Electrochemical Terms Defined

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Activity, a
This is a form of concentration; it is the concentration perceived thermodynamically at an electrode.

Activity coefficient, y±
The ratio of activity a and concentration c, i.e., y± = a / c .

A device for measuring current.

The techniques and methodology of determining a concentration as a function of current; the most popular amperometric measurement technique is voltammetry.

The material to be quantified, characterized or in any other way investigated.

A negatively charged ion.

The electrode at which oxidation occurs during electrolysis.

Anodic Current
An anodic current is the flow of electrical charge (usually carried by electrons) into a working electrode from a second phase (usually an electrolyte solution) as a result of the oxidation of one or more species in the second phase.

Auxiliary redox couples (see Mediation)

Banana Cable
A banana connector (commonly banana plug for the male, banana socket or banana jack for the female) is a single-wire (one conductor) electrical connector used for joining wires to equipment. It is a type of patch connector which is very useful for making connections between electrochemical equipment, electrochemical cells, and other electronic instrumentation The plug is a four-leafed spring tip that snaps into the jack. The plugs are frequently used to terminate patch cords for electronic test equipment.

Banana Connector
Banana connectors are commonly used to make simple and quick connections between scientific instruments. Patch cables terminated with banana plugs (male) fit into banana jacks (female) on instrument panels. The conductive metal portion of the banana plug is about an inch long and bulges slightly to provide a secure mechanical fit when inserted into a banana jack.

Banana Jacks (See Banana Connector)

Banana Plugs (See Banana Connector)

The electrode at which reduction occurs during electrolysis.

Cathodic Current
A cathodic current is the flow of electrical charge (usually carried by electrons) out of a working electrode into a second phase (usually an electrolyte solution) leading to the reduction of one or more species in the second phase.

A positively charged ion.

Channel cell
An electrochemical cell in which analyte solution flows at a velocity Vf over flat stationary electrode(s).

Charge, Q
The quantity of unbalanced electricity in a body such as an electron or an ion.

Charge Density, q
The quotient of charge Q and electrode area A, i.e., q = Q/A .

Chemically modified electrode (CME)
An electrode covered with a special coating (of a conductor or semiconductor unless very thin), thereby altering its electrochemical properties.

The techniques and methodology of studying current as a function of time.

That form of mass transport in which the solution containing electroanalyte is moved: natural convection occurs predominantly by heating of
solution, while forced convection occurs by careful and deliberate movement of the solution, e.g., at a rotated disc electrode or by the controlled flow of
analyte solution over a channel electrode.

A device for measuring the charge Q.

The techniques and methodology of measuring the charge Q.

Counter electrode (CE)
The third electrode in voltammetry and polarography, where the current is measured between the counter and working electrodes. It is rare to monitor the potential of the counter electrode.

Current, I
The rate of charge flow or passage, i.e., I = (dQ/dt).

Current density, i
The quotient of current I and electrode area A, i.e., i = I / A .

Current maximum suppresser
A chemical, usually a surfactant (detergent), added to polarography solutions in order to decrease the incidence of polarographic “peaks”.

Cyclic voltammogram (CV)
A plot of current (as “y” ) against potential (as “x“) obtained during a voltammetric experiment in which the potential is ramped twice, once forward to the switch potential Eλ, and then back again.

Depletion region (depletion layer)
That layer around the electrode within which analyte is consumed during current flow.

Differential pulse voltammetry
A form of voltammetry in which a linear potential ramp of dE/dt is applied to the working electrode, superimposed on which is a succession of pulses.

That form of mass transport in which motion occurs in response to a gradient in concentration or composition, itself caused by a gradient of the
chemical potential p. Diffusion is ultimately an entropy-driven process.

Diffusion coefficient, D
A measure of the velocity of electroanalyte as its diffuses through solution prior to an electron-transfer reaction.

Diffusion current, Id
The maximum current in polarography.

Double-layer capacitance
The capacitance C owing to the two Helmholtz layers at the electrode 1 solution interface.

Dropping mercury electrode (DME)
Historically, a popular choice of working electrode in polarography.

An electroanalytical or electrochemical measurement accompanied by current flow and hence by changes in the concentrations of analyte.

An analyte investigated by electrochemical means.

Electrochemical area
The fractal area of an electrode; the area that an electrode is “perceived” to have.

Electrochemical cell
A pair of electrodes (usually metallic) in contact with a solution containing an electrolyte.

The electrochemical generation of color in accompaniment with a redox reaction, e.g., as displayed by a redox indicator.

