what should the reference electode be set to in cyclic voltammetry

Cyclic Voltammetry Basic Principles, Theory and Setup

Cyclic voltammetry is an electrochemical technique for measuring the current response of a redox active solution to a linearly cycled potential sweep between two or more than fix values. It is a useful method for quickly determining information most the thermodynamics of redox processes, the energy levels of the analyte and the kinetics of electronic-transfer reactions.

Like other types of voltammetry, cyclic voltammetry uses a three electrode organization consisting of a working electrode, reference electrode, and counter electrode.

To perform cyclic voltammetry, the electrolyte solution is commencement added to an electrochemical cell forth with a reference solution and the three electrodes. Apotentiostat is and then used to linearly sweep the potential between the working and reference electrodes until it reaches a pre-fix limit, at which point information technology is swept back in the reverse direction.

This process is repeated multiple times during a scan and the changing current between the working and counter probes is measured past the device in existent time. The issue is a characteristic duck-shaped plot known as a cyclic voltammogram.

Basic theory and principles

The potentiometry principle
Introduction to voltammetry
The 3 electrode system

Circadian voltammograms explained

Cyclic voltammetry of ferrocene

Experimental setup

The electrochemical prison cell
Electrochemical solutions
Selection of electrodes

Applications of cyclic voltammetry
Similar electrochemical methods

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Bones Theory and Principles

Cyclic voltammetry is a sophisticated potentiometric and voltammetric method. During a scan, the chemical either loses an electron (oxidation) or gains an electron (reduction) depending on the direction of the ramping potential.

The Potentiometry Principle

Potentiometry is a manner of measuring the electric potential of an electrochemical prison cell nether static conditions (i.e. no current flow).

For a general reduction or oxidation (redox) reaction, the standard potential is related to the concentration of the reactants (A) and products (B) at the electrode/solution interface co-ordinate to the Nernst equation:

Here,Eastward is the electrode potential, E^0′ is the formal potential, R is the gas constant (8.3145 J·K^-1·mol^-one), T is temperature, n is the number of moles of electrons involved and F is the Faraday constant (96,485 C·mol^-i).

The term [B]^b/[A]^a represents the ratio products to reactants, raised to their respective stoichiometric powers. This tin can be used in place of an activity term when the concentration is sufficiently low (< 0.1 mol·dm^˗3).

Nether standard weather of temperature and force per unit area, the Nernst equation can be written every bit:

Nernst equation

Introduction to Voltammetry

In the general sense, voltammetry is any technique where the current is measured while the potential between two electrodes is varied. Voltammetric methods include cyclic voltammetry, linear sweep voltammetry, and a number of similar electrochemical techniques such as staircase voltammetry, squarewave voltammetry and fast-browse circadian voltammetry.

When performing voltammetry, a current is generated equally the upshot of electron transfer betwixt the redox species and the electrodes. This is carried through the solution by the diffusion and migration of ions.

The Three Electrode Arrangement

Although in principle cyclic voltammetry (and other types of voltammetry) only requires ii electrodes, in practise it is hard to maintain a constant potential and brand certain that the resistance measured is the 1 beyond the working electrode-solution interface. Passing the necessary electric current tin make both hard, while too passing current to counteract the redox events at the working electrode. As a result, a iii electrode system is often used to dissever the role of referencing the potential applied and balance the current produced.

Three electrode cell system — Three electrode cell as used in cyclic voltammetry with an Ossila Potentiostat

To measure and control the potential difference applied the potential of the working electrode is varied while the potential of reference electrode remains fixed by a electrochemical redox reaction with a well-defined value.

To keep the potential fixed, the reference electrode must contain constant concentrations of each component of the reaction, such as a silver wire and a saturated solution of silverish ions.

Importantly, minimal current passes between the reference and working electrodes. The electric current observed at the working electrode is completely balanced by the current passing at the counter electrode, which has a much larger surface expanse.

The electron transfer between the redox species at the working electrode and counter electrode generates electric current that is carried through the solution by the diffusion of ions. This forms a capacitive electrical double layer at the surface of the electrode called the diffuse double layer (DDL). The DDL is composed of ions and orientated electrical dipoles that serve to annul the charge on the electrode.

The measured electric current response is dependent on the concentration of the redox species (the analyte) at the working electrode surface, and is described by a combination of Faraday's law and Fick'southward first law of improvidence:

where i_d is the diffusion-express current, A is the electrode area, D₀ is the improvidence coefficient of the analyte and (∂C₀/∂x)₀ is concentration slope at the electrode surface.

