Analytical Sciences

Abstract − Analytical Sciences, 15(4), 339 (1999).

Features of Phenolsulfonphthalein and Phenolphthalein Substituted at All Ortho-Positions of Phenols with Bromine
Zenzo TAMURA, Rimi TERADA, Kennichi OHNO and Masako MAEDA
School of Pharmaceutical Sciences, Showa University, Hatanodai, Shinagawa, Tokyo 142-8555, Japan
The influence of bromination at all the ortho-positions of phenols on the features of phenolsulfonphtalein and phenolphtalein was investigated. The absorption spectra of Bromphenol Blue (H2BPS) in phosphate buffers demonstrated the complete opening of the lactone ring to form yellow HBPS- at pH 1 where pK2=3.8. The reaction of hydroxide ion with blue BPS2- was found to produce colorless BPS(OH)3- required heating where pK3=~9. On the contrary, the spectra of 3',3",5',5"-tetrabromophenolphtalein (H2BPP) demonstrated that the lactone ring was very stable. More than 99.5% of BPP2- species consisted of a colorless lactone possessing two isolated phenol groups where pK1'=6.0 and pK2'=6.8; the reaction of hydroxide ion with BPP2- to produce BPP(OH)3- was slow where pK3=10.3. These results indicate the increased acidity of phenols and the greater tendency of the central carbon atom to act as an electron acceptor. (Keywords: Bromphenol Blue, 3',3",5',5"-tetrabromophenolphtalein, structural formula and color, dissociation exponent)