A conductor employed either to determine an electrode potential (at zero current, i.e., for potentiometric experiments), or to determine current
during a dynamic electroanalytical measurement. The electronic conductivity of most electrodes is metallic.

Electrode potential, E
The energy, expressed as a voltage, of a redox couple at equilibrium. E is the potential of the electrode when measured relative to a standard (ultimately the SHE). E depends on temperature, activity and solvent. By convention, the half cell must first be written as a reduction, and the potential is then designated as positive if the reaction proceeds spontaneously with respect to the SHE. Otherwise, E is negative.

An ionic salt to be dissolved in a solvent, or the solution formed by dissolving an ionic salt in a solvent.

The process of altering a molecule, ion, etc. of analyte by the passing of a current. The simplest examples are reduction or oxidation

Electron mediation (see Mediation)

The potential that develops between two electrodes in an electrochemical
cell when a state of frustrated equilibrium has been reached; the value of the
emf is always defined as positive (emf derives from the somewhat archaic
term “electromotive force”).

In an electroanalytical context, equilibrium is best defined as zero current during a potentiometric measurement.

Equivalent circuit
In impedance analyses, a collection of electrical components used to mimic the frequency behavior of a cell or electrochemical system.

ex situ
Measurement performed away from the site of formation (i.e., afterwards).

Exchange current, l0
The rate constant of electron transfer (expressed as a current) at zero overpotential.

Exchange current density, i0
The quotient of exchange current l0 and electrode area A, i.e., i0 = l0/A.

Exhaustion (exhaustive electrolysis)
The complete removal of analyte, for example, during stripping.

Faradaic charge
That component of the total charge Q that follows Faraday’s laws; the charge that does not follow these laws is termed “non-faradaic”.

Faradaic efficiency
The quotient of faradaic current to total current passed.

Flow cell
Electrochemical cell in which analyte solution flows at a constant velocity V through stationary tubular electrode(s).

Flux, j
The amount of electroanalyte that reaches an electrode surface (from the solution bulk), usually expressed in moles per unit time.

Formal electrode potential, E0′
An electrode potential measured at STP, in which all reagents and products are present at unit concentration.

Fractal area
The area, e.g., of an electrode, that is greater than the simple geometric area on account of surface roughness.

Glassy carbon (GC)
A form of carbon that is extremely hard and highly conductive to electrons, and is hence a good choice of material for constructing inert electrodes (also known as ‘vitreous’ carbon).

Gran plot
A commonly employed multiple-addition method, used to correct for unknown amounts of contaminant and for dilution errors.

Half-cell reactions
Oxidation or reduction reactions occurring at a single electrode in a cell.

Half-wave potential, El½
An electrode potential from polarography that is characteristic of the analyte .

 Hanging mercury-drop electrode (HMDE)
A commonly employed working electrode in polarography. The HMDE is often preferred to the experimentally simpler dropping mercury electrode (DME) because resultant polarograms do not have a “sawtoothed” appearance and because accumulation (for “stripping” purposes) is readily achieved at its static surface.

A process occurring between phases, as for example, electron transfer between a solid electrode and a solution-phase analyte.

A process occurring in one phase, as for example, electron transfer during a potentiometric titration.

AC resistance.

in situ
Measurement performed in the same place as the site of formation (i.e., performed at the same time).

Indirect coulometry (see Mediation)

Indium-tin oxide (ITO)
Tin-doped indium oxide, used as a thin solid film on glass when constructing optically transparent electrodes.

Inert electrode
An electrode which does not comprise any part of the redox couple under investigation. The best examples are platinum, gold or glassy carbon.

In electrochemistry and electroanalysis, the region between an electrode and the phase containing analyte (usually indicated with a vertical line, I).

A charged species; ions should always be regarded as solvated

Ion-selective electrode (ISE)
In potentiometry, an electrode having a nernstian response to one ion, ideally to the exclusion of others.

IR drop
A potential developed when a current I flows through an electrochemical cell. It occurs as a consequence of the cell resistance R, and has a
magnitude IR (from Ohm’s law). The IR drop is always subtracted from the theoretical cell potential.

International Union of Pure and Applied Chemistry (IUPAC)
The body which defines terminology in chemistry and the other sciences.

Junction potential (see Liquid junction potential)

Kinematic viscosity, v
The quotient of viscosity ns and density ρ, i.e., \nu = ns/ρ

Limiting current, Ilim
Current that is directly proportional to the bulk concentration of analyte.