The product of the diffusion coefficient and concentration gradient can exist idea of as the molar flux (mol·cm^-2·southward^-i) of analyte to the electrode surface.

Cyclic Voltammograms Explained

The 'duck-shaped' plot generated by cyclic voltammetry is chosen a cyclic voltammogram.

Cyclic voltammogram for an electrochemically-reversible one-electron redox process — Example of a cyclic voltammogram for an electrochemically-reversible one-electron redox process

In the example circadian voltammogram shown, the browse starts at -0.4V and sweeps frontward to more positive, oxidative potentials. Initially the potential is non sufficient to oxidise the analyte (a).

Every bit the potential approaches several kT of the standard potential, onset (East_onset) of oxidation is reached and the current exponentially increases (b) as the analyte is being oxidised at the working electrode surface. For a reversible procedure, here the current rises initially equally if there is no change in the concentration of oxidant. The current is dictated by the rate of diffusion of oxidant to the electrode, and the proportion converted to the reduced class according to the Nernst equation. Gradually, as the scan continues more oxidant is depleted, and the concentration gradient arrange to this. It is this change which causes a peak in the voltammogram, where the decrease in current from depletion of the oxidant outweighs the increase from changing the proportion of oxidant oxidised at the electrode.

The current reaches summit maximum at point c (anodic peak current (i_pa) for oxidation at the anodic peak potential (East_pa). Now, more positive potentials cause an increase in electric current that is offset by a decreasing flux of analyte from further and further distance from the electrode surface.

From this betoken the current is limited by the mass transport of analyte from the majority to the DDL interface, which is dull on the electrochemical timescale. This results in a decrease in current (d) as the potentials are scanned more positive until a steady-state is reached where further increases in potential no longer has an effect.

Scan reversal to negative potentials (reductive browse) continues to oxidise the analyte until the applied potential reaches the value where the oxidised analyte which has accumulated at the electrode surface can exist re-reduced (e).

The process for reduction mirrors that for the oxidation, only with an reverse scan direction and a cathodic peak (i_pc) at the cathodic peak potential (E_pc) (f). The anodic and cathodic top currents should be of equal magnitude but with opposite sign, provided that the procedure is reversible (and if the cathodic meridian is measured relative to the base line later on the anodic elevation).

The Randles-Sevcik equation

The peak electric current, i_p, of the reversible redox process is described by the Randles-Sevcik equation.[1]

At 298 Thou, theRandles-Sevcik equation is:

where n is the number of electrons, A the electrode expanse (cm²), C the concentration (mol·cm^-3), D the diffusion coefﬁcient (cm²·south^-1), and five the potential scan charge per unit (5·s^-1).

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Cyclic Voltammetry of Ferrocene

Ferrocene (Fc) is usually used as an internal standard for cyclic voltammetry, and its cyclic voltammogram tin therefore exist considered "typical". Every bit described above for the full general case, at the start of a cyclic voltammetry browse, a positively ramping potential (the frontwards sweep) is applied between the working and reference electrodes. As the potential increases, ferrocene (Fc) physically close to the working electrode is oxidised (i.east. loses an electron). This converts information technology to Fc⁺, and the movement of the electrons creates a measurable electrical current.

Equally united nations-reacted Fc diffuses to the electrode and continues the oxidation process, the electric electric current is increased and at that place is a build upwards of Fc⁺ at the electrode. This build upwardly of Fc+ and depletion of Fc is called the the diffusion layer, and furnishings the charge per unit at which un-reacted material can reach to the electrode. Once the diffusion layers reaches a certain size, the diffusion of Fc to the electrode slows down, resulting in a decrease in the oxidation charge per unit and thus a decrease in electrical current.

Cyclic voltammogram of Ferrocene — Cyclic voltammogram of ferrocene in the Ossila Potentiostat PC software

When the potential ramp switches direction, the process reverses and the reverse sweep begins. Fc⁺ close to the working electrode reduces (i.e., gains an electron), converting it back to Fc. The electric current flows in the opposite direction, creating a negative electric current. The Fc⁺ diffuses to the electrode, reducing to Fc and resulting in a increment in the negative current.

Experimental Setup

The experimental setup for cyclic voltammetry consists of an electrochemical cell containing five major components.

