# Electrochemistry Dictionary - E

• Edison battery: A rechargeable battery developed by Edison. In the charged state, the active material of the positive electrode is nickel oxide while that of the negative electrode is metallic iron, with a basic (potassium hydroxide) electrolyte. During discharging, the nickel oxide is converted to a lower oxidation state oxide, while the iron is converted to iron oxide. It is still used today.

• Electrical current: See current.

• Electrical double layer: The structure of charge accumulation and charge separation that always occurs at the interface when an electrode is immersed into an electrolyte solution. (For a simple example see equilibrium electrode potential.) The excess charge on the electrode surface is compensated by an accumulation of excess ions of the opposite charge in the solution. The amount of charge is a function of the electrode potential. This structure behaves essentially as a capacitor. There are several theoretical models that describe the structure of the double layer. The three most commonly used ones are the Helmholtz model, the Gouy-Chapman model, and the Gouy-Chapman-Stern model.

• Electrical energy: A form of energy. It expresses the ability of an electrical source to carry out useful work or generate heat. E.g., this energy can be used to drive an electrical motor and carry out some mechanical work, or to generate heat with an electrical heater. The electrical energy is usually expressed in units of watt-hour, symbol: "Wh". See also electrical power.

• Electrical potential: The electrical potential difference between two point in a circuit is the cause of the flow of a current. It is somewhat analogous to the difference in height in a waterfall that causes the water to fall, or the difference in pressure in a pipeline that causes the gas to flow. In electrochemistry we typically cannot measure "absolute" potentials, only the "difference" of potential between two points. For similar concepts, see electromotive force (emf) and voltage. These terms are sometimes used interchangeably. However, in electrochemistry "emf" usually refers to the potential difference between the two electrodes of an electrochemical cell when there is no current flowing through the cell, "voltage" refers to same with current flowing, and "potential" is usually used in connection with electrodes (see electrode potential). The measurement unit of the potential is the volt.

• Electrical power: The rate at which an electrical source can supply electrical energy. E.g., a battery may be able to store a large amount of energy, but if it has a small power capability it can provide the energy (do some work) only slowly, and it will take a long time to discharge. Another battery with the same energy storage capability but larger power will provide the energy (do work) faster, but will also be discharged faster. Electrical power is expressed usually in units of watt, symbol: "W". Unfortunately, the terms "power" and "energy" are often used interchangeably in everyday language (and sometimes also in the technical literature) even though they are quite distinct concepts, e.g., when we talk about "energy source" or "power source," we usually mean the same thing. Not only electrical sources but also loads are characterized by a power rating, e.g., an electrical motor or a light bulb is characterized by the power it needs to operate it. The power of a source (or the power need of a load) can be calculated as the product of the current and voltage (watt = ampere × volt). One watt means that one watt-second (coulomb × volt) energy is provided (used) every second. In more practical units, one watt means that one watt-hour (ampere-hour × volt) energy is provided (used) every hour.

• Electrical source (supply): A source of electrical power (electrical energy), a device that supplies electrical current. It can be electrochemical (battery or fuel cell) or an electromechanical device (dynamo) or a specialized electronic instrument. Also called "power source (supply)." Specialized sources can be called "voltage source" or "current source," indicating the characteristic of the electrical power that can be controlled by that device.

• Electroacoustics (electroacoustic effect): The electrokinetic effects arising when soundwaves cause oscillation of small particles suspended in a liquid; particularly, effect analogous to sedimentation potential.

• Electroactive substance: A species in solution that can take part in an electrode reaction or that can be adsorbed on the electrode.

• Electroanalytical chemistry: The application of electrochemical cells and electrochemical techniques for chemical analysis. The analyte is dissolved in the electrolyte of the cell, and one can perform either "qualitative" analysis (determination of the type of constituents present) or "quantitative" analysis (determination of the amount of a given constituent).

• Electrocatalysis: The phenomenon of increasing the rate of an electrode reaction by changing the electrode material. The rate of the electrode reactions (the magnitude of the exchange current density) can strongly depend on the composition and morphological structure of the electrode surface. This is called the "electrocatalytic effect."

• Electrocatalyst: A material that can cause electrocatalysis.

• Electrochemical capacitor: A device that stores electrical energy using electrochemical cells. Large surface area electrodes are used resulting in large double layer capacitance, and much of the storage capacity is due to the charging/discharging of the double layer. Some surface oxidation/reduction also occurs, but in contrast to reactions occurring in batteries, this is limited to a monolayer or two on the electrode surfaces. Consequently, the device behaves more like a capacitor than a battery. It is also called "supercapacitor" and "ultracapacitor". It is not to be confused with electrolytic capacitors. Electrochemical capacitors typically have much larger power density but much smaller energy density than batteries.

