The pH Jump: A Rapid Modulation of pH of Aqueous Solutions by a Laser Pulse

The first electronically excited state of compounds like 8-hydroxypyrene-1,3,6-trisulfonate or 2-naphthol-3,6-disulfonate is characterized by an acid dissociation constant (pX* = 0.5) much lower than that of the ground state (pk° = 7). Thus excitation of such compounds is equivalent to introduction of a strong acid to the solution. In this study we utilized high-intensity (~50 mJ) short laser pulse (50 ns full width at half-maximum) to acidify aqueous solutions by exciting the above proton emitters. The discharged protons were detected both by their reaction with various pH indicators or by following the reprotonation of the proton emitter after its decay to the ground state. The maximal proton concentrations determined by either method are comparable and in typical experiments amount to 10-5 67-5 X 10-5 M. The maximal proton concentration is built up during the laser pulse and decays to the initial level within a few microseconds. The protonation of pH indicators is a second-order diffusion-controlled reaction; the measured rate constants are compatible with the values calculated according to Debye's equation for diffusion-controlled reactions. Though the proton pulse is short, groups which have been protonated during the pulse remain in their protonated state for much longer periods, which are proportional to their intrinsic acid dissociation constant. The applicability of this method as a perturbing system for fast kinetics is discussed.