J. Technology and Education, Vol.28, No.2, pp.65-75 (2021)
総 説
フェノールフタレインの構造研究の歩み
野口 大介
長崎大学大学院工学研究科教育研究支援部（〒852-8521 長崎県長崎市文教町1-14）
*
a.chemist.noguchi.d@gmail.com
A historical overview of studies on the
structures of phenolphthalein
Daisuke
NOGUCHI
Division
of Education and Research Support, Graduate School of Engineering, Nagasaki
University
(Bunkyo-machi
1-14, Nagasaki 852-8521, Japan)
(Received September 11, 2021; Accepted October
4, 2021)
Abstract
Phenolphthalein, one of
the well-known acid-base indicators commonly used in chemical education and
analytical technology, has various applications in industry, such as in
carbonation testing on concrete. Previously, its structural changes in
different pH solutions have remain somewhat unclear, and, regrettably, there
has been no review article dealing with that. In this historical overview, I
describe the details, including its confusion. Phenolphthalein, noted as H2PP,
is a diprotic acid that dissociates H+, producing HPP–
and PP2–. HPP– has a structure of the lactone-ring and,
possibly, ring-opened carboxylate. Around the 1940s, due to a misunderstanding
of resonance theory, ring-opened HPP– was commonly accepted;
however, after the 1980s, probably to avoid the complexity, the representation
of HPP– itself tended to be ignored. By contrast, before the 1980s,
PP2– was consistently shown only as a ring-opened form. After that,
however, it was understood that some PP2– could exist as a
ring-closed structure in a solution and a solid state because of the strength
of the lactone ring as compared with the sulfophthalein dyes. Although inclusion compounds, with PP2–
with β-cyclodextrin and a metal complex, were reported, their X-ray crystal
analyses have not yet been reported. Since the idea of PP2– as a
lactone form seems not to have been accepted commonly yet, it would be helpful
to compare it with research in clathrate chemistry, where ring-closed PP2–
is widely known.
Key words: acid-base indicator, chemical structure, lactone,
triphenylmethane dye.