Electrochemistry Dictionary - S

• Salt bridge: An ionically conducting path between separate compartments of an electrochemical cell. Often the working and reference (and occasionally even the counter) electrodes are in completely separate compartments and the required conducting path between them is provided by a tubing filled with highly conducting electrolyte solution. A common arrangement for a "salt bridge" is an inverted "U" shaped glass tubing with its ends dipped into the solutions of the two cell compartments; however, other materials and shapes are also used. The salt bridge may contain any conducting solution, but very often a highly concentrated potassium chloride solution (often immobilized by some gelling agent) is used.

• Sand equation: An equation relating current density, transition time, and concentration of the reactant in a chronopotentiometric experiment, assuming that the current is sufficiently large to immediately result in diffusion limiting conditions. The equation is valid only for planar electrodes in unstirred solution. The product of the current density and the square root of the transition time divided by the concentration is a constant. The constant is proportional to the square root of the diffusion coefficient of the reactant. Because the equation was derived for an unstirred solution, it ceases to be valid once natural convection starts.

• Saturated solution: See solubility.

• SCE: Stands for "saturated calomel electrode." See calomel electrode.

• Sealed battery: See maintenance-free battery. A battery which can be operated without regard to position.

• Second kind electrode: See electrode of the second kind.

• Secondary battery, storage battery: Cell or battery which can be recharged after discharge, under specified conditions.

• Secondary current distribution: A current distribution that is controlled by the resistivity of the solution (see primary current distribution) and the charge-transfer resistance of the electrode reaction occurring on the working electrode. That is, a current distribution taking into effect the activation overpotential. A large charge-transfer resistance (that is, a slow reaction), compared to the solution resistance, tends to make the current distribution more uniform. This still ignores the effect of the concentration overpotential, see tertiary current distribution.

• Sedimentation potential: An electrical potential difference that arises when small suspended particles move through a liquid (e.g., forced by gravity). Also called "Dorn potential. See electrokinetic effects.

• Self discharge: Capacity loss of a cell or battery under open circuit conditions due to chemical reactions within the cell. A slow discharging of a battery without being connected to an external load. This is caused partly by impurities and side reactions (reactions other than the cell reaction) and partly by the imperfect separation of the active chemicals in the battery causing a slow "direct" reaction between them. The rate of the self discharge determines the shelf life of a non-rechargeable battery.

• Self discharge rate: The rate at which a cell or battery loses service capacity when standing idle.

• Semipermeable membrane: A separator through which certain molecules can pass but others cannot.

• Sensing electrode: See working electrode.

• Separator: Electrically insulating layer of material which physically separates electrodes of opposite polarity. Separators must be permeable to the ions of the electrolyte and may also have the function of storing or immobilizing the electrolyte. Often a thin structural material (usually a sheet) used to separate the electrolyte of a divided electrochemical cell into two or more compartments. A separator is typically either a membrane or a diaphragm. The distinction between these two separators is somewhat blurred. A membrane typically has very small pores that permit only diffusional or conductive motion of the solvent or the electrolyte from one compartment to another. A diaphragm has larger pores so that it permits the flow (see convection) of the electrolyte solution from one compartment to another but still restricts the complete intermixing the two solutions.

• Series connected:A method of connection in which the positive terminal of one device is connected to the negative terminal of another. The voltages add and the current is limited to the least of any device in the string. (See also parallel connected.)

• Series-coupled cells: Individual electrochemical cells can be combined in assemblies by series or parallel coupling (or a combination of the two). In case of "series" coupling, the positive electrode of one cell is connected to the negative electrode of the next cell, and so on. The assembly has only two external terminals. The overall voltage of the assembly is the sum of the individual cell voltages, while the current passing through every cell (and the assembly) is the same. Series coupling can be used in a number of assemblies, such as battery, cell line, and stack. Contrast with parallel coupling.

• Service life: Timescale of satisfactory performance of a battery under a specified operating schedule, expressed in units of time or number of charge/ discharge cycles.

• SHE: Stands for standard hydrogen electrode.

• Shedding: The process whereby poorly adhering active mass (generally in the positive plate of a lead-acid cell) falls from the grid to form a sludge (mud) on the floor of the cell.

• Shelf life: Period of time a cell can be kept idle after manufacture without significant deterioration. The time period a non-rechargeable battery can be stored after manufacturing so that it still can provide a required amount of electricity when connected to a load. The shelf life of modern batteries is many years.

• Short-circuit: The condition when the terminals of a cell or battery are connected directly.

