Voltammetry is a category of
electroanalytical methods used in
analytical chemistry
Analytical chemistry studies and uses instruments and methods to separate, identify, and quantify matter. In practice, separation, identification or quantification may constitute the entire analysis or be combined with another method. Separati ...
and various industrial processes. In voltammetry, information about an
analyte
An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, et ...
is obtained by measuring the current as the potential is varied.
The analytical data for a voltammetric experiment comes in the form of a voltammogram which plots the current produced by the analyte versus the potential of the working electrode.
Theory
Voltammetry is the study of current as a function of applied potential. Voltammetric methods involve electrochemical cells, and investigate the reactions occurring at electrode/electrolyte interfaces.
The reactivity of analytes in these half-cells is used to determine their concentration. It is considered a dynamic electrochemical method as the applied potential is varied over time and the corresponding changes in current are measured.
Most experiments control 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, electric potential difference ( voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
D ...
s) of an electrode in contact with the analyte while measuring the resulting
current (
ampere
The ampere (, ; symbol: A), often Clipping (morphology), 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 amp ...
s).
Electrochemical Cells
Electrochemical cells are used in voltammetric experiments to drive the redox reaction of the analyte. Like other electrochemical cells, two half-cells are required, one to facilitate reduction and the other oxidation. The cell consists of an analyte solution, an ionic electrolyte, and two or three electrodes, with oxidation and reduction reactions occurring at the electrode/electrolyte interfaces.
[Kounaves, S.P., 1997. Voltammetric techniques. ''Handbook of instrumental techniques for analytical chemistry'', pp.709-726.] As a species is oxidized, the electrons produced pass through an external electric circuit and generate a current, acting as an electron source for reduction. The generated currents are Faradaic currents, which follow Faraday’s law. As Faraday’s law states that the” number of moles of a substance, m, produced or consumed during an electrode process is proportional to the electric charge passed through the electron” the faradaic currents allow analyte concentrations to be determined.
Whether the analyte is reduced or oxidized depends on the analyte, but its reaction always occurs at the working/indicator electrode. Therefore, the working electrode potential varies as a function of the analyte concentration. A second auxiliary electrode completes the electric circuit. A third reference electrode provides a constant, baseline potential reading for the other two electrode potentials to be compared to.
Three electrode system
Voltammetry experiments investigate the
half-cell reactivity of an
analyte
An analyte, component (in clinical chemistry), or chemical species is a substance or chemical constituent that is of interest in an analytical procedure. The purest substances are referred to as analytes, such as 24 karat gold, NaCl, water, et ...
. Voltammetry is the study of current as a function of applied potential.
These curves I = f(E) are called voltammograms.
The potential is varied arbitrarily, either step by step or continuously, and the resulting current value is measured as the dependent variable.
The opposite, i.e.,
amperometry
Amperometry in chemistry is 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 t ...
, is also possible but not common.
The shape of the curves depends on the speed of potential variation, (nature of driving force) and whether the solution is stirred or quiescent (mass transfer).
Most experiments control 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, electric potential difference ( voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
D ...
s) of an electrode in contact with the analyte while measuring the resulting
current (
ampere
The ampere (, ; symbol: A), often Clipping (morphology), 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 amp ...
s).
To conduct such an experiment, at least two electrodes are required. The
working electrode, which makes contact with the analyte, must apply the desired potential in a controlled way and facilitate the transfer of charge to and from the analyte. A second electrode acts as the other half of the cell. This second electrode must have a known potential to gauge the potential of the working electrode from; furthermore it must balance the charge added or removed by the working electrode. While this is a viable setup, it has a number of shortcomings. Most significantly, it is extremely difficult for an electrode to maintain a constant potential while passing current to counter redox events at the working electrode.
To solve this problem, the roles of supplying electrons and providing a reference potential are divided between two separate electrodes. The
reference electrode
A reference electrode is an electrode which has a stable and well-known electrode potential. The high stability of the electrode potential is usually reached by employing a redox system with constant (buffered or saturated) concentrations of each ...
is a half cell with a known reduction potential. Its only role is to act as reference for measuring and controlling the working electrode's potential and at does not pass any current. The
auxiliary electrode The auxiliary electrode, often also called the counter electrode, is an electrode used in a three electrode electrochemical cell for voltammetric analysis or other reactions in which an electric current is expected to flow. The auxiliary electro ...
passes the current required to balance the observed current at the working electrode. To achieve this current, the auxiliary will often swing to extreme potentials at the edges of the
solvent window, where it oxidizes or reduces the solvent or supporting electrolyte. These electrodes, the
working,
reference
Reference is a relationship between objects in which one object designates, or acts as a means by which to connect to or link to, another object. The first object in this relation is said to ''refer to'' the second object. It is called a '' name'' ...
