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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.

analytical sciences

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