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Packed-bed electrode: An electrode assembly
consisting of loosely packed small particles of the electrode material (e.g.,
some metal or carbon) with the electrolyte flowing through the bed. This type
of electrode is especially useful for removing small traces of impurities from
the solution by electrolysis (e.g., waste treatment) because the solution is
well stirred and it contacts a large surface of the electrode material.
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Paint electrodeposition: See electrophoretic
deposition.
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Parallel connected: A method of connection
in which positive terminals are connected together and negative terminals are
connected together. Current output adds and voltage remains the same. (See also
series connected.)
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Parallel-coupled cells: Individual electrochemical
cells can be combined in assemblies by parallel or series coupling (or a combination
of the two). In case of "parallel" coupling, the positive electrode of every
cell is connected together and the negative electrode of every cell is connected
together, resulting in two external terminals. The voltage of every cell must
be identical in parallel coupled assemblies. The overall current passing through
the assembly is the sum of the individual cell currents, while the assembly
voltage is identical to the individual cell voltage. Parallel coupling is often
used in batteries. Cell lines and stacks can also be parallel coupled. Contrast
with series coupling.
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Partial current (density): The two current
densities at which the electrode reaction is proceeding in the anodic and cathodic
directions at an electrode potential. The actual (net) current density is the
algebraic sum of the two partial current densities (one is considered positive
the other negative). Electrode reactions are typically "chemically" reversible,
that is, they can proceed both in forward and reverse direction. At equilibrium,
the reaction is proceeding at equal rate in both directions (see exchange current
density): the "anodic partial current density" and the "cathodic partial current
density" are equal and the net current density is zero. When the electrode is
polarized, the partial current densities are unequal and the net current density
is not zero. If the electrode is negatively polarized, the cathodic reaction
speeds up (compared with its rate at equilibrium), while the anodic reaction
slows down and a net cathodic current density results (and vice versa for anodic
polarization).
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Partial reaction: See electrode reaction.
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Passivation: The formation of a thin
adherent film or layer on the surface of a metal or mineral that acts as a protective
coating to protect the underlying surface from further chemical reaction, such
as corrosion, electrodissolution, or dissolution. The passive film is very often,
though not always, an oxide. A passivated surface is often said to be in a "passive
state." The surface oxidation can result from chemical or electrochemical (anodic)
oxidation. During anodic passivation, in the current-potential plots, the current,
instead of increasing with potential, falls to a very small value. See passivation
potential.
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Passivation potential: The most negative
electrode potential at which a passivating film is formed electrochemically.
It is equal to or more positive than the equilibrium potential of formation
of the compound (usually oxide) constituting the passive film. Usually the current
goes through a maximum at the passivation potential. Also called the "Flade"
potential.
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Passivity: The condition
of a metallic material corresponding to an immeasurably small rate of corrosion.
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PEM: Stands for either polymer-electrolyte
membrane or proton-exchange membrane.
-
Permselectivity: The permeation of certain
ions in preference to other ions through an ion-exchange membrane.
-
pH: A measure of the acidity/alkalinity
(basicity) of a solution. The pH scale extends from 0 to 14 (in aqueous solutions
at room temperature). A pH value of 7 indicates a neutral (neither acidic nor
basic) solution. A pH value of less than 7 indicates an acidic solution, the
acidity increases with decreasing pH value. A pH value of more than 7 indicates
a basic solution, the basicity or alkalinity increases with increasing pH value.
The pH of a solution is equal to the negative, ten-based logarithm of the activity
of the hydrogen ions in the solution. Neutral water dissociates into equal amounts
of hydrogen (H+) cations and hydroxyl (OH-) anions. As
the product of the concentrations (activities) of the two ions is always a constant
10-14, water has a pH of 7. In acidic solutions the hydrogen ions are in excess,
while in basic solutions the hydroxyl ions are in excess.
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pH buffer: See buffer solution.
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pH electrode: An electrode assembly
with a pH dependent potential. A variety of different electrodes can be used
for this purpose, the most common one is the glass electrode.