, and
auxiliary make up the modern three-electrode system.
There are many systems which have more electrodes, but their design principles similar to the three-electrode system. For example, the
rotating ring-disk electrode has two distinct and separate working electrodes, a disk, and a ring, which can be used to scan or hold potentials independently of each other. Both of these electrodes are balanced by a single reference and auxiliary combination for an overall four-electrode design. More complicated experiments may add working electrodes, reference, or auxiliary electrodes as required.
In practice it can be important to have a working electrode with known dimensions and surface characteristics. As a result, it is common to clean and polish working electrodes regularly. The auxiliary electrode can be almost anything as long as it doesn't react with the bulk of the analyte solution and conducts well. A common voltammetry method, polarography, uses mercury as a working electrode e.g.
DME and
HMDE, and as an auxiliary electrode. The reference is the most complex of the three electrodes; there are a variety of standards used and it is worth investigating elsewhere. For non-aqueous work,
IUPAC
The International Union of Pure and Applied Chemistry (IUPAC ) is an international federation of National Adhering Organizations working for the advancement of the chemical sciences, especially by developing nomenclature and terminology. It is ...
recommends the use of the
ferrocene
Ferrocene is an organometallic compound with the formula . The molecule is a complex consisting of two cyclopentadienyl rings bound to a central iron atom. It is an orange solid with a camphor-like odor, that sublimes above room temperature, ...
/
ferrocenium couple as an internal standard. In most voltammetry experiments, a bulk
electrolyte (also known as a
supporting electrolyte) is used to minimize solution resistance. It is possible to run an experiment without a bulk electrolyte, but the added resistance greatly reduces the accuracy of the results. With
room temperature ionic liquids, the solvent can act as the electrolyte.
Voltammograms
A voltammogram is a graph that measures the
current of an
electrochemical cell
An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or using electrical energy to cause chemical reactions. The electrochemical cells which generate an electric current are called voltaic o ...
as a function of the potential applied. This graph is used to determine the concentration and the
standard potential of the analyte. To determine the concentration, values such as the limiting or peak current are read from the graph and applied to various mathematical models.
After determining the concentration, the applied standard potential can be identified using the Nernst equation.
There are three main shapes for voltammograms. The first shape is dependent on the diffusion layer.
If the analyte is continuously stirred, the diffusion later will be a constant width and produce a voltammogram that reaches a constant current. The graph takes this shape as the current increases from the background residual to reach the limiting current (i
l). If the mixture is not stirred, the width of the diffusion layer eventually increases. This can be observed by the maximum peak current (i
p), and is identified by the highest point on the graph. The third common shape for a voltammogram measures the sample for change in current rather than current applied. A maximum current is still observed, but represents the maximum change in current (
i
p).
Mathematical Models
To determine analyte concentrations, mathematical models are required to link the applied potential and current measured over time. The Nernst equation relates electrochemical cell potential to the concentration ratio of the reduced and oxidized species in a logarithmic relationship.
The Nernst equation is as follows:
Where:
*
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 ...
*
: standard potential
*
: universal gas constant
*
: temperature in kelvin
*
: ion charge (moles of electrons)
*
:
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of S ...
*
:
Reaction quotient
This equation describes how the changes in applied potential will alter the concentration ratio. However, the Nernst equation is limited, as it is modeled without a time component and voltammetric experiments vary applied potential as a function of time. Other mathematical models, primarily the Butler-Volmer equation, the Tafel equation, and Fick's law address the time dependence.
The
Butler–Volmer equation relates concentration, potential, and current as a function of time.
It describes the non-linear relationship between the electrode and electrolyte voltage difference and the electrical current. It helps make predictions about how the forward and backward redox reactions affect potential and influence the reactivity of the cell.
This function includes a rate constant which accounts for the kinetics of the reaction. A compact version of the Butler-Volmer equation is as follows:
Where:
*
: electrode
current density
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional a ...