• Electrochemical cell: A device that converts chemical energy into electrical energy or vice versa when a chemical reaction is occurring in the cell. Typically, it consists of two metal electrodes immersed into an aqueous solution (electrolyte) with electrode reactions occurring at the electrode-solution surfaces. See also galvanic cell and electrolytic cell. It consist of two electronically conducting phases (e.g., solid or liquid metals, semiconductors, etc) connected by an ionically conducting phase (e.g. aqueous or non-aqueous solution, molten salt, ionically conducting solid). As an electrical current passes, it must change from electronic current to ionic current and back to electronic current. These changes of conduction mode are always accompanied by oxidation/reduction reactions. An essential feature of the electrochemical cell is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water, electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in a galvanic cell the hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. The ions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions, will recombine in the solution to form the final product of the reaction: water. During this process the electrons are conducted from the anode to the cathode through an outside electrical circuit where the electrical current can drive a motor, light a light bulb, etc. The reaction can also be reversed, water can be decomposed into hydrogen and oxygen by the application of electrical power in an electrolytic cell.

• Electrochemical cleaning: See electrolytic degreasing.

• Electrochemical degreasing: See electrolytic degreasing.

• Electrochemical double layer: See electrical double layer.

• Electrochemical drilling: See electrochemical machining.

• Electrochemical equivalent: The weight of a substance (in grams) produced or consumed by the passage of one coulomb in an electrochemical reaction. The gram-equivalent weight divided by the Faraday constant.

• Electrochemical grinding: A combination of electrochemical machining and mechanical grinding. Used when the products of electrochemical dissolution are not easily soluble and must be removed physically from the surface. Used with a metal-bonded and diamond-impregnated grinding wheel. Also called "electrolytic grinding" and "electrogrinding."

• Electrochemical irreversibility: See irreversible electrode reaction.

• Electrochemical machining: A process to produce metallic objects with a technique that is essentially precision electrodissolution. The metal to be machined is made the anode in an electrolytic cell while the cathode (or "tool") is made of inert material and is machined to be the "mirror image" of the desired shape. A very small gap (typically, less than 0.1 cm) is maintained between the electrodes and a large current density is applied with a fast flowing electrolyte. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation from very hard alloys that would be very difficult, if not impossible, to machine with any other metal cutting technique. Some typical applications are the production of turbine blades and the drilling of holes with very large depth-to-diameter ratio. The cathodic reaction is typically hydrogen evolution.

• Electrochemical nose: An instrument which comprises a sampling system, an array of chemical/electrochemical gas sensors with differing selectivity, and a computer with an appropriate pattern-classification algorithm, capable of qualitative and/or quantitative analysis of simple or complex gases, vapors, or odors. See also an Encyclopedia Article.

• Electrochemical pickling: See electrolytic pickling.

• Electrochemical polishing: See electropolishing.

• Electrochemical reaction: An oxidation/reduction reaction that occurs in an electrochemical cell. The essential feature is that the simultaneously occurring oxidation-reduction reactions are spatially separated. E.g., in a spontaneous "chemical reaction" during the oxidation of hydrogen by oxygen to water, electrons are passed directly from the hydrogen to the oxygen. In contrast, in the spontaneous electrochemical reaction in a galvanic cell two separate electrode reactions occur. The hydrogen is oxidized at the anode by transferring electrons to the anode and the oxygen is reduced at the cathode by accepting electrons from the cathode. The overall electrochemical reaction is the sum of the two electrode reactions. The ions produced in the electrode reactions, in this case positive hydrogen ions and the negative hydroxyl (OH-) ions will recombine in the solution to form the final product of the reaction: water. During this process the electrons are carried from the anode to the cathode through an outside electrical circuit where the electronic current can drive a motor, light a light bulb, etc. The reaction can also be reversed, water can be decomposed into hydrogen and oxygen by the application of electrical power in an electrolytic cell.

• Electrochemical reversibility: See reversible electrode reaction.

• Electrochemical series: See electromotive series.

• Electrochemical shaping: A variety of electrochemical techniques used to "shape" metal objects. These include: electrochemical machining, electrochemical drilling, electrochemical grinding, and electropolishing.

• Electrochemical switching: An electrochemically switchable molecule displays a different reactivity toward some other chemical species depending whether the switchable molecule is oxidized or reduced. Consequently, the reactivity of the molecule can be controlled by electrochemical oxidation/reduction. This phenomenon is primarily important in bioelectrochemistry.

• Electrochemical synthesis: See electrosynthesis.

• Electrochemiluminescence: Light emission by excited species produced in an electrode reaction. Also called: "electrogenerated chemiluminescence."

