Bulk electrolysis
Encyclopedia
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
(volt
s) and current
(amp
s) is monitored over time (second
s). In a properly run experiment an analyte
is quantitatively converted from its original oxidation state to a new oxidation state, either reduced
or oxidized. As the substrate is consumed, the current also decreases, approaching zero when the conversion nears completion.
The results of a bulk electrolysis are visually displayed as the total coulombs passed (total electric charge
) plotted against time in seconds, even through the experiment measures electric current
(amps
) over time. This is done to show that the experiment is approaching an expected total number of coulombs.
, number of electron
s in the electrode reaction, and number of electrons passed during the experiment are all related by Faraday's laws of electrolysis
. It follows that, if three of the values are known, then the fourth can be calculated. The bulk electrolysis can also be useful for synthetic purposes if the product of the electrolysis can be isolated. This is most convenient when the product is neutral and can be isolated from the electrolyte solution through extraction
or when the product plates out on the electrode or precipitates in another fashion. Even if the product can not be isolated, other analytical techniques can be performed on the solution including NMR
, EPR
, UV-Vis
, FTIR
, among others techniques depending on the specific situation. In specially designed cells the solution can be actively monitored during the experiment.
and working electrode
, while the other contains the reference electrode
. Strictly speaking, the reference electrode does not require a separate compartment. A Quasi-Reference Electrode such as a silver/silver chloride wire electrode can be exposed directly to the analyte solution. In such situations there is concern that the analyte and trace redox products may interact with the reference electrode and either render it useless or increase drift. As a result even these simple references are commonly sequestered in their own cells. The more complex references such as standard hydrogen electrode
, saturated calomel electrode
, or silver chloride electrode
(specific concentration) can not directly mix the analyte solution for fear the electrode will fall apart or interact/react with the analyte.
A bulk electrolysis is best performed in a three part cell in which both the auxiliary electrode
and reference electrode
have their own cell which connects to the cell containing the working electrode
. This isolates the undesired redox events taking place at the auxiliary electrode. During bulk electrolysis, the analyte undergoes a redox event at the working electrode. If the system was open, then it would be possible for the product of that reaction to defuse back to the auxiliary electrode and undergo the inverse redox reaction. In addition to maintaining the proper current at the working electrode, the auxiliary electrode will experience extreme potentials often oxidizing or reducing the solvent or electrolyte to balance the current. In voltammetry
experiments, the currents (amps) are so small and it is not a problem to decompose a small amount of solvent or electrolyte. In contrast, a bulk electrolysis involves currents greater by several orders of magnitude. At the auxiliary electrode, this greater current would decompose a significant amount of the solution/electrolyte and probably boiling the solution in the process all in an effort to balance the current. To mitigate this challenge the auxiliary cell will often contain a stoichiometric or greater amount of sacrificial reductant (ferrocene
) or sacrificial oxidant (ferrocenium) to balance the overall redox reaction.
For ideal performance the auxiliary electrode
should be similar in surface area, as close as possible, and evenly spaced with the working electrode. This is in an effort to prevent "hot spots". Hot spots are the result of current following the path of least resistance
. This means much of the redox chemistry will occur at the points at either end of the shortest path between the working and auxiliary electrode. Heating associated with the capacitances resistance of the solution can occur at the area around these points, actually boiling the solution. The bubbling resulting from this isolated boiling of the solution can be confused with gas evolution.
of the substrate in the solution to the electrode surface. Rates will increase when the volume of the solution is decreased, the solution is stirred more rapidly, or the area of the working electrode is increased. Since mass transfer is so important the solution is stirred during a bulk electrolysis. However, this technique is generally not considered a hydrodynamic technique
, since a laminar flow of solution against the electrode is neither the objective or outcome of the siring.
Bulk electrolysis is occasionally cited in the literature as means to study electrochemical reaction
rates. However, bulk electrolysis is generally a poor method to study electrochemical reaction rates since the rate of bulk electrolysis is generally governed by the specific cells ability to perform mass transfer. Rates slower than this mass transfer bottleneck are rarely of interest.
as well as hydrogen peroxide
at the anode
. Each of these products is related to its own faradaic efficiency which is tied to the experimental arrangement.
Nor is current efficiency the same as thermodynamic efficiency, since it never address the how much energy (potential in volts) is in the electrons added or removed. The voltage efficiency determined by the reactions overpotential
is more directly related to the thermodynamics of the electrochemical reaction. In fact the extent to which a reaction goes to completion is related to how much greater the applied potential is than the reduction potential of interest. In the case where multiple reduction potentials are of interest, it is often difficult to set an electrolysis potential a "safe" distance (such as 200 mV) past a redox event. The result is incomplete conversion of the substrate, or else conversion of some of the substrate to the more reduced form. This factor must be considered when analyzing the current passed and when attempting to do further analysis/isolation/experiments with the substrate solution.
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....
which generally employees a three electrode system controlled by a potentiostat
Potentiostat
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...