Linear-sweep voltammogram
A normal voltammogram, in which the voltage ramp stops at the switch potential (in contrast to a cyclic voltammogram, in which it does not).

Liquid junction potential, Ej
A potential developed across a boundary between electrolytes which differ in concentration or chemical composition. Ej is caused by differing rates of ionic migration across the boundary, thereby leading to a charge separation. This potential is usually some tens of millivolts, with the magnitude being variable, but it may be minimized by the use of a salt bridge connection containing, e.g., the electrolytes KCl or NH4NO3 (occasionally gelled in agar). With such a bridge, Ej will be about 1–2 mV.

Luggin capillary
A device for allowing the reference electrode to be positioned extremely close to the surface of the worlung electrode, thereby decreasing the IR drop.

Mass transport
The means of effecting movement of electroanalyte to an electrode prior to electromodification. The three forms (“modes”) of mass transport are convection, migration and diffusion.

The electrochemical generation of an oxidant or reductant which can then effect redox changes chemically.

Mediator titration
A titration in which the titer relates to a coulometric titration with a mediator.

Mercury film electrode (MFE)
A thin layer of elemental mercury, usually on graphite, prepared by electrochemical deposition from an aqueous solution of mercury(II).

An electrode of surface area 103 mm2 or less, used because its double-layer charging current is negligible.

That form of mass transport that occurs in response to coulombic attraction between charged ions and an electrode bearing a charge of the opposite
sign to that of the ions that move.

Nernst layer (Nernst depletion layer) (see Depletion region)

Obeying the Nernst equation (equation (3.8)) at all times.

Non-faradaic charge
That component of the total charge Q that does not follow Faraday’s laws; the charge that does follow these laws is termed “faradaic”.

Normal hydrogen electrode (see Standard hydrogen electrode)

Obeying Ohm’s law (equation (2.3)).

Ohmic drop see IR drop

Optically transparent electrode (OTE)
An electrode used for in situ spectroelectrochemistry .

Overpotential, η
Deviation of the potential at an electrode from the equilibrium potential, with the deviation usually being effected at a working electrode by a potentiostat.

The loss of electrons, resulting in the analyte having a higher oxidation number:

R → O + ne¯

for example:

Cu → Cu2+ + 2e¯, or 2Br¯→ Br2 + 2e¯

Peak current, Ip
In voltammetry and cyclic voltammetry, the current that is directly proportional to the bulk concentration of analyte.

A form of potentiostat.

The techniques and methodology of a form of voltammetry in vwhich the working electrode is liquid, elemental mercury, usually employed within a dropping mercury electrode (DME) or a hanging mercury-drop electrode(HMDE).

Potential “window”
That range of potentials in which it is safe and convenient to perform electroanalyses. Either the solvent or the electrolyte are likely to undergo electron-transfer reactions outside of this “window”.

A device for determining a potential (likely to be called a “voltmeter”).

The techniques and methodology of determining an activity as a function of potential (at zero current). Activity and concentration can often be interchanged at low ionic strength.

The instrument employed in dynamic electrochemistry such as voltammetry, allowing three electrodes to be used.

Quiescent (see Still solution)

Quiet solution (see Still solution)

A jargon term, meaning to smoothly increase at a constant rate of d(variable)/dt. A voltage ramp is therefore dE/dt.

Randles-Sevčik plot
A graph derived from the Randles-Sevčik equation, showing a plot of the cyclic voltammetry peak height Ip , (as “y“) as a function of v½ (as “x“), where v is the scan rate.

Random array of microelectrodes (RAM)
A microelectrode system comprising about 1000 carbon fibers embedded randomly within an inert adhesive such as an epoxy resin.

An abbreviation of “reduction and oxidation”; a type of reaction in which both reduction and oxidation occur concurrently.

Redox couple
Two redox states of the same material, e.g., Cu2+ and Cu. A couple sometimes comprises those ions which are necessary to maintain a balanced redox equation, e.g., AgBr + e¯ = Ag + Br¯.

Redox electrode
An electrode, the metal of which comprises one half of a redox couple.

Redox indicator
A chemical that changes color (is “electrochromic”) when undergoing redox change: since the change occurs at a specific potential, the substance is able to function as an indicator.

Redox potential (see Electrode potential)

The gain of electrons, resulting in the analyte having a lower oxidation number:

0 + ne¯→ R

for example:

Cu2+ + 2e‾→ Cu, or Br2 + 2e‾ + 2Br‾

Reduction potential (see Electrode potential)

Reference electrode (RE)
A constant-potential device (e.g., an SHE or SCE) used as a half cell of known potential.

