# Electrochemistry Dictionary - R

• Radical: A molecule or atom possessing an unpaired electron.

• Rate: See C-rate.

• Rate constant: The rate of a chemical reaction is proportional to the product of the concentrations of all the reactants taking part in the reaction, with the "rate constant" the proportionality factor. In other words, the rate constant is the rate of the reaction when all reactants are present in unit concentration. The rate constant of a chemical reaction is a function of temperature and pressure. For an electrode reaction the rate constant is also a function of the electrode potential.

• Rated capacity: The total charge a battery is able to deliver on discharge under some specified conditions. Usually expressed in ampere-hours.

• Rate-determining step: The slowest elementary reaction step in the series of steps comprising the overall reaction. The slowest step will control the rate of the overall reaction. This is analogous to a traffic situation, the time required to drive from the suburbs to downtown may completely depend on the time spent in one traffic jam on the road. Abbreviated as "rds."

• Rational potential: The potential of an electrode expressed against the potential of zero charge (pzc) of the same electrode in the same solution. This provides a potential scale specific for each electrode, with the origin (zero potential) always at the respective pzc. Consequently, two electrodes having the same "rational potential" are typically not at the same "potential" (as measured against any reference electrode). However, the potential of these two electrodes is displaced by the same amount from their respective pzc; therefore, the charge density in the double layer of the two electrodes will be approximately equal. Consequently, the comparison of electrode behavior at the same "rational potential" is more meaningful for some purposes than a comparison at the same "potential on some arbitrary scale." The charge densities of two electrodes at the same rational potential are only approximately equal because the double-layer capacitance and its potential dependence varies from electrode to electrode.

• RDE: Stands for rotating-disk electrode.

• rds: Stands for rate-determining step.

• Reactant: A chemical species that is taking part in a chemical reaction by reacting (sometimes by itself, but usually with other reactants) to form the products of the reaction. (electrode) reaction mechanism: The totality of all the elementary reaction steps occurring in series or parallel that fully defines the overall electrode reaction.

• Reaction overpotential (polarization): The overpotential (alternatively called polarization) associated with a chemical reaction (without charge transfer) step that is an elementary step in the overall electrode reaction.

• Rechargeable battery: A battery in which the chemical reaction system providing the electrical current is easily "chemically" reversible. After discharging, it can be recharged by applying an electrical current to its terminals. Some batteries can be recharged hundreds to thousands times. See, e.g. the lead-acid battery. Also called "secondary" battery, and "accumulator." Contrast with non-rechargeable battery. It operates as a galvanic cell during discharge and as an electrolytic cell during charge. As a consequence, the anode is the negative electrode during discharge, while it is the positive electrode during charge; at the same time, the cathode is the positive electrode during discharge, while it is the negative electrode during charge. This can create a confusing situation, and it is preferable to refer to the electrodes of a rechargeable battery as "positive" and "negative," because this designation is independent of the operational mode. Unfortunately, this nomenclature is not always followed. Often the "negative" electrode is designated as anode and the "positive" electrode is designated as cathode. This naming convention is a carry-over from the convention of the non-rechargeable battery.

• Recombining cell: A secondary cell in which provision has been made for the products of overcharge reactions to recombine so that no net change occurs to the composition of the cell system as a result of overcharging.

• Rectifier: An electrical equipment that converts alternating current into direct current.

• Redox battery: A rechargeable battery with two redox electrodes contained in compartments typically separated by an ion-exchange membrane E.g., a battery with iron and chromium redox couples in the two compartments. During discharge, "ferric" (trivalent iron) cations are reduced to "ferrous" (divalent iron) ions at one of the redox electrodes, while "chromous" (divalent chromium) ions are oxidized to "chromic" (trivalent chromium) ions at the other electrode. The reverse reactions occur during charging. An advantage of this system is that the polarization losses are relatively small because the redox reactions are typically fast and no solid phases are being formed during the reaction. Also, the solutions containing the iron and chromium salts can be stored in separate large tanks and circulated to a small "battery," permitting the storage of large amounts of energy.

• Redox couple: See redox reaction.

• Redox electrode: An inert electrode (e.g., platinum, gold, carbon) the potential of which is controlled by a redox reaction in solution. This is somewhat of a misnomer since all electrodes involve oxidation/reduction. The distinction is that in the case of a redox electrode both oxidized and reduced species are dissolved in the solution surrounding the electrode.

• Redox potential: The potential of a redox electrode.