. In the experiment 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...
is held at a constant potential
Electric potential
In classical electromagnetism, the electric potential at a point within a defined space is equal to the electric potential energy at that location divided by the charge there...
(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 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...
(amp
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) is monitored over time (second
Second
The second is a unit of measurement of time, and is the International System of Units base unit of time. It may be measured using a clock....
s). In a properly run experiment 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...
is quantitatively converted from its original oxidation state to a new oxidation state, either reduced
Redox
Redox reactions describe all chemical reactions in which atoms have their oxidation state changed....
or oxidized. As the substrate is consumed, the current also decreases, approaching zero when the conversion nears completion.
The results of a bulk electrolysis are visually displayed as the total coulombs passed (total electric charge
Electric charge
Electric charge is a physical property of matter that causes it to experience a force when near other electrically charged matter. Electric charge comes in two types, called positive and negative. Two positively charged substances, or objects, experience a mutual repulsive force, as do two...
) plotted against time in seconds, even through the experiment measures electric 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...
(amps
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...
) over time. This is done to show that the experiment is approaching an expected total number of coulombs.
Fundamental relationships and applications
The sample mass, molecular massMolecular mass
The molecular mass of a substance is the mass of one molecule of that substance, in unified atomic mass unit u...
, number of electron
Electron
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 in the electrode reaction, and number of electrons passed during the experiment are all related by Faraday's laws of electrolysis
Faraday's laws of electrolysis
Faraday's laws of electrolysis are quantitative relationships based on the electrochemical researches published by Michael Faraday in 1834.-Statements of the laws:Several versions of the laws can be found in textbooks and the scientific literature...
. It follows that, if three of the values are known, then the fourth can be calculated. The bulk electrolysis can also be useful for synthetic purposes if the product of the electrolysis can be isolated. This is most convenient when the product is neutral and can be isolated from the electrolyte solution through extraction
Liquid-liquid extraction
Liquid–liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
or when the product plates out on the electrode or precipitates in another fashion. Even if the product can not be isolated, other analytical techniques can be performed on the solution including NMR
NMR
NMR may refer to:Applications of Nuclear Magnetic Resonance:* Nuclear magnetic resonance* NMR spectroscopy* Solid-state nuclear magnetic resonance* Protein nuclear magnetic resonance spectroscopy* Proton NMR* Carbon-13 NMR...
, EPR
Electron paramagnetic resonance
Electron paramagnetic resonance or electron spin resonance spectroscopyis a technique for studying chemical species that have one or more unpaired electrons, such as organic and inorganic free radicals or inorganic complexes possessing a transition metal ion...
, UV-Vis
Ultraviolet-visible spectroscopy
Ultraviolet-visible spectroscopy or ultraviolet-visible spectrophotometry refers to absorption spectroscopy or reflectance spectroscopy in the ultraviolet-visible spectral region. This means it uses light in the visible and adjacent ranges...
, FTIR
Fourier transform infrared spectroscopy
Fourier transform infrared spectroscopy is a technique which is used to obtain an infrared spectrum of absorption, emission, photoconductivity or Raman scattering of a solid, liquid or gas. An FTIR spectrometer simultaneously collects spectral data in a wide spectral range...
, among others techniques depending on the specific situation. In specially designed cells the solution can be actively monitored during the experiment.
Cell design
In most three electrode experiments there are two isolated cells. One contains the auxiliaryAuxiliary 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...
and 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...
, while the other contains 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...
. Strictly speaking, the reference electrode does not require a separate compartment. A Quasi-Reference Electrode such as a silver/silver chloride wire electrode can be exposed directly to the analyte solution. In such situations there is concern that the analyte and trace redox products may interact with the reference electrode and either render it useless or increase drift. As a result even these simple references are commonly sequestered in their own cells. The more complex references such as standard hydrogen electrode
Standard hydrogen electrode
The standard hydrogen electrode , is a redox electrode which forms the basis of the thermodynamic scale of oxidation-reduction potentials...
, saturated calomel electrode
Saturated calomel electrode
The Saturated calomel electrode is a reference electrode based on the reaction between elemental mercury and mercury chloride. The aqueous phase in contact with the mercury and the mercury chloride is a saturated solution of potassium chloride in water...
, or silver chloride electrode
Silver chloride electrode
A silver chloride electrode is a type of reference electrode, commonly used in electrochemical measurements. For example, it is usually the internal reference electrode in pH meters...
(specific concentration) can not directly mix the analyte solution for fear the electrode will fall apart or interact/react with the analyte.
A bulk electrolysis is best performed in a three part cell in which both 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...
and 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...
have their own cell which connects to the cell containing 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...