, A/m
2 (defined as j ''= I/S'')
*
:
exchange current density, A/m
2
*
:
electrode potential
In electrochemistry, electrode potential is the electromotive force of a galvanic cell built from a standard reference electrode and another electrode to be characterized. By convention, the reference electrode is the standard hydrogen electrod ...
, V
*
: equilibrium potential, V
*
:
absolute temperature
Thermodynamic temperature is a quantity defined in thermodynamics as distinct from kinetic theory or statistical mechanics.
Historically, thermodynamic temperature was defined by Kelvin in terms of a macroscopic relation between thermodynamic ...
, K
*
: number of electrons involved in the electrode reaction
*
:
Faraday constant
In physical chemistry, the Faraday constant, denoted by the symbol and sometimes stylized as ℱ, is the electric charge per mole of elementary charges. It is named after the English scientist Michael Faraday. Since the 2019 redefinition of S ...
*
:
universal gas constant
*
: so-called cathodic
charge transfer coefficient, dimensionless
*
: so-called anodic charge transfer coefficient, dimensionless
*
: activation
overpotential (defined as
).
At high
overpotentials, the
Butler–Volmer equation simplifies to the
Tafel equation. The Tafel equation relates the electrochemical currents to the overpotential exponentially, and is used to calculate the reaction rate.
The overpotential is calculated at each electrode separately, and related to the voltammogram data to determine reaction rates. The Tafel equation for a single electrode is:
Where:
* the plus sign under the exponent refers to an anodic reaction, and a minus sign to a cathodic reaction
*
:
overpotential, V
* A: "
Tafel slope", V
*
:
current density
In electromagnetism, current density is the amount of charge per unit time that flows through a unit area of a chosen cross section. The current density vector is defined as a vector whose magnitude is the electric current per cross-sectional a ...
, A/m
2
*
: "
exchange current density", A/m
2.
As the redox species are oxidized and reduced at the electrodes, material accumulates at the electrode/electrolyte interface.
Material accumulation creates a concentration gradient between the interface and the bulk solution.
Fick's laws of diffusion
Fick's laws of diffusion describe diffusion and were derived by Adolf Fick in 1855. They can be used to solve for the diffusion coefficient, . Fick's first law can be used to derive his second law which in turn is identical to the diffusion ...
is used to relate the diffusion of oxidized and reduced species to the faradaic current used to describe redox processes. Fick's law is most commonly written in terms of moles, and is as follows:
Where:
* J: diffusion flux (in amount of substance per unit area per unit time)
* D: diffusion coefficient or
diffusivity. (in area per unit time)
* φ: concentration (in amount of substance per unit volume)
* x: position (in length)
Types of voltammetry
History
The beginning of voltammetry was facilitated by the discovery of
polarography in 1922 by the Nobel Prize–winning Czech chemist
Jaroslav Heyrovský
Jaroslav Heyrovský () (December 20, 1890 – March 27, 1967) was a Czech chemist and inventor. Heyrovský was the inventor of the polarographic method, father of the electroanalytical method, and recipient of the Nobel Prize in 1959 for hi ...
. Early voltammetric techniques had many problems, limiting their viability for everyday use in analytical chemistry. In polarography, these problems included the fact that mercury is oxidized at a potential that is more positive than +0.2 Volt, making it harder to analyze the results for the analytes in the positive region of the potential. Another problem included the residual current obtained from the charging of the large
capacitance
Capacitance is the capability of a material object or device to store electric charge. It is measured by the change in charge in response to a difference in electric potential, expressed as the ratio of those quantities. Commonly recognized a ...
of the
electrode
An electrode is an electrical conductor used to make contact with a nonmetallic part of a circuit (e.g. a semiconductor, an electrolyte, a vacuum or air). Electrodes are essential parts of batteries that can consist of a variety of materials ...
surface. When Heyrovsky first recorded the first dependence on the current flowing through the dropping mercury electrode on the applied potential in 1922, he took point-by-point measurements and plotted a current-voltage curve. This was considered to be the first polarogram. In order to facilitate this process, he constructed what is now known as a polarograph with M. Shikata, which enabled him to record photographically the same curve in a matter of hours. He gave recognition to the importance of potential and its control and also recognized the opportunities of measuring the limiting currents. He was also an important part of the introduction of dropping mercury electrode as a modern-day tool.
