Haber-Luggin capillary: See
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
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
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
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.
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
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
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.