A historical overview of the need for distinction between the attainment of thermodynamic equilibrium and a kinetic steady state in experimental studies

In the quantitative characterization of experimental systems the observation of a time-independent response can signify the attainment of thermodynamic equilibrium. However, there are also situations where the time-independence of experimental response merely reflects the attainment of a kinetic steady state. Although quantitative expressions derived on the basis of kinetic steady-state attainment retain validity for a system at thermodynamic equilibrium, those based on attainment of thermodynamic equilibrium do not apply to systems merely reflecting the attainment of a kinetic steady state. This survey of experimental procedures encountered during research endeavours over the past six decades has revealed many situations where the source of the time-independent instrumental response was correctly identified, but also a number where the responses were misinterpreted.