• Electrochromatography: A "chromatographic" separation method with the "mobile," liquid phase forced through the "immobile" phase by the application of an electrical potential difference, that is, by electroosmosis. In some cases, the separation is enhanced by electrophoresis. Chromatography is an analytical (see electroanalytical) separation technique based on the different attraction of the sample components to an immobile/stationary phase through which the sample solution is forced through by a flow of solvent. The sample components are adsorbed/desorbed on the surface of the stationary phase as they are flushed through by the solvent; consequently, they move with speeds inversely proportional to their adsorption strengths and become separated: the least-strongly adsorbed component is flushed out first and the most-strongly adsorbed last. A variety of stationary phases can be employed; the most common ones are: paper, thin layer of gelatinous material, or a column (or capillary) packed with small particles.

• Electroclean: See electrolytic degreasing.

• Electrocoating: See electrophoretic deposition.

• Electroconcentration: Process for increasing the concentration of a trace component in a sample. It can be achieved by a variety of techniques, e.g.: electrochromatography, electrodialysis, electroplating, electroosmosis, and electrophoresis.

• Electrocrystallization: See electroplating. Electroplating typically will result in a crystalline metal deposit; therefore, the two terms can be used interchangeably. The term "electroplating" is mostly used in technological applications, and the term "electrocrystallization" is often used in research studies.

• Electrode: The two electronically conducting parts of an electrochemical cell. See also anode and cathode. These can be simple metallic structures (rods, sheets, etc) or much more complicated, composite structures. E.g., the electrodes in a rechargeable battery will also "contain" the chemicals being converted during its operation. The term "electrode" is also used to denote complex assemblies that include an electrode in a small vessel, which contains an electrolyte and is equipped with an ion-permeable separator. Reference electrodes are such assemblies.

• Electrode kinetics: The application of kinetics to electrode reactions. Not to be confused with electrokinetics.

• Electrode of the first kind: A simple metal electrode immersed in a solution containing its own ion (e.g., silver immersed in a silver nitrate solution). The equilibrium potential of this electrode is a function of the concentration of the cation of the electrode metal in the solution (see Nernst equation). Contrast with electrode of the second kind and electrode of the third kind.

• Electrode of the second kind: A metal electrode assembly with the equilibrium potential being a function of the concentration of an anion in the solution. Typical examples are the silver/silver-chloride electrode and the calomel electrode. Contrast with electrode of the first kind and electrode of the third kind. The assembly consists of a metal, in contact with a slightly soluble salt of this metal, immersed in a solution containing the same anion as that of the metal salt (e.g., silver---silver chloride---potassium chloride solution). The potential of the metal is controlled by the concentration of its cation in the solution, but this, in turn, is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt.

• Electrode of the third kind: A metal electrode assembly with the equilibrium potential being a function of the concentration of a cation, other than the cation of the electrode metal, in the solution. These have been used, with limited success, in sensors for metal ions for metals that are not stable in aqueous solutions, e.g., calcium and magnesium. Contrast with electrode of the first kind and electrode of the second kind. The assembly consists of a metal in contact with two slightly soluble salts (one containing the cation of the solid metal, the other the cation to be determined, with both salts having a common anion) immersed in a solution containing a salt of the second metal (e.g., zinc metal---zinc oxalate---calcium oxalate---calcium salt solution). The potential of the metal is controlled by the concentration of its cation in the solution, but this is controlled by the anion concentration in the solution through the solubility product of the slightly soluble metal salt, which, in turn is controlled by the concentration of the cation of the second slightly soluble salt. These electrodes are very sluggish and unstable due to a series of equilibria to be established to produce a stable potential.

• Electrode potential: The electrical potential difference between an electrode and a reference electrode. We cannot measure the "absolute" potential of an electrode; therefore, the electrode potential must always be referred to an "arbitrary zero point," defined by the potential of the reference electrode. Consequently, it is very important always to note the type of reference electrode used in the measurement of the electrode potential. See also equilibrium electrode potential.

• Electrode reaction: A chemical "half" (or "partial") reaction occurring at the electrode surface. It is called a "half" (or "partial") reaction because only the oxidation or the reduction part of the overall cell reaction occurs at any one electrode. See also electrochemical reaction. Many electrode reactions can proceed either as oxidation or as reduction, depending on the direction of the current flowing through the electrode/electrolyte interface. See, e.g. metal deposition/dissolution or redox reactions. An electrode reaction always occurs in several series and parallel elementary reaction steps. Even in the simplest case there are three steps in series: (1) the reactant must be transported to the electrode surface from the bulk of the electrolyte (usually predominantly by diffusion, but it can also occur by electromigration), (2) a charge-transfer reaction occurs, and (3) the product must be transported from the electrode surface to the bulk of the electrolyte.