Reinmuth notation
A useful shorthand notation for abbreviating the order of a sequence of electrochemical (“E”) and chemical (“C”) reactions. The order of a reaction is read from the left of the Reinmuth code to the right.

Rise time
The length of time needed for the double-layer at the electrodelsolution interface to charge (c. milliseconds).

Rotated ring-disc electrode (RRDE)
Disc electrode within a concentric ring electrode.

Salt bridge
A device employed to minimize the liquid junction potential Ej. The bridge connects two half cells, with one end being immersed in each. The
bridge is occasionally a piece of paper soaked in KCl or NH4NO3, or more commonly a glass U-tube filled with the same electrolytes, gelled with agar or
terminated at either end with a frit.

Sampled DC polarography
A form of polarography in which a linear potential ramp of dE/dt is applied to the working electrode, but where the current is measured only during the last 15% or so of each drop cycle.

Saturated calomel electrode (SCE)
A reference electrode in which the redox couple comprises elemental mercury and mercury(I) chloride, Hg2Cl2 (which also has the archaic name of “calomel”).

Scan rate, v
In cyclic voltammetry, the rate at which the potential of the working electrode is varied, i.e., \nu = (dE/dt). The value of \nu is always cited as positive.

A device having a response (ideally) for one particular analyte. Potentiometric sensors are typically ion-selective electrodes, while amperometric sensors rely on Faraday’s laws.

Silver paint
A conductive material comprising colloidal silver suspended in a polymeric base, which is liquid when applied to form an electrical contact, but then sets rapidly to form a hard and highly conductive contact. The material is commonly applied with a brush, hence the term “paint”.
The removal of oxygen dissolved in a solution of analyte, carried out, e.g., by bubbling purified nitrogen gas through the solution for ca. 10 min before analysis.

Square-wave voltammetry
A form of voltammetry in which the potential waveform applied to the working electrode is a square wave. Pairs of current measurements are made for each wave cycle. The current associated with the forward part of the pulse is termed Iforward, while the current associated with the reverse part is Ireverse
A square-wave voltammogram is then just a graph of the difference between these two currents as a function of the applied potential.

Standard electrode potential, EΘ
An electrode potential measured at standard temperature and pressure, where all products and reagents are present at unit activity.

Standard hydrogen electrode (SHE)
The standard against which redox potentials are measured. The SHE consists of a platinum electrode electroplated with Pt black (to catalyse the electrode reaction), over which hydrogen at a pressure of 1 atm is passed. The electrode is immersed in a solution containing hydrogen ions at unit activity (e.g., 1.228 mol dm–3 of aqueous HCl at 20°C).  The potential of the SHE half cell is defined as 0.000 V at all temperatures.

Still solution
Solution of analyte wholly free of convective effects.

(General), the techniques and methodology of analyte preconcentration used in voltammetry to improve the sensitivity; (specific), the electrochemical re-removal of analyte accumulated at an electrode.

Supporting electrolyte (see Swamping electrolyte)

Swamping electrolyte
An ionic salt added to a solution of analyte in order to minimize migration effects and increase the ionic conductivity.

Sweep rate (see Scan rate)

Switch potential, Eλ
In cyclic voltammetry, the potential at which the voltage ramp changes direction.

Tafel relationship
The equation which relates the current I and overpotential

η= a + b log i

Temperature voltage coefficient (see Voltage coefficient)

Theoretical cell potential
The algebraic sum of the individual redox potentials of an electrochemical cell at zero current, i.e., emf = Epositive elecvde – Enegative electrode. In practice, when current flows in a cell, a liquid junction potential is present, and the cell potential is larger than this theoretical value.

Transfer coefficient, α
A measure of the symmetry within the transition state “complex” formed during an electron-transfer reaction between an electrode and a moiety of electroanalyte. A wholly symmetrical complex has α = 0.5.

Voltage coefficient
In thermodynamics, the quantity (d emf /dT).

The techniques and methodology of measuring current as a function of applied potential.

A trace of current (as “y“) against applied potential (as “x“).

A device for measuring a potential. A good-quality as high an internal resistance (“impedance”) as possible.

Wall-jet electrode
Electrochemical cell in which analyte solution is squirted as high an internal resistance (“impedance”) as possible.

Window (see Potential window)

Working electrode (WE)
The principal electrode during dynamic analyses. The potential of the WE is measured with respect to a reference electrode, while the current passing through the WE is measured with respect to the counter electrode.

Glossary of Electrochemical Terms taken from Analytical Techniques in the Sciences