-
pH meter: Volt meter that measures the
electrical potential difference between a pH electrode and a reference electrode
and displays the result in terms of pH value of the sample solution in which
they are immersed.
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Photoelectrochemical cell: A galvanic
cell in which usable current and voltage are simultaneously produced upon absorption
of light by at least one of the electrodes. Also called "photogalvanic cell."
-
Photoelectrochemistry: Chemistry resulting
from the interaction of light with electrochemical systems. See also photoelectrochemical
cell and photoelectrolytic cell.
-
Photoelectrolytic cell: An electrolytic
cell in which the production of chemicals is caused by or speeded up by the
absorption of light by at least one of the electrodes. The process occurring
in such cell is called "photoelectrosynthesis."
-
Photoelectrosynthesis: Production of
chemicals in a photoelectrolytic cell, where the production is caused by or
speeded up by the absorption of light by at least one of the electrodes.
-
Photogalvanic cell: See photoelectrochemical
cell.
-
Photovoltaic cell:
The semiconductor device that converts light into dc electricity.
The building block of photovoltaic modules.
-
pH standard: See buffer solution.
-
Pickling: Process for removal of oxide
scales from metal surfaces in preparation for electroplating. Typically, the
metal is immersed in hot, strongly acidic solution that dissolves the oxide
scales. The solution usually also contains some corrosion inhibitor to avoid
dissolution of the metal itself. See also electrolytic pickling.
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Pile: An archaic name for a battery
or other series-coupled electrochemical cells. See, e.g. the voltaic pile.
-
Plate: In the terminology
of secondary batteries, this has the same meaning as 'electrode'.
-
Plating: See electroplating.
-
Platinized platinum electrode: A platinum
metal electrode that is covered with a rough, large surface area platinum coating.
The purpose is to produce an electrode with a large true area that will be relatively
non polarizable. See also platinum black.
-
Platinum black: A rough, large surface
area platinum coating usually deposited on a platinum metal electrode. See also
platinized platinum electrode.
-
Polarizable electrode: An electrode
that is easily polarizable. That is, the potential of the electrode will change
significantly from its equilibrium potential with the application of even a
small current density. The reason for this behavior is that the electrode reaction
is inherently slow (has a small exchange current density). See also overpotential
and ideal polarized electrode. Opposite: non-polarizable electrode.
-
Polarization:
The change of potential of an electrode from its equilibrium potential upon
the application of a current. See overpotential for a more detailed description.
Somewhat confusingly, the term "polarization" is often also used in place of
overvoltage. Deviation from equilibrium conditions in an electrode or galvanic
cell caused by the passage of current. It is related to the irreversible phenomena
at the electrodes (electrode polarization) or in the electrolytic phase (concentration
polarization).
-
Polarization loss: Reduction
in the voltage of a cell delivering current from its equilibrium value. : positive:
Positively charged electrode, usually of a secondary cell; acts as cathode during
discharge and anode during charge.
-
Polarization curve: Alternative name
for current-potential plot.
-
Polarogram: The graphical representation
of the result of polarography.
-
Polarograph: An instrument used in carrying
out polarographic analysis.
-
Polarography: A classical electroanalytical
technique discovered in 1922 by J. Heyrovsky, for which he was awarded the Nobel
Prize for Chemistry in 1959. Essentially, it is linear-sweep voltammetry using
a dropping-mercury electrode for working electrode and a large mercury pool
as counter electrode. See also Ilkovic equation.
-
Pole: Alternative name of a terminal.
-
Polymer-electrolyte membrane: An ion-exchange
membrane that is used both as a "separator" and as the electrolyte in some fuel
cells. Abbreviated as "PEM."
-
Porous electrode: An electrode consisting
of a highly porous solid. This is often used in fuel cells with gaseous reactants.
In this case, the charge-transfer reaction proceeds mainly at the triple interface
formed by the electrode material, the electrolyte, and the gaseous reactant.
The pores of the structure are partly filled by the electrolyte and partly by
the gas. A porous electrode provides a much larger area for reaction than a
solid electrode with the gas bubbled around it. A porous electrode can also
be used as a flow-through electrode.
-
Post: See terminal.