• Electrodeposition: A process for depositing solid materials on an electrode surface using electrolysis. It is a somewhat loosely used term that is applied to many technologies. There are a number of metal deposition technologies. However, not only metals but also different compounds can be electrodeposited. This is used most often for the formation of oxides (such as manganese dioxide and lead dioxide) by anodic oxidation of dissolved salts. Deposition can also be achieved electrophoreticly. See also an Encyclopedia Article on electroplating.

• Electrodics: The part (sub-discipline) of electrochemistry that deals with phenomena occurring at the surface of electrodes, particularly charge-transfer reactions. See also ionics.

• Electrodissolution: The reverse reaction of metal deposition.

• Electrodialysis: A process to move ions from one solution into another using an electrolytic cell. An example is the electrochemical desalination of seawater. In its simplest form, the cell is separated into three compartments by appropriate ion-exchange membranes with electrodes placed in the two outer compartments, and all compartments are fed seawater. As an electrical current is forced through the cell, anions will move from the central compartment through an anion-exchange membrane into the anode compartment and the cations will move through an cation-exchange membrane into the cathode compartment. Since the ion-exchange membranes are appropriately ion selective, the ions cannot move from the edge compartments to the central compartment, resulting in a desalinated effluent from that compartment. In practice, more than one cell will be connected in series, and the process will be carried out in several stages since it would not be efficient to remove all the salt in one step. This process is also used to remove industrial pollutants from waste streams.

• Electroendosmosis: See electroosmosis.

• Electroextraction: See electrowinning.

• Electroflotation: An electrolytic process for particle separation. An aqueous solution containing dispersed solid particles is electrolyzed to produce hydrogen and oxygen gas bubbles, the rising bubbles carry particles adhering to them to the surface where they can be skimmed off. This process is routinely used in processing minerals (ores) for separating the light and heavy particles, and in waste treatment to remove solids.

• Electroforming: A process to produce metallic objects with a technique that is essentially precision electroplating. The metal is deposited onto a "mandrel" or "former" of suitable shape to a desired thickness, followed by the removal of the mandrel to produce a free standing metal object. One of the advantages of this production technique is that very complicated shapes can be produced with a single operation. It is often used to produce very precise optical elements, and solid-state electronic devices (integrated circuit boards, computer chips). Other applications are the production of flat or perforated metal sheets, seamless perforated metal tubes, and metal bellows. Two very prominent past applications of this technique were the production of "stampers" for the old-fashioned musical (phonograph) records and "electrotypes" for the printing industry. Practically any metal or alloy that can be electroplated can also be used for electroforming. The preparation of the removable mandrel is an important step in this process. One example is the use of machined copper or brass that is surface treated to permit electroplating that will closely follow the mandrel surface but will not permit strong adhesion of the electroformed piece.

• Electrogalvanizing: See galvanizing.

• Electrogenerated chemiluminescence: See electrochemiluminescence.

• Electrogenerated species: A chemical species produced at an electrode surface by a charge-transfer reaction.

• Electrogravimetry: An electroanalytical technique in which the substance to be determined (usually a metal) is deposited out on an electrode which is weighed before and after the experiment. The potential of the electrode must be carefully chosen to ensure that only the metal do be determined will deposit. Under favorable conditions, two or more metals can be determined by successive depositions at different potentials.

• Electrogrinding: See electrochemical grinding.

• Electrokinetic effects (electrokinetics): Phenomena that arise due to a charge separation caused by the relative motion of a solid and liquid phase. A portion of the Gouy-Chapman diffuse layer is sheared off as the two phases move relative to each other, resulting in a charge separation. The hydrodynamic boundary layer remains attached to the solid surface while the rest of the liquid moves separately; consequently, electrokinetic effects arise when the "diffuse double layer" is thicker that the "hydrodynamic boundary layer." The electrical potential difference between the bulk solution and the "shear plane" is the "electrokinetic potential," often called the "zeta potential." Two types of effects arise: an electrical potential difference will arise between the two phases if they move relative to each other due to an external force (streaming potential and sedimentation potential) or a movement of the two phases will arise relative to each other if an electrical potential is applied parallel to the phase boundary (electroosmosis and electrophoresis). Accordingly, "electrokinetics" includes the following four "electrokinetic effects:" Streaming potential arises when liquid is flowing by a solid surface, e.g., when liquid is forced through a capillary tubing or porous solid by a pressure differential. Sedimentation potential arises when small suspended particles move through a liquid (e.g., forced by gravity). This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "eletrophoretic potential" or "Dorn potential." Electroosmosis is the movement of a liquid through a capillary tubing or porous solid driven by an electrical potential difference. Also called "electroendosmosis." Electrophoresis is the movement of small suspended particles in a liquid driven by an electrical potential difference. This can occur in "dispersions" (suspended solid particles) or "emulsions" (suspended immiscible liquid droplets). Also called "cataphoresis." Electrokinetics should not be confused with electrode kinetics.