The working electrode, where the compound of interest is reduced (Cn+ → C(north−i)+ ) or oxidised (Cn+ → C(northward+1)+).
The counter electrode, which completes the circuit with the potentiostat (see figure below).
The reference electrode, used to mensurate the potential.
The studied solution containing the chemical to be studied.
The reference electrode solution (optional, meet choice ofreference electrode).

The potential of the studied solution is measured relative to the potential between the reference solution and reference electrode.

The Electrochemical Jail cell

An electrochemical cell is a device in which a chemical reaction generates an electric response or, conversely, an electrical current is used to trigger a chemic reaction. The simplest possible electrochemical cell consists of 2 connected electrodes in an electrolyte solution. In cyclic voltammetry, iii electrodes are used.

The physical setup of an electrochemical cell is relatively simple. The working and counter electrodes sit in an electrochemical solution, and the reference electrode sits in a separate tube within the prison cell containing the reference solution. The reference electrode tube should be approximately ii thirds full - a syringe and needle can be used to add the solution.

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Degassing

In addition to having holes for each electrode, electrochemical glassware typically has gas intakes to permit for an inert gas (ordinarily nitrogen or argon) to be bubbled through the solution to remove its oxygen. This procedure is known every bit degassing.

Degassing is of import because molecular oxygen is electrochemically active, and if not removed will create unwanted redox processes. In improver, the products of this reaction (hydrogen peroxide) tin can likewise collaborate with the compound and further interfere with the results of the experiment.

In one case the oxygen has been removed, it can be kept out with a continuous stream of inert gas.

Adventure of contamination

When preparing an electrochemical cell, it is important to minimise the gamble of any contamination with water, equally water can form reactive species when reduced or oxidised. This tin can be washed by heating the components in a glassware oven prior to utilize.

Electrochemical solutions

The electrochemical solution used for cyclic voltammetry typically consists of three components.

The chemical compound of involvement (x^-3 – x^-5 Yard)
An electrolyte (0.ane Grand)
A solvent which dissolves both the compound of involvement and the electrolyte

Solvent pick

The choice of solvent and electrolyte is dictated by the solubility of the studied chemical (and so that it tin be dissolved at the concentration needed) and the desired potential range.

Reference tables of the potential range of diverse solvent and electrolyte pairs are widely available [1] only these ranges are highly dependent on the purity and dryness of both the electrolyte and solvent. For the all-time results, choose a loftier purity solvent and electrolyte and oven dry all components before utilize.

Be aware when manually purifying and drying your components by standard procedures [2] that Grubbs purification apparatuses tin add undesirable electroactive impurities [three].

Solvent reference table

A brusque table of potential ranges is listed beneath based on the values given past A.J. Bard and L.R. Faulkner [2]. Values are given relative to the Standard Calomel Electrode (SCE) (see option of reference electrode).

Electrode	Solvent	Electrolyte	Positive Range Relative to SCE / Five	Negative range Relative to SCE / V
Pt	Water	1 M H₂SO_iv	+ i.iii	− 0.3
Pt	Water	pH 7 buffer	+ i.0	− 0.7
Pt	Water	1 M NaOH	+ 0.half dozen	− 0.9
Hg	Water	1 1000 H₂SO_four	+ 0.3	− 1.1
Hg	H2o	1 M KCl	+ 0.0	− ane.9
Hg	Water	i M NaOH	− 0.ane	− ii.0
Hg	Water	0.1 One thousand Et₄NOH	− 0.1	− 2.4
C	Water	1 Grand HClO₄	+ 1.5	− 0.ii
C	H2o	0.1 Grand KCl	+ one.0	− i.3
Pt	MeCN	0.1 Thou TBANF₄	+ 2.five	− 2.5
Pt	DMF	0.1 M TBAP	+ 1.five	- 2.8
Pt	Benzonitrile	0.one One thousand TBANF₄	+ 2.5	− 2.four
Pt	THF	0.1 M TBAP	+ 1.four	− 3.one
Pt	PC	0.1 Thousand TEAP	+ 2.two	− 2.5
Pt	CH₂Cl₂	0.1 M TBAP	+ 1.8	− 1.7
Pt	Then₂	0.one Yard TBAP	+ 3.four	− 0.0
Pt	NH₃	0.1 Yard KI	+ 0.1	− 3.0

Insoluble chemicals

It is possible to written report the electric response of materials, like polymers, which cannot be sufficiently dissolved in standard electrochemical solvents. To practise this, glaze the working electrode with the material by depositing it with a solvent.

