ABSTRACT
The thermodynamic standard state for the element hydrogen is the diatomic molecule in the gaseous state at 298.15 K containing an equilibrium mixture of the ortho and para isomers (normal hydrogen). By convention, the term normal hydrogen does not necessarily refer to isotopically pure hydrogen (protium), but is taken to mean hydrogen that contains deuterium at the levels of natural abundance or less. Hydrogen forms covalent bonds with many elements, for which no oxidation state is normally given. The hydrogen ion (proton) in water is the predominant instance of the +1 state, and, by convention, is the reference state for establishing the energy levels of ions in solution. The use of hydrogen gas is undesirable in many laboratory situations, and this has prompted the development of nonequilibrium hydrogen reference electrodes. Another type of hydrogen electrode that can be used without hydrogen gas bubbling is the palladium hydride-hydrogen electrode.