• Electrokinetic potential: Alternative name for "zeta potential." See electrokinetic effects.

• Electrokinetic remediation: See electroremediation.

• Electroless plating: Process to produce thin metallic coatings on objects without the application of external current. The plating bath contains a dissolved salt of the metal and a reducing agent. However, the reduction of the metal cation to metal occurs only on the surface of the object to be coated due to the catalytic nature of the surface. The advantages of this process over electroplating are the possibility to produce coatings on insulator materials, and to produce uniform thickness coatings on geometrically complex surfaces.

• Electrolysis: A process that decomposes a chemical compound into its elements or produces a new compound by the action of an electrical current. The electrical current is passed trough an electrolytic cell and oxidation/reduction reactions occur at the electrodes. E.g., water can be decomposed into hydrogen and oxygen, or a metal can be electroplated by electrolysis.

• Electrolyte: A chemical compound (salt, acid, or base) that dissociates into electrically charged ions when dissolved in a solvent. The resulting electrolyte (or electrolytic) solution is an ionic conductor of electricity. Very often, the so formed solution itself is simply called an "electrolyte." Also, molten salts and molten salt solutions are often called "electrolyte" when used in electrochemical cells, see ionic liquid.

• Electrolyte solution: See electrolyte.

• Electrolytic capacitor: A storage device similar to any other type of electrical capacitor. However, only one of its conducting phases is a metallic plate, the other conducting phase is an electrolyte solution. The dielectric is a very thin (passive) oxide film on the surface of the metal (typically aluminum or tantalum) that constitutes one conducting phase of the capacitor. There is also another metal electrode immersed in the solution, which serves only as the electrical contact to the solution. Electrolytic capacitors typically have much larger capacitance than classical capacitors because the dielectric is very thin (on the order of millionth of centimeter). There are no economical ways to produce dielectric films that thin in any other way. The electrolytic capacitor is not to be confused with the electrochemical capacitor. The overall capacitance of this device is the sum of two series coupled capacitances because the metal electrical contact in the solution will have an electrical double layer. The capacitance of the double layer is even larger than that of the oxide covered electrode because the "dielectric" in the double layer is only a few molecular layers of the solvent. Consequently, the overall capacitance is dominated by the lower value of the oxide-covered electrode. An electrochemical capacitor, depending completely on double-layer capacitance, will have much larger capacitance than an electrolytic capacitor, however it can be operated only in a few volts potential range because of the limitation of the double-layer range. An electrolytic capacitor can be operated to tens or hundreds of volts, depending on the thickness of the dielectric oxide film.

• Electrolytic cell: An electrochemical cell that converts electrical energy into chemical energy. The chemical reactions do not occur "spontaneously" at the electrodes when they are connected through an external circuit. The reaction must be forced by applying an external electrical current. It is used to store electrical energy in chemical form, see rechargeable battery. It is also used to decompose or produce (synthesize) new chemicals by application of electrical power. This process is called electrolysis, e.g., water can be decomposed into hydrogen gas and oxygen gas.The free energy change of the overall cell reaction is positive.

• Electrolytic cleaning: See electrolytic degreasing.

• Electrolytic degreasing: Process for removal of grease, oil, etc from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly basic (sometimes hot) solution that dissolves these coatings. The strong hydrogen evolution occurring on the cathode may reduce some of the coatings, and the strong bubble evolution removes the coatings mechanically, while the agitation of the solution helps the chemical dissolution of the coatings by the base. Also called: "electrolytic cleaning," "electrochemical cleaning," "electrocleaning," and "electrochemical degreasing." See also degreasing.

• Electrolytic grinding: See electrochemical grinding.

• Electrolytic hydrogen: See water electrolysis.

• Electrolytic oxygen: See water electrolysis.

• Electrolytic machining: See electrochemical machining.

• Electrolytic pickling: Process for removal of oxide scales from metal surfaces in preparation for electroplating. The metal is made the cathode in an electrolytic cell containing strongly acidic (sometimes hot) solution that dissolves the oxide scales. The strong hydrogen evolution occurring on the cathode may reduce some of the oxides, and the strong bubble evolution removes the scales mechanically, while the agitation of the solution helps the chemical dissolution of the scales by the acid. See also pickling.

• Electrolytic polishing: See electropolishing.

• Electrolytic refining: See electrorefining.

• Electrolytic solution: See electrolyte.

