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

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  • Spectrophotometric Analysis of the Rela

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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.

Journal

  • Analytical Sciences

    Analytical Sciences 12 (6), 927-930, 1996

    The Japan Society for Analytical Chemistry

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