Electroanalytical method
Encyclopedia
Electroanalytical methods are a class of techniques in analytical chemistry
which study an analyte
by measuring the potential
(volt
s) and/or current
(ampere
s) in an electrochemical cell
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), coulometry
(the cell's current is measured over time), and voltammetry
(the cell's current is measured while actively altering the cell's potential).
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
.
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. Common forms of coulometry include bulk electrolysis
, also known as Potentiostatic coulometry or controlled potential coulometry, as well as a variety of coulometric titrations.
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 electrode
s. 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 and 10 mmol/L.
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.
s, 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 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 current readings 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
.
in analytical methods
Analytical chemistry
Analytical chemistry is the study of the separation, identification, and quantification of the chemical components of natural and artificial materials. Qualitative analysis gives an indication of the identity of the chemical species in the sample and quantitative analysis determines the amount of...
which study an analyte
Analyte
An analyte, or component , is a substance or chemical constituent that is of interest in an analytical procedure. Grammatically, it is important to note that experiments always seek to measure properties of analytes—and that analytes themselves can never be measured. For instance, one cannot...
by measuring the potential
Potential
*In linguistics, the potential mood*The mathematical study of potentials is known as potential theory; it is the study of harmonic functions on manifolds...
(volt
Volt
The volt is the SI derived unit for electric potential, electric potential difference, and electromotive force. The volt is named in honor of the Italian physicist Alessandro Volta , who invented the voltaic pile, possibly the first chemical battery.- Definition :A single volt is defined as the...
s) and/or current
Electric current
Electric current is a flow of electric charge through a medium.This charge is typically carried by moving electrons in a conductor such as wire...
(ampere
Ampere
The ampere , often shortened to amp, is the SI unit of electric current and is one of the seven SI base units. It is named after André-Marie Ampère , French mathematician and physicist, considered the father of electrodynamics...
s) in an electrochemical cell
Electrochemical cell
An electrochemical cell is a device capable of either deriving electrical energy from chemical reactions, or facilitating chemical reactions through the introduction of electrical energy. A common example of an electrochemical cell is a standard 1.5-volt "battery"...
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
Ion selective electrode
An ion-selective electrode , also known as a specific ion electrode , is a transducer that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the...
(the difference in electrode potentials is measured), coulometry
Coulometry
Coulometry is the name given to a group of techniques in analytical chemistry that determine the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity consumed or produced....
(the cell's current is measured over time), and voltammetry
Voltammetry
Voltammetry is a category of electroanalytical methods used in analytical chemistry and various industrial processes. In voltammetry, information about an analyte is obtained by measuring the current as the potential is varied.- Three electrode system :...
(the cell's current is measured while actively altering the cell's potential).
Potentiometry
PotentiometryIon selective electrode
An ion-selective electrode , also known as a specific ion electrode , is a transducer that converts the activity of a specific ion dissolved in a solution into an electrical potential, which can be measured by a voltmeter or pH meter. The voltage is theoretically dependent on the logarithm of the...
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
Glass electrode
A glass electrode is a type of ion-selective electrode made of a doped glass membrane that is sensitive to a specific ion. It is an important part of the instrumentation for chemical analysis and physico-chemical studies. In modern practice, widely used membranous ion-selective electrodes are part...
even though it actually contains two electrodes: an indicator electrode and a reference electrode (distinct from the reference electrode
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 concentrations of each participants of the redox reaction.There are many ways reference...
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
PH meter
A pH meter is an electronic instrument used for measuring the pH of a liquid...
.
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 electronElectron
The electron is a subatomic particle with a negative elementary electric charge. It has no known components or substructure; in other words, it is generally thought to be an elementary particle. An electron has a mass that is approximately 1/1836 that of the proton...
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. Common forms of coulometry include bulk electrolysis
Bulk electrolysis
Bulk electrolysis is also known as potentiostatic coulometry or controlled potential coulometry. The experiment is a form of coulometry which generally employees a three electrode system controlled by a potentiostat. In the experiment the working electrode is held at a constant potential and...
, 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 potentialReduction potential
Reduction potential is a measure of the tendency of a chemical species to acquire electrons and thereby be reduced. Reduction potential is measured in volts , or millivolts...
of an analyte and its electrochemical reactivity
Electrochemical reaction mechanism
In chemistry, an electrochemical reaction mechanism is the step by step sequence of elementary steps, involving at least one outer sphere electron transfer, by which an overall chemical change occurs .- Overview :...
. 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
Working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
and auxiliary electrode
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 electrical current is expected to flow...
s. In practice the analyte solutions is usually disposed of since it is difficult to separate the analyte from the bulk electrolyte
Electrolyte
In chemistry, an electrolyte is any substance containing free ions that make the substance electrically conductive. The most typical electrolyte is an ionic solution, but molten electrolytes and solid electrolytes are also possible....
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.
Polarometry
Polarometry is a subclass of voltammetry that uses a dropping mercury electrodeDropping mercury electrode
The dropping mercury electrode is a working electrode made of mercury and used in polarography. Experiments run with mercury electrodes are referred to as forms of polarography even if the experiments are identical or very similar to a corresponding voltammetry experiment which use solid working...
as the working electrode
Working electrode
The working electrode is the electrode in an electrochemical system on which the reaction of interest is occurring. The working electrode is often used in conjunction with an auxiliary electrode, and a reference electrode in a three electrode system...
. The auxiliary electrode
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 electrical current is expected to flow...
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 potentiostatPotentiostat
A potentiostat is the electronic hardware required to control a three electrode cell and run most electroanalytical experiments. A Bipotentiostat and polypotentiostat are potentiostats capable of controlling two working electrodes and more than two working electrodes, respectively.The system...
s, 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
Differential pulse voltammetry
Differential Pulse Voltammetry is often used to make electrochemical measurements. It can be considered as a derivative of linear sweep voltammetry or staircase voltammetry, with a series of regular voltage pulses superimposed on the potential linear sweep or stair steps...
is also referred to as differential pulse amperometry. This experiment can be seen as the combination of linear sweep voltammetry
Linear sweep voltammetry
Linear sweep voltammetry is a voltammetric method where the current at a working electrode is measured while the potential between the working electrode and a reference electrode is swept linearly in time...
and chronoamperometry
Chronoamperometry
Chronoamperometry is an electrochemical technique in which the potential of the working electrode is stepped and the resulting current from faradic processes occurring at the electrode is monitored as a function of time...
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 current readings 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
Accuracy and precision
In the fields of science, engineering, industry and statistics, the accuracy of a measurement system is the degree of closeness of measurements of a quantity to that quantity's actual value. The precision of a measurement system, also called reproducibility or repeatability, is the degree to which...
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
Amperometric titration
Amperometric titration refers to a class of titrations in which the equivalence point is determined through measurement of the electric current produced by the titration reaction. It is a form of quantitative analysis.-Background:...
, 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
Coulometry
Coulometry is the name given to a group of techniques in analytical chemistry that determine the amount of matter transformed during an electrolysis reaction by measuring the amount of electricity consumed or produced....
.
in analytical methods