
working electrode
Working may refer to:
* Work (human activity), intentional activity people perform to support themselves, others, or the community
Arts and media
* ''Working'' (musical), a 1978 musical
* ''Working'' (TV series), an American sitcom
* ''Workin ...
.
Coulometry
Coulometry uses applied current or potential to convert an analyte from one oxidation state to another completely. In these experiments, the total current passed is measured directly or indirectly to determine the number of
electron
The electron (, or in nuclear reactions) is a subatomic particle with a negative one elementary charge, elementary electric charge. It is a fundamental particle that comprises the ordinary matter that makes up the universe, along with up qua ...
s 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. Typical forms of coulometry include
bulk electrolysis, also known as ''Potentiostatic coulometry'' or ''controlled potential coulometry'', as well as a variety of coulometric titrations.
References
Bibliography
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