Due to the complex nature with which charges diffuse through the solid and the various distortions which occur within the deposited compound, the normal equations and mathematical proofs do not strictly apply nether these circumstances. By approximating the onset potential as the redox potential of that procedure, still, the technique withal gives a good approximation of the energy levels for insoluble materials.

Internal standards

Internal standards, usually ferrocene (come across below), are ofttimes used to calculate the value of the oxidation and reduction potentials. Internal standards are compounds which oxidise or reduce in solution, ideally somewhat independently of the organization (although ferrocene does vary betwixt solutions).

This oxidation or reduction provides a voltammogram which can be used to reference the position of the oxidation or reduction of the compound of involvement.

It is common exercise to study these standards immediately after the chemic of interest, using the same solutions. Recent reviews, even so, suggest that it is ameliorate to ever have the internal standard nowadays in society to prevent changes in the position of the voltammograms [6]. This is particularly true for quasi reference electrodes where large shifts accept been observed.

Why do we apply supporting electrolyte for cyclic voltammetry?

Inert ions are added to the electrochemical solution in tooth excess to the analyte in order to provide enough ionic strength to the solution for information technology obey the Nernst equation. The backlog of electrolyte decreases the thickness of the lengthened double layer so that the applied potential decreases to a negligible level within nanometers of the working electrode surface. The issue is that the current response at the electrode surface is well defined.

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Working and counter electrodes

The counter electrode and working electrode must be conductive so that charges can motility to and from the solution, and they must not cause any chemical reaction in the solution. Inertness is usually achieved by making them out of unreactive cloth such as platinum.

A big counter electrode surface area makes sure that the measured current corresponds to the current flow between the working and counter electrode [5].

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Pick of reference electrode

Reference electrodes are designed so that an equilibrium is setup with known potential between the metal wire and the surrounding solution. In cyclic voltammetry, all electrochemical processes occur relative to this potential.

The reference electrode is setup in the cell and so that it is in a excursion with the reference electrode and working electrode in opposing directions. In one management, the working electrode goes from the solid state into the solution and the reference electrode goes from the solution to the solid state.

The outcome of this (along with Kirchhoff's voltage constabulary and cypher solution resistivity) is that the measured potential is aught when the working electrode potential is equal to the reference electrode potential.

The about common reference electrodes are the standard calomel electrode, the normal hydrogen electrode, the silverish/argent chloride (Ag/AgCl) electrode in saturated potassium chloride and the Ag/Ag⁺ (0.01M, normally AgNO₃) electrode in acetonitrile. Their standard reduction potentials are listed beneath.

Information technology should be noted that the Ag/Ag⁺ electrode is usually setup with the same electrolyte solution that is used in the studied solution. This is to minimise the junction potentials (the potential between the reference solution and the studied solution). Take this into consideration when choosing your electrolyte and your solvent every bit well every bit when estimating the book of solution that you require for your experiment.

Electrode	Standard reduction potential / eV
Normal Hydrogen Electrode	0.000 (by definition)[1]
Standard Calomel Electrode	0.242[1]
Ag / Ag⁺ 0.01 Grand (ordinarily AgNO₃) in CH₃CN	Variable dependent on setup[5]
Ag/AgCl, KCl(saturday. in H₂O) *	0.197[1]

* Note: the AgCl coats the argent electrode

The reference electrode is setup so that the reference solution is separated from the studied solution via a frit.

This arrangement allows an electric connectedness which permits a measurement of voltage. The tiresome movement of liquid through the frit (a porous glass membrane that allows liquid to flow through it at a deadening rate) reduces the mixing of the reference solution and the studied solution to a minimum.

Fifty-fifty with the use of a frit, however, some mixing to be expected. For this reason, an Ag/Ag⁺ electrode is sometimes favoured over the Ag/AgCl, KCl(sat. in H₂O) as the Ag/AgCl, KCl(sat. in H₂O) will slowly leak water over time and water impurities in the studied solution atomic number 82 to the narrowing of the potential window.

In addition, the AgCl in solution may likewise be reactive to certain studied chemicals. A double frit can be employed to prevent this, with an interior reference solution and an exterior studied solution separated from the bulk studied solution. This prevents the studied solution almost the working electrode from being contaminated with h2o.

Annotation: Frits should always be stored in liquid between uses to foreclose deposition. Never shop a frit in air.