-
Pot: An alternative name of an industrial
electrolytic cell used in aluminum production.
-
Potential: See electrical potential.
-
Potential of zero charge: The electrode
potential where the charge in the electrical double layer is zero. Abbreviated
as "pzc."
-
Potential-pH
diagram: A diagram often used in the field of corrosion to indicate
the corrosion tendency and stability of a metal in aqueous solutions. The equilibrium
potential of the metal is plotted against the pH of the solution, usually for
a series of concentrations of the metal ion. The curves demarcate potential-pH
domains where a species of the metal is predominant in equilibrium, this can
be the metal, its ion, oxide, or hydroxide. In simplified version, the diagram
can indicate the potential-pH domains where the metal is immune to corrosion,
corrodes, or passivates. The diagrams must be used with some caution because
they represent equilibrium conditions and the corrosion tendency is also influenced
by kinetic effects. Also called "Pourbaix diagram." See also an Encyclopedia
Article.
-
Potential-ramp technique: Alternative
name for voltammetry.
-
Potential-step voltammetry: Alternative
name for chronoamperometry.
-
Potential-sweep technique: Alternative
name for voltammetry.
-
Potentiodynamic technique: Alternative
name for linear-sweep voltammetry. This expression is primarily used in the
field of corrosion.
-
Potentiokinetic technique: Alternative
name for linear-sweep voltammetry. This expression is primarily used in the
field of corrosion.
-
Potentiometer: Can be used in more than
one meaning: 1. A continuously variable resistor. More precisely, a resistor
with continuously variable tap. This can provide three resistance values, a
fixed resistance between the two end connectors, and two variable resistances,
one between either end connector and the variable tap connector. The sum of
the two variable resistances is the fixed resistance. 2. A somewhat archaic
measurement system, based on a resistor with a continuously variable tap, that
can be used to measure the electromotive force of electrochemical cells that
can be easily polarized by current. It uses a comparison technique to compare
a "standard" voltage source to the unknown, under conditions of practically
zero current. It is seldom used today because high input resistance voltmeters
and electrometers are readily available.
-
Potentiometry: An electroanalytical
technique based on the measurement of the electromotive force of an electrochemical
cell comprised of a measuring and a reference electrode. The simplest example
of a measuring electrode is a metal electrode whose potential depends on the
concentration of the cation of the electrode metal (see Nernst equation).
-
Potentiostat:
An electronic instrument that controls the electrical potential between the
working and reference electrodes of a three-electrode cell at a preset value.
It forces whatever current is necessary to flow between the working and counter
electrodes to keep the desired potential, as long as the needed cell voltage
and current do not exceed the compliance limits of the potentiostat.
-
Potentiostatic technique: An electrochemical
measuring technique for electrochemical analysis or for the determination of
the kinetics and mechanism of electrode reactions based on the control of the
electrode potential.
-
Pourbaix diagram: See potential-pH diagram.
-
Power: See electrical power.
-
Power density: Characteristic parameter
of a battery indicating its electrical power per unit weight or volume. The
terminology is not strictly defined. Weight based power density is often called
"specific power" or "gravimetric power density." Volume based power density
is often called "power density" or "volumetric power density. The power density
is typically expressed as watt/kilogram or watt/liter.
-
Power source (supply): See electrical
source (supply).
-
Preparative electrochemistry: See electrosynthesis.
-
Primary battery:
A cell or battery whose useful life is over once its reactants have been
consumed; i.e. One not ed to be recharged.
-
Primary current distribution: A current
distribution that is completely controlled by the resistivity of the electrolyte
solution between the working and counter electrodes. The current always follows
the least resistive path; consequently, a non-uniform current distribution will
result if the geometry of the electrodes is such that the resistivity of the
current path is not the same to every point on the working electrode. Every
other effect that may influence the current distribution is ignored in this
case, or assumed to be negligible. See also secondary and tertiary current distribution.
-
Proton: See atomic structure.
-
Proton-exchange membrane: An ion-exchange
membrane that is used both as a "separator" and as the electrolyte in some fuel
cells. Abbreviated as "PEM."
-
PZC: Stands for potential of zero charge.