. This isolates the undesired redox events taking place at the auxiliary electrode. During bulk electrolysis, the analyte undergoes a redox event at the working electrode. If the system was open, then it would be possible for the product of that reaction to defuse back to the auxiliary electrode and undergo the inverse redox reaction. In addition to maintaining the proper current at the working electrode, the auxiliary electrode will experience extreme potentials often oxidizing or reducing the solvent or electrolyte to balance the current. In 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 :...
experiments, the currents (amps) are so small and it is not a problem to decompose a small amount of solvent or electrolyte. In contrast, a bulk electrolysis involves currents greater by several orders of magnitude. At the auxiliary electrode, this greater current would decompose a significant amount of the solution/electrolyte and probably boiling the solution in the process all in an effort to balance the current. To mitigate this challenge the auxiliary cell will often contain a stoichiometric or greater amount of sacrificial reductant (ferrocene
Ferrocene
Ferrocene is an organometallic compound with the formula Fe2. It is the prototypical metallocene, a type of organometallic chemical compound consisting of two cyclopentadienyl rings bound on opposite sides of a central metal atom. Such organometallic compounds are also known as sandwich compounds...
) or sacrificial oxidant (ferrocenium) to balance the overall redox reaction.
For ideal performance 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...
should be similar in surface area, as close as possible, and evenly spaced with the working electrode. This is in an effort to prevent "hot spots". Hot spots are the result of current following the path of least resistance
Path of least resistance
The path of least resistance describes the physical or metaphorical pathway that provides the least resistance to forward motion by a given object or entity, among a set of alternative paths. The concept is often used to describe why an object or entity takes a given path.In physics, the path of...
. This means much of the redox chemistry will occur at the points at either end of the shortest path between the working and auxiliary electrode. Heating associated with the capacitances resistance of the solution can occur at the area around these points, actually boiling the solution. The bubbling resulting from this isolated boiling of the solution can be confused with gas evolution.
Rates and kinetics
The rate of such reactions/experiments is not determined by the concentration of the solution, but rather the mass transferMass transfer
Mass transfer is the net movement of mass from one location, usually meaning a stream, phase, fraction or component, to another. Mass transfer occurs in many processes, such as absorption, evaporation, adsorption, drying, precipitation, membrane filtration, and distillation. Mass transfer is used...
of the substrate in the solution to the electrode surface. Rates will increase when the volume of the solution is decreased, the solution is stirred more rapidly, or the area of the working electrode is increased. Since mass transfer is so important the solution is stirred during a bulk electrolysis. However, this technique is generally not considered a hydrodynamic technique
Hydrodynamic technique
Hydrodynamic technique is a subcategory of electroanalytical methods in which the analyte solution flows relative to a working electrode. In many voltammetry techniques, the solution is intentionally left still to allow diffusion controlled mass transfer...
, since a laminar flow of solution against the electrode is neither the objective or outcome of the siring.
Bulk electrolysis is occasionally cited in the literature as means to study electrochemical reaction
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 :...
rates. However, bulk electrolysis is generally a poor method to study electrochemical reaction rates since the rate of bulk electrolysis is generally governed by the specific cells ability to perform mass transfer. Rates slower than this mass transfer bottleneck are rarely of interest.
Efficiency and thermodynamics
Electrocatalytic analyzes will often mention the current efficiency or faradaic efficiency of a given process determined by a bulk electrolysis experiment. For example if one molecule of hydrogen results from every two electrons inserted into an acidic solution then the faradaic efficiency would be 100%. This indicates that the electrons did not ended up performing some other reaction. For example the oxidation of water will often produce oxygenOxygen
Oxygen is the element with atomic number 8 and represented by the symbol O. Its name derives from the Greek roots ὀξύς and -γενής , because at the time of naming, it was mistakenly thought that all acids required oxygen in their composition...
as well as hydrogen peroxide
Hydrogen peroxide
Hydrogen peroxide is the simplest peroxide and an oxidizer. Hydrogen peroxide is a clear liquid, slightly more viscous than water. In dilute solution, it appears colorless. With its oxidizing properties, hydrogen peroxide is often used as a bleach or cleaning agent...
at the anode
Anode
An anode is an electrode through which electric current flows into a polarized electrical device. Mnemonic: ACID ....
. Each of these products is related to its own faradaic efficiency which is tied to the experimental arrangement.
Nor is current efficiency the same as thermodynamic efficiency, since it never address the how much energy (potential in volts) is in the electrons added or removed. The voltage efficiency determined by the reactions overpotential
Overpotential
Overpotential is an electrochemical term which refers to the potential difference between a half-reaction's thermodynamically determined reduction potential and the potential at which the redox event is experimentally observed. The term is directly related to a cell's voltage efficiency...
is more directly related to the thermodynamics of the electrochemical reaction. In fact the extent to which a reaction goes to completion is related to how much greater the applied potential is than the reduction potential of interest. In the case where multiple reduction potentials are of interest, it is often difficult to set an electrolysis potential a "safe" distance (such as 200 mV) past a redox event. The result is incomplete conversion of the substrate, or else conversion of some of the substrate to the more reduced form. This factor must be considered when analyzing the current passed and when attempting to do further analysis/isolation/experiments with the substrate solution.