descriptions differentiate catalytic activity of material surfaces for the
water oxidation reaction by the stability of the reactive oxygen (O*)
intermediate. The underlying conjecture
is that there are several meta-stable steps of the reaction, each connected by
free energy differences critically dependent on O*. Recently in-situ,
time-resolved spectroscopy of the (photo
)-electrochemical water oxidation reaction identified the vibrational and optical signatures of O* time-evolution. However, there has been little connection between these inherently kinetic experiments and the underlying thermodynamic parameters of the theory. Here, we discover that picosecond optical spectra of the O* population modulated by a shift in reaction equilibria defines an effective equilibrium constant (Keff) containing the relevant free-energy differences. A Langmuir isotherm as a function of electrolyte pH extracts Keff using a model titania system (SrTiO3). The results show how to obtain equilibrium constants of individual reaction steps on material surfaces, which had not been experimentally accessible previously. Further, we find that for a photo-excited reaction on a semiconductor surface tuning past a pH defined by Keff doubles the initial O* population. That the free energies of the catalytic surface are definable through a time-resolved spectroscopy, alongside the finding that the surface recollects its explicit equilibrium with the electrolyte, provides a new and critical connection between theory and experiment by which to tailor the pathway of water oxidation and other surface reactions.