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Electrochemistry Dictionary - H

  • Haber-Luggin capillary: See Luggin capillary.

  • Half cell: A somewhat archaic term, indicating a structure that contains an electrode and the surrounding electrolyte. Electrochemical cells are often divided, containing two separate electrolytes (one surrounding each electrode, e.g., the Daniell cell). In these cases the electrode and its electrolyte can be considered "half" of the cell. Commercially available reference electrodes can be considered "half cells."

  • Half-cell reaction: A not incorrect, but somewhat archaic term for electrode reaction.

  • Half reaction: See electrode reaction.

  • Hanging-mercury-drop electrode: A variation of the dropping-mercury electrode, where the mercury is not flowing continuously. A "hanging drop" is formed at the end of the capillary and is used as a working electrode. The advantage of this electrode arrangement is that the droplet can easily be renewed by feeding some mercury if the electrode surface becomes contaminated. Also called "static-mercury-drop electrode." Abbreviated as "hmde."

  • Helmholtz layer: See the Helmholtz model of the double layer.

  • Helmholtz model of the double layer: The simplest model of the electrical double layer. The excess ions in the solution side of the double layer line up in one plane ("Helmholtz plane") very close to the electrode surface. In a somewhat more complex model there are two planes of closest approach of the ions. Ions in the "outer Helmholtz plane" are about two solvent-molecule diameters away from the electrode surface because both the ions and the electrode surface are solvated. Ions in the "inner Helmholtz plane" have shed their solvation layer (these are usually the weekly solvated, large anions) and penetrated the solvent layer on the electrode; these, so called contact adsorbed, ions are sitting directly on the electrode surface. The ionic portion of the Helmholtz model is often called the "Helmholtz layer" or "compact layer."

  • Helmholtz plane: See the Helmholtz model of the double layer.

  • Henderson equation: An equation that predicts the value of the liquid junction potential for simple cases.

  • HER: Stands for hydrogen evolution reaction.

  • Heterogeneous charge-transfer reaction: A charge-transfer reaction with the charge transferred across a phase boundary, typically between a solid and a liquid phase. Contrast with homogeneous charge-transfer reaction.

  • Hittorf method: An experimental method for the determination of transport numbers. Electrolysis is carried out in a three-compartment cell and the concentration changes occurring in the anode and cathode compartments can be used to calculate the transport numbers. The concentration in the center compartment should remain unchanged.

  • HMDE: Stands for hanging-mercury-drop electrode.

  • Homogeneous charge-transfer reaction: A charge-transfer reaction with both reactants present in the same phase. Typically both reactants are dissolved species in a solution while the charge is transferred from one to the other. Contrast with heterogeneous charge-transfer reaction.

  • Hybrid cell: Electrochemical cell in which one of the two active reagents is in the gas phase and may be supplied from an external source. A hybrid cell occupies an intermediate position between closed cells and fuel cells.

  • Hybrid electric vehicle: A vehicle that has more than one type of power supply to support the drive: e.g. Battery/motor plus fossil fuel/internal combustion engine.

  • Hybrid system: A power system consisting of two or more power generating subsystems (e.g., the combination of a wind turbine and a photovoltaic system).

  • Hydration: Solvation occurring in an aqueous solution.

  • Hydration number: The number of water molecules associated with an ion in the process of solvation in aqueous solutions.

  • Hydrodynamic boundary layer: A thin immobile layer of fluid that always exists at a solid/moving-fluid interface. Whether the movement of the fluid is due to "forced" or "natural" convection, a thin layer of fluid will always remain completely immobile at the surface of the solid due to the solid-liquid interactive forces.

  • Hydrodynamic voltammetry: Voltammetry under conditions of convective mass transport to/from the surface of the working electrode.

  • Hydrogen economy: A proposed, new energy distribution system based on hydrogen gas as the energy carrier and hopefully on a renewable energy supply. Hydrogen could be generated (using e.g., solar energy) in a variety of ways, one of them being water electrolysis. Hydrogen would be distributed to the end users through a system similar to today's gas pipelines. The hydrogen could be used either by burning to generate heat or by fuel cells to generate electricity.

  • Hydrogen electrode: A redox electrode with dissolved hydrogen gas being the reduced species and hydrogen ions the oxidized species. Hydrogen gas (or a gas mixture containing hydrogen) is bubbled through the electrolyte to keep a desired dissolved hydrogen content. The inert metallic electrode is usually platinized platinum. The equilibrium potential of this electrode depends on the concentration (strictly speaking, activity) of both the hydrogen ions and the dissolved hydrogen gas (controlled by the hydrogen gas pressure), see Nernst equation. The electrode can be used as a measuring electrode in a sensor to determine the hydrogen ion concentration (pH), or it can be used as a reference electrode if all the concentrations are known and constant. It is used equally often for both purposes. It is also the most fundamental reference electrode as the standard hydrogen electrode. See also the dynamic hydrogen electrode and the reversible hydrogen electrode.

  • Hydrogen evolution reaction: An electrode reaction in which hydrogen gas is produced at the cathode of an electrolytic cell by the reduction of hydrogen ions or the reduction of the water molecules of an aqueous solution. Abbreviated as "her." See also water electrolysis.

  • Hydrogen production: See water electrolysis.

  • Hydrogen scale of electrode potentials: See standard hydrogen electrode.

  • Hydrolysis: A chemical reaction in which water reacts with another substance and gives decomposition or other products, often a reaction of water with a salt to create an acid or a base.

  • Hydrous: A substance that contains water. The opposite of anhydrous.

  • Hypochlorite production: See brine electrolysis.