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
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
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.
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.
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
Timescale of satisfactory performance of a battery under a specified
operating schedule, expressed in units of time or number of charge/ discharge
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.
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.
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
Sodium chlorate production: See brine
Sodium chloride solution, electrolysis:
See brine electrolysis.
Sodium hydroxide production: See brine
Sodium hypochlorite production: See
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
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.
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 energy: The energy
output of a battery per unit weight, usually expressed as Wh kg-1.
Specific ion electrode: See ion-selective
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
Spectroelectrochemistry: The simultaneous
application of electrochemical and optical spectroscopic techniques to investigate
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
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
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
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
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.