• Electromachining: See electrochemical machining.

• Electrometallurgy: Branch of metallurgy (science dealing with the production of metals) using electrochemical processes, that is electrowinning.

• Electrometer: A voltmeter with very large input resistance. A typical modern voltmeter has an input resistance of around ten million ohms, an electrometer can have ten million times more. Electrometers are used to measure the electromotive force of electrochemical cells that can be easily polarized by current. The voltmeter always draws a small current, the magnitude depends on the ratio of the resistance of the cell and the voltmeter. High resistance cells (e.g., one containing a glass electrode, must be measured with an electrometer.

• Electromigration: The movement of ions under the influence of electrical potential difference.

• Electromotive force: The cell voltage of a galvanic cell measured when there is no current flowing through the cell. In other words, the equilibrium electrode potential difference between the two electrodes of the cell. Abbreviated as "emf."

• Electromotive series: A tabulation on which various substances, such as metals or elements, are listed according to their chemical reactivity or standard electrode potential. It is usually ordered with increasing standard electrode potentials (most negative on top). For metals, the order indicates the tendency to spontaneously reduce the ions of any other metal below it in the series (see cementation). During electrolytic reduction of cations (e.g., electroplating) an element lower in the series (more positive) will deposit first, and an element higher in the series (more negative) will deposit only when the solution is practically depleted of the ions of the first element. Also called "electrochemical series" and "galvanic series."

• Electron: See atomic structure.

• Electronation: Alternative name of a reduction process.

• Electroneutrality condition: The expression of nature's tendency to keep any system electrically neutral, that is, if it contains electrically charged particles the total sum of negative charges will be equal to the total sum of positive charges. Applying this condition to a solution of electrolytes implies the equality of the total positive ionic charges to the total negative ionic charges. This equality should hold even as we subdivide the solution into smaller and smaller volume elements. This condition results from the statistical distribution of the ions around each other considering the attractive tendency of oppositely charged particles and the repulsive tendency of similarly charged particles. Consequently, there is a statistical limit of the size of the volume element to which it applies. At the extreme, a volume small enough to contain only a single ion obviously cannot be electrically neutral.

• Electronic conductor: A material that conducts electricity with electrons as charge carriers.

• Electronic nose: See electrochemical nose.

• Electron-transfer reaction: See charge-transfer reaction.

• Electroorganic: Relating to organic electrochemistry.

• Electroosmosis: The movement of a liquid through a capillary tubing or porous solid driven by an electrical potential difference. See electrokinetic effects. Also called "electroendosmosis."

• Electroosmotic dewatering: Compaction of slurries by electroosmosis.

• Electroosmotic remediation: See electroremediation.

• Electrooxidation: Oxidation carried out with an electrochemical reaction.

• Electropainting: Alternative name for electrophoretic painting.

• Electrophoresis: As a phenomenon: the movement of small suspended particles or very large molecules in a liquid driven by an electrical potential difference. Also called "cataphoresis." See electrokinetic effects. As an electroanalytical technique: a separation method for very large organic molecules (usually of biological origin) based on their different electrophoretic velocities through an "immobilized" liquid phase. The liquid can be immobilized by a variety of "supports", e.g.: paper, gelatinous material, capillary tubing.

• Electrophoretic deposition (painting): Deposition of particles carried to a surface by electrophoresis. The loosely formed deposit layer typically needs compacting that can partially occur by electroosmotic removal of the liquid, or by other means. Electrode reactions occurring on the substrate surface can take part in "binding" the coating. Practical applications are surface coating and paint deposition (practiced on large scale in the automotive industry) and fabrication of ceramic products. Also called "electrocoating."

• Electrophoretic potential: Alternative name for "sedimentation potential." See electrokinetic effects.

• Electrophysiology: The study of the electrical properties of living tissue.

• Electroplating: The process that produces a thin, metallic coating on the surface on another metal (or any other conductor, e.g., graphite). The metal substrate to be coated is made the cathode in an electrolytic cell where the cations of the electrolyte are the positive ions of the metal to be coated on the surface. When a current is applied, the electrode reaction occurring on the cathode is the reduction of the metal ions to metal. E.g., gold ions can be discharged form a gold solution to form a thin gold coating on a less expensive metal to produce "custome" jewelry. Similarly, chromium coating is often applied to steel surfaces to make them more "rust resistant." Electroplating is also used in the production of integrated circuits on computer chips and for other modern electronic instrumentation. The anode material can either be the metal to be deposited (in this case the electrode reaction is electrodissolution that continuously supplies the metal ions) or the anode can be of nonreactive material and the anodic reaction is oxygen evolution (in this case the plating solution is eventually depleted of metal ions). Also called "electrodeposition." See also an Encyclopedia Article.

