# Electrochemistry Dictionary - P

• 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.

• Paint electrodeposition: See electrophoretic deposition.

• 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.)

• 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.

• 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).

• Partial reaction: See electrode reaction.

• 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.

• 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.

• Passivity: The condition of a metallic material corresponding to an immeasurably small rate of corrosion.

• 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.

• pH buffer: See buffer solution.

• 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.

• 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.

• 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.