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Electrochemistry Dictionary - L
Lead-acid battery: A rechargeable battery.
During discharging, the reaction on the positive electrode is the conversion
of lead dioxide to lead sulfate, while on the negative electrode it is the conversion
of metallic lead to lead sulfate. The reactions are reversed during charging.
The current collector can be lead in both electrodes. The electrolyte is sulfuric
acid. While it is one of the earliest practical storage batteries (1866), it
is still very widely used today, e.g. as automobile starter battery.
Leakage current: See current leakage.
Leclanche cell (battery): One of the
earliest practical non-rechargeable batteries (Georges-Lionel
Leclanche). It uses a zinc anode (negative electrode) and a manganese dioxide
cathode (positive electrode) with ammonium chloride solution as electrolyte.
The initially liquid electrolyte was later "immobilized," and this system became
the first dry cell. It is still widely used.
Leveling agent, leveler: Small amounts
of (usually organic) compounds added to an electroplating solution that changes
the mechanism of the plating to produce a metal deposit smoother than the original
Limiting current density: The maximum
current density that can be achieved for an electrode reaction at a given concentration
of the reactant in the presence of a large excess of supporting electrolyte.
The mass transport occurs exclusively through diffusion in the diffusion layer,
driven by the concentration difference of the reactant between the edge of the
diffusion layer and the electrode surface. As the current density is increased
(usually by changing the electrode potential), the surface concentration of
the reactant must decrease so that the concentration difference driving the
diffusion can increase and provide the required flux of the reactant. However,
the surface concentration obviously cannot decrease below zero, thereby a situation
is reached when further change of the electrode potential cannot increase the
reactant flux, and correspondingly the current density. The concept of "limiting
current density" is valid even in the absence of supporting electrolyte. However,
the situation is more complex in this case because electromigrational effects
must also be taken into consideration.
Linear-sweep voltammetry: See voltammetry.
Liquid-junction potential: A potential
difference between two solutions of different compositions separated by a membrane
type separator. The simplest example is the case of two solutions containing
the same salt in different concentrations. The salt will diffuse from the higher
concentration side to the lower concentration side. However, the diffusion rate
of the cation and the anion of the salt will very seldom be exactly the same
(see mobility). Let us assume for this example that the cations move faster;
consequently, an excess positive charge will accumulate on the low concentration
side, while an excess negative charge will accumulate on the high concentration
side of the junction due to the slow moving anions. This sets up a potential
difference that will start an electromigration of the ions that will increase
the net flux of the anions and decrease the net flux of the cations. In steady-sate
conditions, the two ions will move at the same speed and a potential difference
will be created between the two solutions. This "steady-sate" potential difference
seems constant, but this is misleading because it slowly changes as the concentrations
between the two solutions equalize. The diffusion process will "eventually"
result in equal concentrations of the salt in the two solutions separated by
the membrane, and the liquid-junction potential will vanish. For a simple case,
the value of the liquid junction potential can be calculated by the so called
Load: A device that consumes electrical
power, e.g. a motor or a light bulb.
Local cell: See corrosion.
LSV: Stands for "linear-sweep voltammetry,"
Luggin probe or
Luggin-Haber capillary: a
device used in measuring the potential of an electrode with a significant current
density imposed on its surface. (The probe minimizes the IR drop that would
otherwise be included in the measurement and without significantly disturbing
the current distribution on the specimen. A salt bridge with a thin, capillary
tip at one end. This can be useful for minimizing the solution ir drop by placing
the fine capillary tip very close to the surface of the working electrode, when
the salt bridge is used to connect the working and reference electrode compartments
of a three-electrode cell. The solution distance causing the ir drop can be
easily limited to a few millimeters; and, in specially designed cells, often
to a much smaller distance.
LV: Stands for "linear-sweep voltammetry,"