• Electroporation: The application of very brief, carefully controlled, pulsed, rotating electrical fields to human cells, a process that causes pores to open in the cell membrane and allows pharmaceuticals or genes to gain access to the cell's interior.

• Electropolishing: A process that produces a bright, shiny surface on a metal. The metal is anodically dissolved in an electrolytic cell under conditions that projections in the surface are dissolved faster than the smoother areas.

• Electroreduction: Reduction carried out with an electrochemical reaction.

• Electrorefining: An electrochemical process that produces a purified metal from a less pure metal. The metal to be purified is made the anode in an electrolytic cell and it is dissolved by the application of a current into a usually acidic aqueous electrolyte or a molten salt. At the same time, the pure metal is deposited on the cathode. The process is carried out under conditions that most impurities will either precipitate as "sludge" or remain dissolved in the electrolyte. Copper is one metal that is often electrorefined in aqueous solutions, and aluminum is electrorefined using a molten salt electrolyte. Also called "electrolytic refining" and "metal refining."

• Electroremediation: An electroosmotic process for removing soluble contaminants from soil. Electrosmosis is carried out in the wet soil with strategically placed electrodes, resulting in a movement of the contaminants toward the electrodes thereby cleansing the soil and concentrating the contaminants in a small volume of soil around the electrodes from where they can be easily removed. Also called "electrokinetic remediation" "electroosmotic remediation" and "soil remediation."

• Electrorheology: See electroviscosity.

• Electroseparation: A process that uses electrolysis to selectively remove a constituent from solution.

• Electrosorption: Adsorption at electrode surfaces. Generally, adsorption at electrically charged interfaces.

• Electrosynthesis: Production of chemicals in an electrolytic cell. See also Encyclopedia Articles on electrosynthesis of inorganic and organic compounds.

• Electrotype: See electroforming.

• Electroviscosity: The phenomenon of a change in viscosity due to the presence of charge on particles suspended in a solvent.

• Electrowinning: An electrochemical process that produces metals from their ores. Most metals occur in nature in oxidized form in their ores. While numerous ways exist to reduce the ores, for many metals electrochemical reduction is the most practical. The ore is dissolved (often following some chemical purification or preprocessing) in an acidic aqueous solution or in a molten salt and the resulting electrolyte solution is electrolyzed. The metal is deposited on the cathode (either in solid or in liquid form), while the anodic reaction is usually oxygen evolution. Copper and zinc are two metals that are often produced by aqueous electrolysis. Aluminum, magnesium, and sodium are some metals that can be produced by molten salt electrolysis. For aluminum, this is the only practically used production process (see an Encyclopedia Article). Also called "electroextraction."

• Element, chemical: A substance that cannot be decomposed into simpler substances by chemical means.

• Elementary reaction step: Chemical reactions usually take place in a number of simple ("elementary") reaction steps proceeding in series. The overall reaction is the sum of the elementary reactions. E.g., the electroplating of copper on some metal involves three elementary steps: (1) a redox reaction in which the double positively charged copper cation reacts with an electron from the metal electrode to form a single charged copper ion, followed by (2) a metal deposition reaction in which the single charged copper ion reacts with a second electron to form a copper atom on the surface of the metal, and finally (3) an electrocrystallization step in which the copper atom becomes incorporated into the crystalline structure of the underlying metal. The rate-determining step in copper deposition is usually the first of these steps. Some complicated reactions can also involve parallel paths, each proceeding through a different series of elementary steps (different reaction mechanisms). The sum of the series elementary steps in each parallel path must add up to the same overall reaction.

• emf: Stands for electromotive force.

• Energy: The energy of a system expresses the ability of that system to do some useful work or generate heat. Energy can be in many forms; e.g., mechanical energy, chemical energy, heat energy, electrical energy, etc. The different forms of energy can be converted into each other. It is a fundamental law of nature that energy can never be converted from one form to another 100%, some of the energy is always converted into heat energy during the conversion. Also, heat can never be converted 100% into any other form of energy.

• Energy conversion: A process in which energy is converted from one of its many forms to another. The fuel cell is an electrochemical energy conversion device.

• Energy density: Characteristic parameter of a battery indicating the amount of electrical energy stored per unit weight or volume. The terminology is not strictly defined. Weight based energy density is often called "specific energy" or "gravimetric energy density." Volume based energy density is often called "energy density" or "volumetric energy density." The energy density is typically expressed as watt-hour/kilogram or watt-hour/liter.

