# Electrochemistry Dictionary - I

• Ideal non-polarizable electrode: An electrode that is practically not polarizable. That is, the potential of the electrode will not change from its equilibrium potential with the application of even a large current density. The reason for this behavior is that the electrode reaction is extremely fast (has an almost infinite exchange current density). Also called "ideal depolarized electrode." Contrast with ideal polarizable electrode.

• Ideal polarized (polarizable) electrode: An electrode is called "ideal polarizable" if no electrode reactions can occur within a fairly wide electrode potential range. Consequently, the electrode behaves like a capacitor and only capacitive current ( no faradaic current) is flowing upon a change of potential. Many electrodes can behave as an ideal polarized electrode but only within an electrode potential range called the "double-layer range." Also called "completely-polarizable electrode" and "totally-polarized electrode." Contrast with ideal non-polarizable electrode.

• IHP: Stands for "inner Helmholtz plane." See the Helmholtz model of the double layer.

• Ilkovic equation: A relation between diffusion limited current density and time in a polarograhic experiment. The diffusion current density is proportional to the growth-time of the mercury drop on the one-sixth power, and to the mass-flowrate of the mercury on the two-third power. The proportionality constant contains the product of the concentration of the reactant and the square root of the diffusion coefficient of the reactant.

• Immobilized electrolyte: See dry cell.

• Impedance: Impedance is the analogue of the resistance or resistivity when applied to alternating current. That is, it is a measure of a material's inability to carry the electrical current. In many materials, the impedance varies as the frequency of the applied electrical potential changes, due to the properties of the liquid or solid. In electrochemistry, the impedance of the electrode reaction is also frequency dependent.

• Indicator electrode: See working electrode.

• Indifferent electrolyte: See supporting electrolyte.

• Indirect electrolysis: The production of chemicals in an electrolytic cell through intermediate electrolysis products. It is often used in the oxidation/reduction of organic compounds that would otherwise react very slowly at the electrode surface. An intermediate oxidizing/reducing agent is produced at the electrode surface and the agent reacts with the organic in the bulk solution. The agent is continuously regenerated by the electrolysis. A typical oxidizing agent is the ferric (trivalent iron) ion, and an example of the reducing agent is the cerous (trivalent cerium) ion. The reactive intermediate is often called a "mediator," and the overall reaction a "mediated reaction."

• Indirect electrosynthesis: See indirect electrolysis.

• Inert electrode: An electrode that serves only as a source or sink for electrons without playing a chemical role in the electrode reaction. Noble metals, mercury, and carbon are typically used as inert electrodes. See for example the redox electrode. The "inert" nature of the electrode can sometimes be questioned. While the electrode may not take part in the reaction as a reactant or product, it still can act as an electrocatalyst.

• Inert electrolyte: See supporting electrolyte.

• Inhibitor: A substance added to the electrolyte which prevents an electrochemical process, generally by modifying the surface state of an electrode. A well known example is that of corrosion inhibitors which prevent metal corrosion.

• Initial drain: Current that a cell or battery supplies when first placed on a fixed load.

• Inner Helmholtz plane: See the Helmholtz model of the double layer. Abbreviated as "IHP."

• Inner-sphere charge-transfer reaction: A charge-transfer reaction with the reactants in direct contact with each other, without any intervening solvent molecules. Note that a "reactant" can also be an electrode. Contrast with outer-sphere charge-transfer reaction.

• Insulator (electrical): A material that will not carry any electrical current. It has zero conductivity and infinite resistivity.

• Interconnect: An electrically conductive structural part that connects series-connected cells in a fuel cell stack.

• Intermediate: A molecular or ionic species that is formed (directly or indirectly) from the reactants and reacts further (directly or indirectly) to form the products of the reaction. It does not accumulate during the course of the reaction.

• Internal electrolyte: The electrolyte solution inside a reference electrode assembly such as the silver/silver-chloride electrode. (Also called "filling solution.") Internal electrolytes are used also in membrane electrodes. Contrast with external electrolyte.

• Internal reference electrode: A reference electrode used inside a membrane electrode assembly as an electrical contact, with stable potential, to the internal electrolyte.

