All articles A–Z

Hydrogen isotope exchange

Hydrogen isotope exchange (HIE) replaces carbon-bound hydrogen in organic molecules with deuterium or tritium, typically catalytically, and is a central tool for labeling drugs and radiotracers.

Hydrogen jet fire

A hydrogen jet fire is a turbulent diffusion flame from an ignited pressurized hydrogen release, controlled by jet momentum and radiating little heat, so it is nearly invisible.

Hydrogen leak detection is the use of sensor, optical, acoustic, and tracer-gas methods to find unintended hydrogen releases, which matter because hydrogen is highly flammable and burns with a nearly invisible flame.

Hydrogen line

The hydrogen line, also called the H I line or 21 cm line, is a spectral line of neutral hydrogen from the hyperfine spin-flip transition, widely used to map galaxies.

Hydrogen liquefier

A hydrogen liquefier is an industrial refrigeration system that cools gaseous hydrogen to about −253 °C, producing liquid hydrogen for rocket propellant and energy transport.

Hydrogen lithography

Hydrogen depassivation lithography (HDL) is an atomic-scale lithography technique, developed at the University of Illinois in the mid-1990s, in which a scanning tunneling microscope tip removes individual hydrogen atoms from a silicon surface.

Hydrogen maser

A hydrogen maser is an atomic clock that uses hydrogen's 1420 MHz hyperfine transition as its reference, prized for stability in VLBI, deep-space tracking, and national time scales.

Hydrogen mass repartitioning

Hydrogen mass repartitioning (HMR) is a molecular dynamics technique that transfers mass from heavy atoms to bonded hydrogens, allowing simulations with a 4 fs time step.

Hydrogen molecular ion

The hydrogen molecular ion, H₂⁺, is a molecule made of two protons bound by a single electron, recognized as the simplest molecule and the reference molecule of quantum chemistry.

Hydrogen negative ion

The hydrogen negative ion, or hydride ion, H⁻, is the anion of hydrogen: a proton bound to two electrons, unbound in the Hartree–Fock approximation and a standard test case for electron correlation.

Hydrogen oxidation reaction

The hydrogen oxidation reaction (HOR) is the fuel cell anode half-reaction converting hydrogen to protons and electrons; it is far slower in alkaline media than in acid, raising platinum demand.

Hydrogen permeation testing

Hydrogen permeation testing is a family of laboratory methods measuring how hydrogen moves through metals, used to assess hydrogen embrittlement and standardized by ISO and ASTM.

Hydrogen peroxide

Hydrogen peroxide (H₂O₂), also called dihydrogen dioxide, is a colorless liquid inorganic compound and powerful oxidizing agent used chiefly for bleaching, disinfection, and chemical synthesis.

Hydrogen peroxide–urea

Hydrogen peroxide–urea, also called carbamide peroxide or urea peroxide, is a crystalline 1:1 adduct of urea and hydrogen peroxide used as a stable solid peroxide source.

Hydrogen phase III

Hydrogen phase III is an orientationally ordered solid molecular phase of hydrogen, stable above roughly 150 GPa and marked by a sharp change in infrared vibron absorption.

Hydrogen phase IV

Hydrogen phase IV is a high-pressure solid phase of dense hydrogen, identified in 2012 to 2013 at roughly 220 to 230 GPa, mixing molecular hydrogen with weakly molecular layers.

Hydrogen phase V

Hydrogen phase V is a proposed solid phase of dense hydrogen, reported in 2016, appearing above 325 gigapascals and seen as a step toward metallic hydrogen.

Hydrogen pipeline transport

Hydrogen pipeline transport moves gaseous hydrogen through dedicated pipelines, the lowest-cost delivery option over land, though hydrogen embrittlement limits which steel pipes can carry it.

Hydrogen plasma reduction of iron

Hydrogen plasma reduction of iron (HPR), or hydrogen plasma smelting reduction (HPSR), reduces iron ore to iron using hydrogen plasma, emitting water vapor instead of CO2.

Hydrogen plasma smelting reduction

Hydrogen plasma smelting reduction (HPSR) is an experimental ironmaking process that reduces iron ore to liquid iron in one step, emitting water vapor instead of carbon dioxide.

Hydrogen processing of magnet scrap

Hydrogen processing of magnet scrap (HPMS) is a recycling method that uses hydrogen gas to turn sintered rare earth magnets like NdFeB into demagnetized powder for new magnets.

Hydrogen production

Hydrogen production, also called hydrogen generation, is the industrial manufacture of hydrogen gas, mostly from fossil fuels, with 97 million tonnes produced globally in 2023.

Hydrogen Production Model

The hydrogen production model is a combined experimental and analytical representation, built by JAERI and JAEA, of a steam methane reforming plant coupled directly to Japan's HTTR high-temperature gas-cooled reactor.

Hydrogen recovery from refinery purge gas

Hydrogen recovery from refinery purge gas is the capture and purification of unreacted hydrogen from refinery purge streams, mainly by pressure swing adsorption, membranes, or cryogenic separation.

Hydrogen safety

Hydrogen safety is the set of properties, engineering practices, and codes for handling hydrogen fuel safely, addressing its wide flammability range, low ignition energy, and embrittlement of metals.

Hydrogen Shot

Hydrogen Shot is a US Department of Energy initiative launched in June 2021 to cut the cost of clean hydrogen production 80%, to $1 per kilogram in a decade.

Hydrogen society

A hydrogen society is an energy system using hydrogen as a major energy carrier, a goal Japan has pursued with policy since its 2017 strategy.

Hydrogen spectral series

The hydrogen spectral series, or hydrogen lines, are the emission and absorption lines of atomic hydrogen, grouped by lower energy level into six named series.

Hydrogen spectroscopy

Hydrogen spectroscopy is the measurement of light emitted or absorbed by atomic and molecular hydrogen, a precision testing ground for quantum electrodynamics since Balmer's 1885 formula.