Know - How

Glossary

 Anode

The electrode where oxidation occurs in an electrochemical cell. By common convention, the current on the anode is considered a positive current; however, in electroanalytical chemistry  the anodic current is often considered negative.

 Auxiliary Electrode

See counter electrode 

Bipotentiostat

A potentiostat that controls the potential of two working electrodes independently in the same cell Mainly used for rotating-ring-disk electrodes.

Cathode

The electrode  where reduction occurs. It is negative in an electrolytic cell, but positive in a galvanic cell. By common convention, the current on the cathode is considered a negative current; however, in electroanalytical chemistry  the cathodic current is often considered positive.

Cell Voltage

The electrical potential difference between the two electrodes of an electrochemical cell. For a three-electrode cell, it is the voltage between the working electrode and the counter electrode. The cell voltage ishould not be confused with the working electrode potential.

Charge-Transfer Reaction

A chemical reaction where electrical charges are passed from one reactant to another. If the charge is passed from an electrode to a dissolved species, it is considered as a heterogeneous charge-transfer reaction.

Chronoamperometry

An electrochemical technique used for analysis or for kinetics studies. The potential of the working electrode of an electrochemical cell is rapidly changed by a rectangular pulse, and the resulting current is measured as a function of time. See Cottrell equation.

Chronocoulometry

An electrochemical technique used for analysis or for kinetics studies. The potential of the working electrode of an electrochemical cell is rapidly changed by a rectangular pulse, and the resulting charge is measured as a function of time.

Chrono - potentiometry

An electrochemical technique used for analysis or for kinetics studies. The current through the working electrode of an electrochemical cell is rapidly changed by a rectangular pulse, and the resulting potential is measured as a function of time. See Sand equation.

Corrosion Current Density

The corrosion current per area unit (current density) is directly proportional to the corrosion rate. See also Tafel plot.

Corrosion potential

The rest potential of a corroding material is also called corrosion potential.  It is a mixed potential with a value between the equilibrium potentials of the anodic and cathodic corrosion reactions.

Cottrell equation

The C. describes the relation between diffusion limited current and time in a chronoamperometry. It is valid if the potential jump is large enough to result immediately in limiting current.  It is applicable to planar electrodes in unstirred solutions only. Simplified, it says that i(t) t0.5 is a constant.

Coulometer

Also designed as Integrator: used to measure the electrical charge
Coulometry see Chronocoulometry

Counter electrode

The counter electrode is an auxiliary electrode which shall not react with the electrolyte. It is only used to force a current through the working electrode. It is made of inert materials (Pt, Au, Graphite, or platinised Ti). Also called auxiliary electrode.

Current Density

The electrochemical reaction rate is defined by the current I passing an electrode, divided by the electrode area: This is the current density (abbr. i)

Current Sink

A current sink is an ampere meter which has practically no internal resistance. Therefore, current sinks also are called "zero - ohm - ammeters". Current sinks are valuable if e.g. galvanic currents are to be measured without producing a voltage drop across the ammeter. So, they are capable to measure extremely small currents down below the pA - range.

Donnan Potential 

The electrical potential difference between two solutions separated by an semi-permeable wall (membrane) in the absence of any current flowing through the membrane.

Double-junction Reference Electrode

A common calomel (or AgCl) electrode is contained in another vessel containing a chloride-free solution (e.g. conc. KNO3) in order to prohibit chloride contamination of the main electrolyte.

Electrometer

An electrometer is a special kind of voltmeter, having extremely high internal resistance (typically > 1000 GOhms). It is used to measure equilibrium potentials.

Electroosmosis

E.o. is the movement of a liquid through a capillary tubing or porous solid driven by an electrical potential difference

Electrophoresis

E.p. is the movement of suspended particles in a liquid driven by an electrical potential difference. Used to deposit material on an electrically charged surface in an electrolyte (painting, ceramic coating).

Equilibrium Potential 

The equilibrium potential is the potential which is established at an electrode as long as a chemical equilibrium is established at the surface of the electrode, and no external current passes the electrode, also called "open circuit potential" OCP.

Galvanometer

A very sensitive ammeter used to measure currents in the range of A down to  pA and below.

Galvanostat

A galvanostat is a device used to control the electric current passing an electrode. Indeed, each potentiostat can act as galvanostat, depending on the manner to connect the cell. Many of our potentiostats however, can be switched from the potentiostatic to the galvanostatic mode without changing the cell connections.

Haber - Luggin Capillary

A salt bridge from the reference electrode to the main electrolyte with a capillary tip at one end. The tip is placed close to the working electrode to minimise IR drop error. Also called Luggin Probe.

Nernst Equation

The N.E. defines the equilibrium potential of an electrode. E = Eo + (RT/nF) * logQ, where Q is the quotient of  oxidised and reduced species.

NHE

Normal hydrogen electrode, another name for the standard hydrogen electrode

OCP

(abbr.) open-circuit potential.

Overvoltage

The difference between the cell voltage, while a current is passing the cell, and the open-circuit voltage

Polarization

The change of potential of an electrode from its equilibrium potential upon the application of a current. Confusingly, the term "polarisation" is also used in optics and biology, having another meaning then.

Polarization Curve

Another name for current-potential plot

Potentiostat

A potentiostat is a device used to keep a working electrode at desired potential with respect to a reference electrode. This is done by a current which is passed from the working electrode to a counter electrode. From a more electronic point of view, a potentiostat is a special kind of feed-back amplifier.

Potential Meter

Usually, reference electrodes have high source resistances: some kOhms up to MOhms. If you use a com­mon voltmeter with an internal resistance of some MOhms, appreciable potential errors may occur.

Potential meters therefore have input resistances of more GOhms.

Polarisation

Polarisation means to force the working electrode from its rest potential (or equilibrium potential) to another potential. 

Polarisation Resistance

The polarisation resistance is defined by the slope DE / DI of the current - potential curve in the vicinity of the rest potential. It depends on the electrode area and the rate of the potential shift (or current shift) used to measure the curve (see also impedance measurements)

Polarography

An electroanalytical technique, introduced by J. Heyrovsky, for which he was awarded the Nobel Prize for Chemistry in 1959. It is basically 2-electrode sweep voltammetry using a dropping-mercury electrode for working electrode and a mercury pool as counter electrode.

Potential

The voltage difference to a standard level is called potential (abbr. E). In electrochemistry, all potentials refer to the reaction 2H+ + 2 e-   ---> H2 at standard conditions (1 molar concentration, 1 bar pressure, 20C).

Potentiokinetic Technique

Alternative name for linear-sweep voltammetry, mainly used for obtaining polarisation curves at low dE/dt rates.

Potentiometry

Measurement of the electromotive force between a working electrode and a reference electrode.

Potentiostat

An electronic device controlling the electrical potential between the working electrode and a reference electrodes of a three-electrode cell to a selected value. This condition requires that current is passed through the working electrode. The potentiostatic condition can be established as long as the cell voltage can be established and the required current can be delivered by the potentiostat.

Reference electrode

A reference electrode is an electrode which always has a constant electrical potential (you may also say voltage difference) with respect to the standard hydrogen electrode as the reference point for all electro­chemical potentials.

Rest Potential

The potential of a electrode is called the rest potential (also called open circuit potential), if no externally forced current passes this el


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