• Internal resistance: Resistance to the flow of direct current within a cell, causing a drop in closed circuit voltage proportional to the current drain from the cell.

• Ion: An electrically charged chemical particle (atom, molecule, or molecule fragment). "Anions" are negatively charged, and "cations" are positively charged.

• Ion-exchange membrane: A plastic sheet formed from ion-exchange resin. The utility of such membranes is based on their property that they are permeable preferentially only to either positive ions (cation-exchange membrane) or to negative ions (anion-exchange membrane).

• Ion-exchange resin: A polymeric resin that contains electrically charged fragments ("fixed ions") permanently attached to the polymer backbone, electrical neutrality is achieved by attached mobile "counterions" in the solution phase the resin is immersed into. A practical use of such resin is the removal of unwanted ions from a solution by replacing them with other ions. E.g., a cation exchange resin containing fixed negative charges with attached mobile sodium ions can be used to remove "hardness" from water if the calcium and magnesium ions are more strongly attracted to the resin and therefore will replace the sodium ions. Eventually all the sodium ions will go into solution and the ion-exchange process terminates. The resin can be regenerated by soaking in a high concentration sodium salt solution. Such process can also be used to remove unwanted ions from polluted water streams.

• Ionic conductor: A material that conducts electricity with ions as charge carriers. See also electrolyte.

• Ionic liquid: A liquid containing mostly ions, a molten salt in which the molecules are fully (or almost fully) dissociated. Contrast with electrolyte solution in which the dissociated salt is dissolved in a solvent, with the solvent not (or only slightly) dissociated. Corresponding examples are molten sodium chloride (table salt) and aqueous solution of sodium chloride.

• Ionic mobility: A quantitative measure of an ion's ability to move under the influence of a potential difference in solution. (See also electromigration.) It is the speed of movement under the influence of unit potential difference. While the mobility is defined in terms electromigration, it also affects the speed of diffusion.

• Ionics: Part (sub-discipline) of electrochemistry that deals with the behavior of ions in liquid solutions, ionic liquids, and solids ("solid-state ionics"). See also electrodics.

• Ion-selective membrane: See ion-exchange membrane.

• Ion-selective electrode: An electrode or electrode assembly with a potential that is dependent on the concentration of an ionic species in the test solution and is used for electroanalysis. Ion-selective electrodes are often membrane type electrodes. Abbreviated as "ISE."

• Ion-sensing electrode: See ion-selective electrode.

• ir (drop) compensation: Some potentiostats are equipped with an optional ir compensation. The potentiostat electronically corrects for the solution ir drop and the potential of the working electrode is controlled (at least in principle) at the correct value. Unfortunately, most potentiostats become unstable at full compensation, so one can only make a partial compensation, resulting in an uncompensated ir drop and an error in the potential control. The user must provide the solution resistance value, though some potentiostat setups will measure it automatically. Contrast with ir (drop) correction.

• ir (drop) correction: A numerical correction of measured potential of the working electrode for the solution ir drop. (One must know the value of the current and the value of the resistance of the electrolyte between the working and the reference electrodes.) It cannot be simply stated whether this correction is positive or negative because of the contradictory conventions used for the anodic and cathodic currents. In either case, the absolute value of the corrected potential must be smaller than that of the uncorrected potential. Contrast with ir (drop) compensation.

• ir drop: The electrical potential difference between the two ends of a conducting phase during a current flow. It is the product of the current (i) and the resistance (r) of the conductor. In electrochemistry, it refers to the solution ir drop, or to the ohmic loss in an electrochemical cell. See also Ohm's law.

• ir loss, ir drop: Decrease in the voltage of a cell during the passage of current due to the internal resistance of the bulk phases within the cell ­mainly that of the electrolyte and the separators. Also known as 'ohmic loss'.

• Irreversible electrode: An electrode with an irreversible electrode reaction.

• Irreversible electrode reaction: A qualitative term for a slow electrode reaction. An electrode reaction having a small exchange current density. Opposite: reversible electrode reaction. See also quasi-reversible electrode reaction.

• ISE: Stands for ion-selective electrode.

• Isoelectric focusing: A variation of the electrophoretic separation technique. The separation of molecules occurs in a combination of potential an pH gradients resulting in sharper separations compared to simple electrophoresis.