The Master Key to the Problem of Reversible Chemical Hydrogen Storage is 12 kJ (mol H₂)^-1

This article unveils on basis of the ideal gas law, the atomic conception of matter and classic equilibrium thermodynamics the ideal final regularity of reversible hydrogen mass transfer. This result allows to clarify problems of metal hydride chemistry which otherwise are impossible to understand e.g. why the substitution of 4 mol % Na by K in Ti-doped NaAlH₄ raises the reversible hydrogen capacity by 42 % at no substantial change to thermodynamic reaction parameters or how the dopants take effect in (Rb/K)-co-doped Mg(NH₂)₂/2LiH; both cases are discussed in this context. This ideal final regularity is a hitherto missed out superposition of physical chemistry fundamentals and defines the maximum specific energy at distinct conditions: directly for two-phase hydrogen storage methods and indirectly for electrochemical systems due to the normative role of hydrogen electrode potentials.