Spectrophotometric Analysis of the Relationship between Dissociation and Coloration, and of the Structural Formulas of Phenolphthalein in Aqueous Solution

Abstract

The absorption spectra of phenolphthalein (H²PP) in phosphate buffers of pH 5 to 13 were analyzed in order to determine the features of the dissociation and coloration of H²PP. The spectra demonstrated that HPP^-, while keeping a γ-lactone ring, was colorless, and that only PP^2- was red, where pK₁=9.05, and pK₂=9.50; further, the reaction of hydroxide ion with PP^2- to produce colorless PP(OH)^3- was rather slow, where pK₃=12. On the contrary, the color of the aqueous solution of phenolsulfonphthalein (H₂⁺PS^-), having a zwitter ion structure and no lactone ring, was orange-red, which dissociated to yellow HPS^- and finally to PS^2-, having a similar red color and absorption spectrum to PP^2-. These results clearly demonstrate the relationship between the color and the prolonged conjugated system.