Protonation constant of caffeine in aqueous solution

Abstract

The decreases in the conductivities of dilute aqueous solutions of hydrochloric acid and of sodium chloride on the addition of various concentrations of caffeine have been measured from 25 to 50 °C. The viscosities of the caffeine solutions have also been determined. The results, interpreted by the isodesmic model for caffeine self-association, showed that only the monomeric caffeine molecules in solution were protonated. The dissociation constant Hcaf⁺ was found to be 0.82, 0.76 and 0.69 mol kg^–1 at 25, 38, and 50 °C, respectively. The dimerisation model for caffeine association failed to fit the conductivity results.