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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 substrate.

  • 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 "Henderson" equation.

  • 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," see 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," see voltammetry.