Invariance of activity coefficients for the hydrogen ion with charge-type of fluorescent indicators in aqueous-methanol solutions

Steady-state fluorimetric pH titrations of three fluorescent indicators of differing charge-type were carried out in aqueous methanol solutions containing up to 64 mol% methanol. The plot of the ratio of the relative fluorescence efficiency of the indicator acid to that of its conjugate base versus the product of the formal hydrogen ion concentration and the appropriate Bronsted kinetic activity factor gives straight lines for titrations in each mixed solvent, just as it does in water, but only if the constant terms in the Bronsted factor are modified to account for the bulk dielectric constant in each solvent. The product of the formal charges on the proton and the particular conjugate base studied in each titration must also be included in the Bronsted factor. This supports the hypothesis that a valid operational pH can be defined and kinetic parameters related to proton transfer can be extracted from the fluorimetric titrations in mixed solvents.