• Energy efficiency: For a rechargeable battery: the fraction, usually expressed as a percentage, of the electrical energy stored in a battery by charging that is recoverable during discharging. For an electrolytic cell: the fraction, usually expressed as a percentage, calculated as the theoretically required energy divided by the energy actually consumed in the process (production of a chemical, electroplating, etc). Inefficiencies arise from current inefficiencies and the inevitable heat losses due to polarization. See also coulometric efficiency.

• Energy source (supply): See electrical source (supply).

• Energy storage: A process in which energy is stored in some form, ready for future use on demand. The time scale of storage can extend to many years if needed. The battery is an electrochemical energy storage device.

• E-nose: See electrochemical nose.

• Equalization: The process by which all cells of a multi-cell rechargeable battery are brought to the same state of charge.

• Equalizing charge: Passage of an amount of charge by which the undercharged cells of a battery are brought up to a fully charged condition without damaging those already fully charged.

• Equilibrium: An electrode or an electrochemical cell is said to be in "equilibrium" when there is no net current flowing and there are no net electrode reactions taking place in the system. (See, however, exchange current density.) In equilibrium, the potential of the electrodes is the equilibrium potential and the cell voltage is the electromotive force.

• Equilibrium cell voltage: See electromotive force.

• Equilibrium electrode potential: The electrical potential of an electrode measured against a reference electrode when there is no current flowing trough the electrode. In other words, the electromotive force of an electrochemical cell consisting of the electrode in question and a reference electrode. Also called: "open circuit potential (ocp)." See also equilibrium and standard electrode potential. The concept of equilibrium potential is probably easiest to demonstrate with a simple metal/metal-ion electrode system. When a metal (e.g., silver) is immersed in a solution containing its ion (e.g., silver nitrate solution) metal ions will cross the metal/solution interface. They will pass from the phase where the "chemical energy" of the ion is large to the phase where the "chemical energy" of the ion is smaller. Depending on the system, this can occur in either direction. However only the positively charged (e.g., silver) cations can pass through the interface. The negatively charged electrons cannot pass into the solution, and the anions (e.g., nitrate) cannot pass into the metal. Consequently, charge accumulation occurs at the interface forming an electrical double layer. Consider an example when the metal ions move preferentially from the metal into the solution: the metal surface becomes negatively charged because of the accumulation of the electrons left behind, while the solution layer near the metal surface becomes positively charged because of the accumulation of silver ions. This process produces a potential difference between the two phases that will slow and eventually stop the passage of the metal ions. At "equilibrium" the chemical driving force and the opposing electrical force are equal. The potential difference between the metal and the solution phases under these conditions is the "equilibrium potential difference." This potential difference cannot be measured because there is no way to make an electrical connection to the solution phase without setting up another electrode potential. Consequently, electrode potentials are always measured against a reference electrode whose potential is known on an arbitrary scale. See standard hydrogen electrode.

• Equilibrium potential: See equilibrium electrode potential.

• Equilibrium voltage: See electromotive force.

• Equivalent circuit: An electrical circuit (usually comprised of series and parallel coupled resistors and capacitors) that models the fundamental properties and behavior of electrodes or electrochemical cells.

• Equivalent weight: A characteristic weight of a substance relating to a specific reaction the substance participates. In electrochemical reactions, the molecular weight or atomic weight divided by the number of electrons transferred during the reaction. Consequently, the equivalent weight of a substance can be different for different reactions. E.g. the equivalent weight of the cuprous ion (singly charged copper ion) is equal to its atomic weight, independently whether it is oxidized to cupric ion (doubly charged copper ion) or it is reduced to (electrically neutral) copper metal. On the other hand, the equivalent weight of the cupric ion is one half of its atomic weight if it is reduced to copper metal, but it is the atomic weight if it is reduced only to cuprous ion. In a wider sense, the molecular weight or the atomic weight divided by the valence change occurring during the reaction. For acid/base reactions, the molecular weight divided by the number of hydrogen ions produced or consumed during the reaction.

• Exchange current density: At the equilibrium potential there is no net current flowing through the electrode. However, the equilibrium is a dynamic one, that is, the electrode reaction proceeds at "equal rates" both in the forward and in the reverse direction, resulting in a zero "net" reaction rate and a zero "net" current. The rate of the electrode reaction can be expressed as an equivalent current density and the "exchange current density" of a reaction is the current density flowing "equally" in both directions in equilibrium. A large exchange current density indicates a fast reaction (see also non-polarizable electrode), while a small exchange current density indicates a slow reaction (see also polarizable electrode)

• Expander: A substance added in small amount to the active materials of a lead-acid battery to improve the service life and capacity of the electrodes. In particular, an expander prevents the increase in crystal grain size of lead in the negative electrode.

• External electrolyte: The electrolyte solution in the electrochemical cell into which the reference electrode is immersed. Contrast with the internal electrolyte of the reference electrode.