• Redox reaction: A class of electrode reactions involving oxidation/reduction of two dissolved species. E.g., iron metal can exist in solution as a doubly positively charged ("ferrous") ion or a triply positively charged ("ferric") ion. Such a system is often called a "redox couple," such as the "ferrous/ferric" couple. The ferric ions can be cathodically reduced to ferrous ions, or the ferrous ions can be anodically oxidized to ferric ions. With these reactions, an inert electrode is used that does not take part in any reactions under the conditions of the oxidation/reduction of the ions. This electrode then acts only as a source or sink of electrons; examples are: carbon, graphite, platinum, gold. Compare with a metal deposition/dissolution reaction where one of the reacting species is a solid metal and the other species is in solution.

• Reducing agent: A substance that is affecting reduction by donating electrons to another substance. See oxidation/reduction. Also called "reductant."

• Reductant: Alternative expression for reducing agent.

• Reduction: The gain of electrons by a chemical species. See oxidation/reduction

• Reference electrode: An electrode that has a well known and stable equilibrium electrode potential or a nonpolarizable electrode with a known and highly reproducible potential. It is used as a reference point against which the potential of other electrodes (typically that of the working electrode or measuring electrode) can be measured in an electrochemical cell. In principle it can be any electrode fulfilling the above requirements. In practice, there are a few commonly-used (and usually commercially-available) electrode assemblies that have an electrode potential independent of the electrolyte used in the cell. For some common reference electrodes see e.g., the silver/silver-chloride electrode, calomel electrode, and hydrogen electrode. Strictly speaking, there can be a small change in the potential of these electrodes depending on the electrolyte because the presence of a liquid-junction potential. This is very often (justifiably or not) ignored. The liquid-junction potential is also minimized by the use of high concentration potassium chloride as the filling solution of the reference electrodes, because the diffusion rate of both ions is very closely the same in these solutions.

• Reserve battery: A non-rechargeable battery that is stored in an "inactive" form until its intended, immediate use. E.g., the battery is stored "dry," and it can be activated by injection of the electrolyte. Or, a battery operating with molten salt electrolyte is stored at a temperature below the electrolyte melting point, and it is activated by the sudden application of heat to melt the electrolyte. The advantages of such arrangements are an extremely long shelf life without practically any loss of stored energy, and the possibility to produce a very powerful battery with very reactive chemicals that would otherwise cause a very fast self discharge. Most reserve batteries are made for military applications.

• Residual current (density): A small faradaic current density flowing through an electrode under conditions when zero faradaic current is expected (e.g., within the double-layer range). It is caused by traces of impurities in the electrolyte. Also called "background current (density)."

• Resistance: See resistivity.

• Resistance overpotential (polarization): See solution ir drop.

• Resistivity (electrical): The measure of a material's inability to carry electrical current. The measurement unit of the resistivity (resistance) is the ohm. See also impedance. The reciprocal of conductivity.

• Resistor: An electrical circuit element with a fixed resistivity.

• Rest potential: See equilibrium potential.

• Reversal: See cell reversal.

• Reversible electrode: An electrode with a reversible electrode reaction.

• Reversible electrode reaction: A qualitative term for a fast electrode reaction. There are, unfortunately, several meanings attributed to the term "reversibility," resulting in possibly confusing situation. An electrode reaction is considered reversible in the "electrochemical sense" if the reaction is fast, that is, if the exchange current density of the electrode reaction is large. In contrast, in the "chemical sense," reversibility indicates that the reaction can proceed both in forward and backward (reverse) direction. Also called nernstian reaction. Opposite: irreversible electrode reaction. See also quasi-reversible electrode reaction. Both of the above described meanings of reversibility are different from the meaning in the "thermodynamic sense."

• Reversible hydrogen electrode: A commonly used reference electrode. A hydrogen electrode immersed directly into the electrolyte of the electrochemical cell and usually (unless otherwise sated) operated with one atmosphere pressure hydrogen gas. The equilibrium potential depends on the hydrogen ion concentration (strictly speaking, activity) of the cell electrolyte.

• Rotating-disk electrode: A specialized hydrodynamic electrode used in the study of the kinetics and mechanism of electrode reactions and in electroanalysis for ensuring a known and controllable flow of solution over the electrode. The flow control is achieved by using a flat disc electrode that is rotated in the solution resulting in a defined hydrodynamic boundary layer. Abbreviated as "RDE."

• Rotating-ring-disk electrode: A variant of the rotating-disk electrode which includes a second electrode - a concentric ring electrode - that is placed outside the disk and used to analyze the species generated on the disk. The ring is electrically insulated from the disk so that their potentials can be controlled independently. Abbreviated as "RRDE."

• Rotating-wire electrode: An electrode made of metal wire (often platinum) rotated about its axis at a known and constant velocity. It is used in the study of the kinetics and mechanism of electrode reactions and in electroanalysis.

• Roughness factor: The ratio between the true electrode area and the geometric electrode area.

• RRDE: Stands for rotating-ring-disk electrode.