pH jump: A relaxational approach

Back-protonation of the 8-hydroxy-1,3,6-pyrenetrisulfonate anion (RO^- + H⁺ → ROH (k_b)) initiated by a laser pulse photolysis of the neutral molecule (the pH jump method, pH_j) is used in order to establish the chemical equivalence between the generated protons and water equilibrium protons. The diffusion-controlled nature of the back-reaction is established by checking the temperature and isotope (H/D) dependence of the reaction. A relaxation kinetics analysis is carried out in order to follow the time evolution of [H⁺]. The same analysis is proved to be applicable for the description of the bleaching kinetics of an acid-base indicator (BCG) present in the solution at the time of the pH jump.