An operational pH in aqueous dimethylsulfoxide based upon the acidity dependence of the rate of a simple ionic recombination reaction in the lowest excited singlet state

7-Hydroxy-1-naphthalenesulfonate (2-naphthol-8-sulfonate) monoanion demonstrates proton transfer in the lowest excited singlet state in DMSO–water mixtures as well as in pure water. The dissociation reaction of the directly excited monoanion is strongly solvent-dependent, and independent of solution acidity. The reprotonation of the conjugate base, however, depends predominantly on the acidity of the solution and only on the continuum properties of the solvent. The separability of the dissociation and reprotonation reactions, using steady-state methodology, allows the reprotonation to be treated independent of the dissociation. The linear relationship between the ratio of the relative fluorescence efficiencies of acid and conjugate base, and the hydrogen ion concentration is obtained only if proper Brönsted activity factors are included in the relationship. These factors can be calculated from classical electrostatics and are the fourth powers of the activity coefficients necessary to convert the formal hydrogen ion concentration to hydrogen ion activity. Using this approach, pH was calculated from hydrogen ion concentration in DMSO–water solutions, containing a mole fraction of DMSO up to about 0.4.