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 PP²⁻ was red, where pK₁=9.05, and pK₂=9.50; further, the reaction of hydroxide ion with PP²⁻ 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 PS²⁻, having a similar red color and absorption spectrum to PP²⁻. These results clearly demonstrate the relationship between the color and the prolonged conjugated system.