• Siemens: The measurement unit of electrical conductance. Symbol: "S". The reciprocal of ohm, and sometimes called "mho."

• Silver/silver-chloride electrode: A commonly used reference electrode. The electrode assembly consists of a silver metal electrode in contact with solid silver chloride (usually as a coating on the silver metal) immersed in an aqueous chloride salt solution saturated with silver chloride. All these are contained in a small vessel, typically made of glass tubing. The internal electrolyte of the reference electrode assembly and the external electrolyte into which the whole assembly is immersed are in ionic contact through a separator. A typical separator is a small porous ceramic plug sealed into the end of the glass tubing. The operating principle of this electrode is that of an electrode of the second kind. The equilibrium electrode potential is a function of the chloride concentration of the internal electrolyte ("filling solution"). The most commonly used electrolyte is 4 molar potassium chloride, producing a potential of 0.222 volt against the standard hydrogen electrode at 25 oC. Occasionally, other concentrations of potassium chloride or other chloride salts are used.

• SLI battery: A battery of usually 12 V or 24 V used for Starting, Lighting and Ignition in vehicles with internal combustion engines.

• Solar constant: The rate at which energy is received from the sun just outside the earth's atmosphere on a surface perpendicular to the sun's rays. Approximately equal to 1.36 kW/m2.

• SMDE: Stands for static-mercury-drop electrode.

• Sodium chlorate production: See brine electrolysis.

• Sodium chloride solution, electrolysis: See brine electrolysis.

• Sodium hydroxide production: See brine electrolysis.

• Sodium hypochlorite production: See brine electrolysis.

• Soil remediation: See electroremediation.

• Solid-oxide fuel cell: A fuel cell that employs a solid, ionically conductive material as electrolyte. Due to the typically low ionic conductivity of solid oxides, these fuel cells must operate at very high temperatures.

• Solubility: The maximum amount of a species that can be dissolved in a given solvent. It is usually expressed as the maximum achievable concentration. A solution is called "saturated" if it contains the maximum dissolvable amount.

• Solubility product: The solubility of slightly soluble salts is often expressed as the product of the solubility concentrations of its ions. E.g., the solubility product of silver chloride is the product of the concentrations of the silver and chloride ions in the saturated solution of this salt. The significance of the solubility product is that its value cannot be exceeded even in the presence of other dissolved salts. Consequently, the solubility of silver chloride is less in a solution containing potassium chloride than in pure water. This is because in the calculation of the solubility product one must use the "total" chloride concentration in the solution, therefore a silver concentration lower than in water is needed to satisfy a constant solubility product. The solubility (the saturated solution concentration) of the salt, in the absence of any other dissolved species in the solution, is the square root of the solubility product for a salt like the silver chloride. Strictly speaking, activities should be used instead of concentrations.

• Solute: The dissolved species (e.g., a salt) in a solution.

• Solution ir drop: The ir drop in the electrolyte solution of a three-electrode cell between the working and the reference electrodes. This ir drop (which is expressed as a potential) is always included in the measured potential of the working electrode. Therefore, it is important to minimize this error, and to place the reference electrode as close as possible to the working electrode (see Luggin tip). It is also called "ohmic overpotential (or polarization)" or "resistance overpotential (or polarization)." One can correct for the ir drop to obtain the real electrode potential, or in some cases one can compensate for the ir drop during potential control. During the measurement of an electromotive force (potential measurement without any current flowing), the ir drop is always zero, and the position of the reference electrode is immaterial. See also ohmic loss.

• Solvation: Ions in solution are always surrounded by solvent molecules. A few of these molecules will be more or less strongly attached to the ion (mainly because of the attraction of the charged ion and the dipole of the solvent molecule) and this assembly may be considered as a single unit for some purposes. E.g., the solvent molecules will move together with the ion during diffusion and electromigration. The number of solvent molecules so attached to an ion is called the "solvation number." The surface of an electrode also can, and usually is, solvated. Since the electrodes usually have some excess charge (see electrical double layer,) they also attract the solvent dipoles, and the electrode surface is usually covered by a monolayer of strongly oriented solvent molecules. Under certain extreme conditions, a solution can contain free electrons that are stabilized by solvation. The solvation number is not very exactly defined since its value may depend on the measurement technique.

• Solvation number: See solvation.

• Sonoelectrochemistry: Electrochemical phenomena occurring under the influence of soundwaves (typically ultrasound).

• Source: See electrical source (supply).

• Specific conductance: The quantitative and characteristic measure of the conductivity of a given substance. This characteristic constant is the numerical value of the conductivity between two opposite sides of a unit cube (usually a cube of one centimeter) of the substance. Also called "specific conductivity."