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The quasi reference electrode

An alternative reference electrode in cyclic voltammetry experiments is the quasi reference electrode (also known as a pseudoreference electrode). This is a reference electrode (usually argent wire) which does not have a surrounding solution with ions to grade the half equation.

Considering the potential of this reference electrode is not defined by ions of known concentration, the utilise of an internal standard such as ferrocene is vital. In add-on, because the point which is being referenced against can shift depending on the contents of the solution, it is important that the internal standard is present during the reduction / oxidation of the studied chemical.

There are some disadvantages to using a quasi reference electrode. While they can reproduce the results of a standard reference electrode and are much easier to setup, they are also much more susceptible to potential drift [vi].

A large standard difference has also been reported when using a quasi reference electrode. This can exist reduced by separating the electrode from the rest of the solution using a frit (with the reference solution the same as the studied solution) [6] [7].

Circadian Voltammetry Applications

For applications, please see 'Cyclic Voltammetry Applications and Voltammograms'.

Like Electrochemical Methods

Broadly speaking, voltammetric techniques can be categorised equally being either sweep blazon or polarography-like. The old refers to methods like cyclic voltammetry where the solution is not stirred later each prepare potential, and the latter refers to techniques where it is. Other types of voltammetry change these methods, for example, with the utilize of a rotating electrode.

The types of voltammetry folio gives more data on the advantages, disadvantages, and applications of each technique.

Sweep type methods

Cyclic voltammetry
Linear sweep voltammetry
Staircase voltammetry

Polarography similar methods

Conventional polarography
Normal pulse voltammetry
Chronoamperometry
Opposite pulse voltammetry
Differential pulse voltammetry
Squarewave pulse voltammetry

Other types of voltammetry

Rotated electrode voltammetry
Stripping voltammetry
Ultramicroelectrodes
Electrode impedance spectroscopy
Alternating current polarography
Alternating electric current voltammetry
Ultramicroelectrodes

Circadian voltammetry remains the most widely used voltammetric technique due to its speed, range of uses, and the ease with which the data can exist analysed.

References

Sevćik, A. Collection of Czechoslovak Chemical Communications 1958, xiii, 349
A. L. Bard and Fifty. Faulkner Electrochemical methods: Fundamentals and Applications, 2nd ed. John Wiley & Sons 2001
Due west. L. G. Armarego and C. L. L. Chai Purification of Laboratory Chemicals, 7th ed. Butterworth-Heinemann 2012
L. J, Fifty. B, and P. G Advanced Practical Organic Chemistry, 3rd edition. Manipal: Routledge 2013
J. L. Brédas, R. Silbey, D. Southward. Boudreaux, and R. R. Risk Chain-Length Dependence of Electronic and Electrochemical Properties of Conjugated Systems: Polyacetylene, Polyphenylene, Polythiophene, and Polypyrrole J. Am. Chem. Soc., vol. 105, no. 22, pp. 6555–6559, 1983
N. Elgrishi, K. J. Rountree, B. D. McCarthy, E. S. Rountree, T. T. Eisenhart, and J. Fifty. Dempsey A Practical Beginner's Guide to Cyclic Voltammetry J. Chem. Educ., vol. 95, no. 2, pp. 197–206, 2018
Yard. A. Snook, A. S. Best, A. G. Pandolfo, and A. F. Hollenkamp Evaluation of a Ag/Ag⁺ reference electrode for use in room temperature ionic liquids Electrochem. commun., vol. 8, no. 9, pp. 1405–1411, 2006
Five. M. Hultgren, A. W. A. Mariotti, A. Yard. Bond, and A. G. Wedd Reference potential calibration and voltammetry at macrodisk electrodes of metallocene derivatives in the ionic liquid [bmim][PF6] Anal. Chem., vol. 74, no. 13, pp. 3151–3156, 2002
J. Heinze, Angew. Chemie Int. Ed. English, 1984, 23, 831–918
K. A. Mabbott, J. Chem. Educ., 1983, 60, 697
R. Southward. Nicholson, Anal. Chem., 1966, 38, 1406
R. S. Nicholson and I. Shain, Anal. Chem., 1964, 36, 706–723
R. S. Nicholson, Anal. Chem., 1965, 37, 1351–1355
J. Heinze, B. A. Frontana-Uribe and Southward. Ludwigs, Chem. Rev., 2010, 110, 4724–4771

Contributing Authors

Harry Robson
Max Reinhardt
Chris Bracher

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