In 1942, the English electrochemist
Archie Hickling
Archie is a masculine given name, a diminutive of Archibald. It may refer to:
People Given name or nickname
*Archie Alexander (1888–1958), African-American mathematician, engineer and governor of the US Virgin Islands
* Archie Blake (mathemati ...
(
University of Leicester
, mottoeng = So that they may have life
, established =
, type = public research university
, endowment = £20.0 million
, budget = £326 million
, chancellor = David Willetts
, vice_chancellor = Nishan Canagarajah
, head_la ...
) built the first three electrodes
potentiostat, which was an advancement for the field of electrochemistry. He used this potentiostat to control the voltage of an electrode. In the meantime, in the late 1940s, the American biophysicist
Kenneth Stewart Cole invented an electronic circuit which he called a voltage clamp. The voltage clamp was used to analyze the ionic conduction in
nerve
A nerve is an enclosed, cable-like bundle of nerve fibers (called axons) in the peripheral nervous system.
A nerve transmits electrical impulses. It is the basic unit of the peripheral nervous system. A nerve provides a common pathway for the ...
s.
The 1960s and 1970s saw many advances in the theory, instrumentation, and the introduction of computer added and controlled systems. Modern polarographic and voltammetric methods on mercury electrodes came about in three sections.
The first section includes the development of the mercury electrodes. The following electrodes were produced:
dropping mercury electrode, mercury steaming electrode, hanging mercury drop electrode, static mercury drop electrode, mercury film electrode, mercury amalgam electrodes, mercury microelectrodes, chemically modified mercury electrodes, controlled growth mercury electrodes, and contractible mercury drop electrodes.
There was also an advancement of the measuring techniques used. These measuring techniques include : classical DC polarography, oscillopolarography, Kaloussek's switcher, AC polarography, tast polarography, normal pulse polarography, differential pulse polarography, square-wave voltammetry,
cyclic voltammetry,
anodic stripping voltammetry
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry (anodic, cathodic and adsorptive) have been ...
, convolution techniques, and elimination methods.
Lastly, there was also an advancement of preconcentration techniques that produced an increase in the sensitivity of the mercury electrodes. This came about through the development of
anodic stripping voltammetry
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry (anodic, cathodic and adsorptive) have been ...
,
cathodic stripping voltammetry
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry (anodic, cathodic and adsorptive) have bee ...
and
adsorptive stripping voltammetry
Electrochemical stripping analysis is a set of analytical chemistry methods based on voltammetry or potentiometry that are used for quantitative determination of ions in solution. Stripping voltammetry (anodic, cathodic and adsorptive) have bee ...
.
These advancements improved sensitivity and created new analytical methods, which prompted the industry to respond with the production of cheaper
potentiostat, electrodes, and cells that could be effectively used in routine analytical work.
Applications
Voltammetric sensors
A number of voltammetric systems are produced commercially for the determination of species that are of interest in industry and research. These devices are sometimes called
electrodes but are actually complete voltammetric cells, which are better referred to as
sensors
A sensor is a device that produces an output signal for the purpose of sensing a physical phenomenon.
In the broadest definition, a sensor is a device, module, machine, or subsystem that detects events or changes in its environment and sends ...
. These sensors can be employed for the analysis of organic and inorganic analytes in various matrices.
The oxygen electrode
The determination of dissolved oxygen in a variety of aqueous environments, such as sea water, blood, sewage, effluents from chemical plants, and soils is of tremendous importance to industry, biomedical and environmental research, and clinical medicine. One of the most common and convenient methods for making such measurements is with the
Clark oxygen sensor, which was patented by
L.C. Clark, Jr. in 1956.
See also
*
Current–voltage characteristic
A current–voltage characteristic or I–V curve (current–voltage curve) is a relationship, typically represented as a chart or graph, between the electric current through a circuit, device, or material, and the corresponding voltage, or ...
*
Neopolarogram
The term neopolarogram refers to mathematical derivatives of polarography, polarograms or cyclic voltammetry, cyclic voltammograms that in effect deconvolute diffusion and electrochemical kinetics. This is achieved by analog or digital implementati ...
References
Further reading
*
*
*
External links
* http://new.ametek.com/content-manager/files/PAR/App%20Note%20E-4%20-%20Electrochemical%20Analysis%20Techniques1.pdf
{{Electroanalytical
Electroanalytical methods