• Specific energy: The energy output of a battery per unit weight, usually expressed as Wh kg-1.

• Specific ion electrode: See ion-selective electrode.

• Specific power: The power output of a battery per unit weight, usually expressed as W kg-1.

• Specific resistance: The quantitative and characteristic measure of the resistivity of a given substance. This characteristic constant is the numerical value of the resistivity between two opposite sides of a unit cube (usually a cube of one centimeter) of the substance. Also called "specific resistivity."

• Spectroelectrochemistry: The simultaneous application of electrochemical and optical spectroscopic techniques to investigate a phenomenon.

• Stack: A series-coupled assembly of cells, a term used primarily for fuel cells.

• Standard cell: A non-rechargeable cell (battery) whose emf is accurately known and remains sufficiently constant. It is less and less used nowadays because the availability of electronic voltage standards.

• Standard electrode potential: The equilibrium potential of an electrode when both the oxidized and the reduced species are present in unit concentration (strictly speaking, activity) in the solution; if the "reduced" form is a metal, a pure metal (not alloyed with other metals) is considered to be at unit concentration. (See also the Nernst equation.) The standard potentials are always expressed against the standard hydrogen electrode the potential of which is zero "by definition." Standard potentials are a function of the temperature, they are usually tabulated for 25oC. Also called "normal electrode potential." The standard potential is the electromotive force of an electrochemical cell comprised of the electrode in question and the standard hydrogen electrode. Strictly speaking, one must use unit activities rather than concentrations.

• Standard hydrogen electrode: The most fundamental reference electrode in electrochemistry. "By definition" its equilibrium potential is considered zero at any temperature, because this electrode was chosen as an arbitrary zero point for electrode potentials. A zero point is needed since the potential of a single electrode cannot be measured, only the difference of two electrode potentials is measurable. All electrode potentials are expressed on this "hydrogen scale." It is a hydrogen electrode with an electrolyte containing unit concentration of hydrogen ions and saturated with hydrogen gas at unit atmosphere pressure. This electrode can be somewhat inconvenient to use because of the need to supply hydrogen gas. Therefore, other reference electrodes (e.g., calomel or silver/silver chloride) are often used instead, but the measured electrode potentials can be converted to the "hydrogen scale." Abbreviated as "SHE." Also called "normal hydrogen electrode." Strictly speaking, one must use unit activity rather than concentration of hydrogen ions and unit fugacity rather than unit pressure of hydrogen gas.

• Standard rate constant of electrode reaction: The rate constant of an electrode reaction at the standard electrode potential.

• State of charge: For a rechargeable battery: the fraction, usually expressed as a percentage, of the total electrical energy stored in a battery by charging that is still available for discharging at a certain point of time. Contrast with depth of discharge.

• Static-mercury-drop electrode: Alternative name for hanging-mercury-drop electrode. Abbreviated as "smde."

• Stationary state: See steady state.

• Steady state: A state of a system in which the conditions do not change in time, or at least they do not seem to change with time. That is, the change occurs on a time scale longer than the time scale of the observations. A good example of the creation and slow vanishing of a steady-state condition is the liquid-junction potential.

• Stern modification of the double layer theory: See the Gouy-Chapman-Stern model of the double layer.

• Storage battery: See secondary battery.

• Streaming potential: An electrical potential difference that 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. See electrokinetic effects.

• Stripping analysis: A group of electroanalytical techniques for the determination of trace amounts of substances, consisting of two steps: preconcentration and analysis. The preconcentration involves the electrodeposition or adsorption of the substance to be determined on the surface of an electrode. This is followed by the "stripping" analysis of the substance by an electroanalytical technique. For example, traces of metal ions can be preconcentrated by cathodic electrodeposition followed by anodic dissolution (stripping). Or traces of halides (e.g., chloride) can be anodically preconcentrated at a mercury electrode as mercury salts, followed by cathodic stripping.

• Supercapacitor: See electrochemical capacitor.

• Supply: Alternative expression for source.

• Supporting electrolyte: An electrolyte added to the solution for the sole purpose to increase the solution conductivity, while the electrolyte does not take part in any reactions. Also called "inert," "indifferent," or "swamping" electrolyte.

• Surface active agent: A substance which modifies the behaviour of a phase by interacting with its surface. For example, in the case of lead-acid batteries, the morphology of the active materials deposited at the electrodes may be strongly affected by the addition of surface-active agents.

• Swamping electrolyte: Alternative expression for supporting electrolyte.